3 −1 Globally, salinization affects between 100 and 1000 billion m a of irrigation water. The discovery that zero valent iron (ZVI, Fe ) could be used to desalinate water (using intra-particle catalysis in a diffusion environment) raises the possibility that large-scale in situ desalination of aquifers could be undertaken to support agriculture. ZVI desalination removes NaCl by an adsorption–desorption process in a multi-stage cross-coupled catalytic process. This study considers the potential application of two ZVI desalination catalyst types for in situ aquifer desalination. The feasibility of using ZVI catalysts when 3 −1 placed in situ within an aquifer to produce 100 m d of partially desalinated water from a saline aquifer is considered. Keywords Zero valent iron (ZVI) · Irrigation · Aquifer · Catalyst · Desalination List of symbols and abbreviations E Measured apparent activation energy −1 A Required aquifer abstraction rate (kJ mol ) 3 −1 −1 (m d ) EC Electrical conductivity (mS cm ) 3 o A The gross rock aquifer volume (m ), ΔE Standard potential (V) required to produce AF Farad a Year F Faraday constant −1 a ZVI particle size (nm) (96,485.33289 C mol ) 2 −1 2 −1 a ZVI surface area (m g or cm g )g Grams + −3 2 c Concentration of Na (mol cm ) G The gross area (m ), encompassed by the C Charge associated with Cl removal aquifer treatment zone −1 (Coulombs) G Gibbs free energy (kJ mol ) −1 o C Capacitance (F g ) ΔG Standard Gibbs free energy −1 C(NaCl) Desorbed removed NaCl product (kJ mol ) = − RT ln(K). C Concentration of NaCl at equilibrium ΔG = ΔH − TΔS (Ebbing and Gammon −1 (g L ) 2005) −1 C Feed water salinity (mol L )ha Hectare t = 0 −1 C = NaCl Concentration of NaCl in the feed water ΔH Enthalpy (kJ mol ) 0 t = 0 −1 (g L ) ΔH Enthalpy associated with Stage 1 1 −1 C = NaCl Concentr ation of NaCl in the product (kJ mol ) t t = n −1 water (g L ) ΔH Enthalpy associated with Stage 2 −1 d Day (kJ mol ) D Required level of desalination I Current associated with desalination (A) (1 − C /C ) k Rate constant = Ln[C /C ]/t and is the t 0 0 t −1 E Activation energy for Stage 1 (kJ mol ) dimensionless logarithmic change in the −1 E Activation energy for Stage 2 (kJ mol ) ratio Ln[C /C ] per unit time 2 0 t k The observed rate constant associated −1 with a water flow rate of 0.37 L m and * David D. J. Antia a single Type B catalyst charge contain- email@example.com ing 57.6 g Fe k Pseudo-first-order cathodic rate constant DCA Consultants Ltd., Haughend, Bridge of Earn Rd., Dunning, Perthshire PH2 9BX, UK Vol.:(0123456789) 1 3 71 Page 2 of 19 Applied Water Science (2018) 8:71 k Normalized rate constant. It is calcu- ORP Oxidation–reduction potential [Volts lated as either k = k/(P a ) or k = k/(P ) (mV)]. ORP is translated to Eh, V, n w s n w (Wilkin and McNeil 2003; Antia 2015b); using the quinhydrone method docu- k is the rate constant following normali- mented in Antia (2016a, 2017), i.e., Eh, zation for the amount of catalyst placed mV = − 65.667 pH + 744.67 + ORP in the reaction environment (P ), i.e., P The effective pseudo-specific capacitance w sc rate constant per unit volume of water (normalized charge) associated with −1 processed (L) per unit weight of ZVI desalination (F g ) −1 −1 catalyst (g L ), k = k/P ; since catalytic P ZVI concentration (g L ) n w w reaction rates can be expressed as a func-PES Polyether sulphone tion of the surface area of the catalyst, k Q Reaction quotient, e.g., 1/(NaCl / n t = n can be expressed (Antia 2015b) in terms NaCl ) (Pourbaix 1974; Ebbing and t = 0 of the catalyst surface area (m ) per unit Gammon 2005) volume of water (L), i.e., k = k/(P a ); q Equilibrium absorbance of NaCl by n w s e −1 the actual surface area (a ) is not known ZVI (catalyst), g NaCl g Fe, q for a s e in most ZVI studies and changes during Type A catalyst is typically in the range catalysis (Antia 2015b). P is always 0 < q < 0.3 (Antia 2015b), q for a w e e known and is simple to measure and Type B catalyst is typically in the range define (Antia 2015a, b) 0 < q < 1000 (Antia 2015b, 2016a, 2017, k Rate constant at temperature T 2018a, b, c) 1 1 k Rate constant at temperature T r The radius (m) of the treatment zone 2 2 K Dimensionless equilibrium constant around the abstraction well (Ebbing and Gammon 2005) = rate R External cell resistance ext constant forward reactions/rate constant R Coefficient of determination −1 −1 reverse reactions = 1/(NaCl / R Gas constant (8.3144598 J mol K ) equilibrium −1 NaCl ) = 1/(C /C ) ΔS Entropy (J K ) t = 0 e 0 kW Kilowatt S Catalytic site kWh Kilowatt hour S Aquifer mobile water saturation L Litres T, T , T Temperature (K) 1 2 LDH Layered double hydroxide T Aquifer thickness (m) m Minutes t Time spent in the reaction environment m Number of protons transferred (s) m Cubic metre t Time spent in the reaction environment M Molecular weight of [Cl ], within the aquifer (s) −1 35.453 g molt Tonnes n Number of 0.8 m batches of saline water V The volume of water required within the processed by a single Type B catalyst aquifer in the ZVI-influenced desalina- charge tion zone (m ) − −1 n Number of electrons transferred W Weight of Cl removed (g L , i.e., con- e r −1 n Number of samples or trials considered centration reduction, g L ) NaClS Catalytic site with adsorbed NaCl Z The amount of ZVI catalyst required (t) 1 1 NaCl C to desalinate the aquifer t = 0 0 NaCl C ZVI Zero valent iron, Fe t = n t NaCl N aCl concentration in the saline aquifer $ US dollar aquifer −1 (g L ) ϕ Aquifer porosity NaCl Required NaCl concentration in the prod- product water −1 uct water (g L ) N Net to gross ratio within the aquifer [i.e., Introduction aquifer/(aquifer + aquitard ratio) within the gross aquifer unit] About 17% of global arable cropland is irrigated, of which at least 20% is adversely affected by salinization (Lekakis and Antonopoulis 2015). Irrigated crop land accounts for more than 30% of global agricultural production (Lekakis 1 3 Applied Water Science (2018) 8:71 Page 3 of 19 71 and Antonopoulis 2015) and more than 65% of global of producing this desalinated water falls within the range −3 −3 3 −1 anthropogenic water usage (e.g., Knapp and Baerenklau $3 m –$5 m for a plant producing about 100,000 m d 2006; FAO 2011; Amarasinghe and Smakhtin 2014; Wada (Antia 2017), but may, in the future, reduce to the range −3 −3 and Bierkens 2014; Panta et al. 2014; Lekakis and Anto- $0.7 m –$4 m (Bitar and Ahmad 2017). nopoulis 2015). In 2010, it was discovered that zero valent iron (Fe , ZVI) Globally salinization affects between 100 and 1000 bil- can be used to partially desalinate water (Fronczyk et al. 3 −1 lion m a of irrigation water (e.g., Knapp and Baerenklau 2010, 2012; Antia 2010, 2015a, b, 2016a, b, 2017, 2018a, 2006; FAO 2011; Amarasinghe and Smakhtin 2014; Wada b, c; Hwang et al. 2015; Noubactep, 2018). The desalination and Bierkens 2014; Panta et al. 2014). Salinized irriga- process is a complex, multi-stage, cross-coupled, catalytic, tion water adversely affects crop yields and the long-term adsorption and desorption series of reactions which con- + − viability of agricultural land, and the underlying aqui-centrate Na and Cl ions in the vicinity of the ZVI and in fers receiving salinized infiltration water (e.g., Ayers and related products (Fronczyk et al. 2010, 2012; Antia 2010, Westcot 1994). A small decrease in salinity (e.g., 25–50% 2015a, b, 2016a, b, c, 2017, 2018a, b, c; Hwang et al. 2015). reduction) can have the potential to increase crop yields The simplified, desalination, cross-coupled, catalytic process (depending on the crop, and initial water salinity) by (Antia 2016c) is summarized in Fig. 1. The catalytic pro- between 25 and 10,000% (e.g., Ayers and Westcot 1994; cess cycles (Fig. 1) between oxidative addition of Fe (i.e., Antia 2015b, 2017, 2018a, b, c). an increase in oxidation number) and reductive elimination −1 Agricultural crops have a low value ($ ha ) and can (i.e., a decrease in oxidation number). The oxidation number 3 −1 have a high irrigation requirement (m ha ). This irri- for Fe oscillates within the range − 2 to + 8 during catalysis gation requirement may (depending on crop type and (Antia 2015a, b, 2016a, 2017, 2018a, b). This cross-coupled 3 −1 location) fall within the range 100–10,000 m ha (e.g., catalytic process has been demonstrated to also remove (dur- Tabieh et al. 2015; Antia 2015b). It is unlikely that wide- ing desalination) nitrates, nitrites, fluorides, phosphates, sul- spread usage, for irrigation, of partially desalinated water, phates, Fe, Al, Cu, Mg, Mn, As, K, Ca, B, Ba, Sr from water will occur if the delivery cost of the irrigation water (Antia 2010, 2014, 2015a, b, 2016c). The desalination pro- −3 is > $0.2 m (e.g., Tabieh et al. 2015; Antia 2015b, 2016a, cess has been demonstrated to operate over the temperature b, 2017, 2018a, b, c). The economics of irrigation (and range − 10 to 90 °C (Fronczyk et al. 2010, 2012; Antia 2010, increased agricultural crop yields) using partially desali- 2015a, b, 2016a, b, 2017, 2018a, b, c; Hwang et al. 2015). nated water is addressed elsewhere (Antia 2017, 2018a, In 2013, small-scale trials using a 300 cm pres- b, c). sured dead-end filtration cell established that polyether Conventional desalination plants use a physical process sulphone (PES) membranes impregnated with 5–10% (e.g., membrane separation (reverse osmosis), or thermal Fe O nanoparticles could achieve NaCl rejection rates of 3 4 distillation (e.g., multi-stage flash distillation) to produce 62–82% from nitrogen-saturated pressured water contain- −1 desalinated (or partially desalinated) water containing a ing < 2 g NaCl L (Abuhabid et al. 2013; Alam et al. 2013). 50–99% reduction in water salinity. The full cycle cost A control PES membrane with no Fe O nanoparticles 3 4 Fig. 1 Schematic flowsheet n+/- Fe Reactant 1 [M-X], e.g. for each desalination catalytic - - H O, OH , HCO , CO , 2 3 2 cycle. The cations M and N can [M-R] CO, C H x y vary on each cycle. The anions R and X can vary on each cycle. A similar cross-coupled cata- Oxidave Addion lytic cycle has been proposed for water treatment using ZVI (Antia 2016c) Reducve Eliminaon (n+p)+ M-[Fe ]-X Metal Salt [N-R] (n+p)+ M-[Fe ]-R Transmetallaon [N-X] 1 3 71 Page 4 of 19 Applied Water Science (2018) 8:71 − − showed no NaCl rejection. These trials established that (i) is: 4OH → O + 2 H O + 4 e . The associated capaci- 2 2 − − Fe O selectively removes NaCl from a flowing body of tor recharge reaction is O + 2H O + 4e → 4OH . x y 2 2 water, and (ii) NaCl removal is a function of particle size, Within the recharge–discharge reaction, O can pore size, particle surface area, porosity, particle concentra- be replaced by one or more of CO, CO, CH , 2 4 tion, fluid pressure, and gas partial pressure. Similar obser - C H, C H O, C H O N , or C H O N Cl , e.g., x y x y z x y z a x y z a b + − + − vations have been made in the ZVI desalination studies 2H O + C → CO + 4H + 4e. CO + 4H + 4e → 2H O + C 2 2 2 2 + − (Fronczyk et al. 2010, 2012; Antia 2010, 2015a, b, 2016a, (Antia 2015b, 2018b); 2H O + CO → H CO + 2H + 2e . 2 2 3 + − b, 2017, 2018a, b, c; Hwang et al. 2015). H CO + 2H + 2e → 2H O + C O (Antia 2 3 2 + − The desalination catalysts are [Fe O (OH) (H O) ] pol- 2015b). CO + 8H + 8e → CH + 2H O , x y z 2 r n 2 4 2 + − ymers (Antia 2015b, 2018a, b, c). These particles can be CH + 2H O → C O + 8H + 8e (Antia 2 015b ) ; 4 2 2 m+∕− + − charged, [Fe O (OH) (H O) ] and can incorporate one 2CO + 12H + 12e → 2CH + 2H O (Antia 2011a, 2015b, x y z 2 r 4 2 or more anions (A ), or cations (C ) (Antia 2015b, 2018a). 2018b). n a They exhibit capacitance, and are conductive (Antia 2015b, This oscillating process (Antia 2010, 2014) allows the −1 2018a). This capacitance can exceed 300 F g (Antia ZVI catalyst to remove, or transform, a variety of other pol- 2015b) and the associated pseudo-capacitance increases as lutants within the water (e.g., pesticides, herbicides, fungi- the nano-porosity and surface area increases (Antia 2015b, cides, nitrates, hormonal pollutants, chlorates, phosphates, 2018a). The capacitance increases as the Debye length nitrogenous pollutants, As, B, Ba, Ca, Cu, Cd, Fe, Mg, Mn, decreases, the surface area of the reactive surfaces increases Pb, Sr, Se, etc.). A more detailed list of the pollutants that (Antia 2018a). The desalination rate constant is a function of can be removed, or transformed, is provided elsewhere the capacitance (Antia 2015b, 2018a). Capacitance increases (Antia 2010, 2011a, 2014, 2016c). The removal process as the dead-end porosity of the catalyst increases (Antia includes one or more of direct reaction to an alternative 2015b, 2018a). product, precipitation, reduction, oxidation, adsorption, or Desalination is associated with capacitor discharge (Antia adsorption/desorption. The removal of pollutants increases 2015b, 2018b). The catalyst complex (capacitor) comprises with increased contact time in the reaction environment to n+ a central metal atom (e.g., F e ) and neutral molecules or a new equilibrium level (Pourbaix 1974; Antia 2010). The ions attached to it (Antia 2018b, 2018c). In saline water con- equilibrium level is a function of the Eh and pH of the prod- taining Fe, the dominant acids (within the catalyst) include uct water and the nature of the other components in the water + + + n+ n+ n+ n+ n+ one or more of H, Na, K, Fe, Ca, Al, Mg, Cu , (Pourbaix 1974; Antia 2010, 2011a, b, 2014). The removal and CO (Antia 2015b, 2016a, 2017, 2018b). The domi- process is commonly multi-stage (Antia 2011a, 2016c) with nant bases (within the catalyst) include one or more of H O, numerous pollutants being removed simultaneously (Antia − − − n − n− n− 2− − Cl , HCO ,O , O , C H ,C H O , CO , NO , 2015a, b). x x y 3 2 y z 3 3 − 2− − − − − − n− n− HS, H (e), OH , HO , ClO, ClO , SO , Fe , This study considers the potential application of two 2 4 − − 2− N , NO , SO , and CO. groups of ZVI desalination catalysts (termed Catalysts A 3 2 3 The oscillation between capacitor discharge and recharge and B) for in situ aquifer desalination. drives the catalytic desalination reaction (Antia 2010, Type A catalysts (Figs. 2, 3, 4, 5, 6, 7) operate as cata- 2015b, 2018b). In acidic water, the basic capacitor dis- lyst pellets (or powders), which are placed in water (Antia + − charge reaction is: 2H O → O + 4H + 4e . The associ- 2015b). These catalyst pellets slowly remove NaCl from 2 2 + − ated capacitor recharge reaction is O + 4H + 4e → 2H O. a water body over a period of 50–1200 d (Antia 2015a, 2 2 In alkali water, the basic capacitor discharge reaction b). The NaCl is concentrated within the dead-end pores 40 20 Barrier 1Barrier 2 Barrier 1Barrier 2 NaCl + S 1 E NaCl + S 20 1 1 E -10 a -20 ΔH ΔH -20 -30 1 1 -40 -40 -50 NaClS ΔH ΔH 2 2 -60 -60 NaClS C(NaCl) + S -70 1 1 C(NaCl) + S -80 -80 Reaction Co-ordinate Reaction Co-ordinate (b) (a) Fig. 2 Schematic energy versus reaction co-ordinate graphs illustrating the desalination process. a E is positive. b E is negative a a 1 3 -1 Energy, kJ M -1 Energy, kJ M Applied Water Science (2018) 8:71 Page 5 of 19 71 -6 -8.0 -7 -8.5 -1 P = 20 gL -8 -9.0 -9 -9.5 -10 -1 P = 100 gL -11 -10.0 0246 81012 0 500 1000 1500 2000 2500 (a) Feed Water Salinity, g/L (b) Number of Days in the Reaction Environment -6 10000 -1 P = 30 gL -1 P = 30 gL -7 -1 P = 10 gL -8 -1 -9 P = 1 gL -10 -11 1 0% 20% 40% 60% 80% 100% 0% 20% 40% 60% 80% 100% (c) Probability of a Lower Value (d) Probability of a Higher Value Fig. 3 Catalyst A: rate constants. a k and k versus feed water salin- of the trials are provided in Antia (2015b). Probability (P) calculated ity. b k versus time required to reduce the water salinity by 50%. c by ranking values, where P = rank number/(total number of sam- k versus probability of a lower value. d Probability versus residence ples + 1): methodology after Antia (1986) time required to achieve a 50% reduction in salinity. Further details -6.2 -6.2 -6.4 -6.4 y = -0.0749x - 6.3654 -6.6 -6.6 R = 0.3364 -6.8 -6.8 -7.0 -7.0 -7.2 -7.2 -7.4 -7.4 y = 5740.8x - 27.389 -7.6 2 -7.6 R = 0.3312 -7.8 -7.8 05 10 15 0.0035 0.00360.0037 Temperature, C 1/T (a) (b) Fig. 4 Catalyst A: activation energy assessment. a Isothermal tem- n = 73, k = k/P . The saline water was constructed by adding NaCl to n w perature versus k natural spring water. The composition of the natural spring water is and b log (k ) versus 1/T. Trial details: reactor size n n −1 −1 0.2 L, P provided in Antia (2015a, b) = 18.91 g L , C = 8.2 g L , t = 24 h, number of analyses, w 0 within the catalyst as both hypersaline water and halite Type B catalysts (Figs. 8, 9, 10, 11) gradually remove − + (Antia 2017, 2018a, b). The salinity of the water is reduced the Cl and Na ions from a batch of water over a period of to an equilibrium level which is typically between 5 and 1–36 h (Antia 2015b, 2016a, 2017, 2018b, c). The salinity 80% of the feed water salinity (Antia 2015a, b, 2016a, b, of the water is reduced to an equilibrium level which is typi- 2017, 2018a, b, c). The required pellet concentration in cally between 10 and 80% of the feed water salinity (Antia −1 the water is in the range 20–100 g Fe L (Antia 2015b), 2017, 2018a, b, c). Each batch of catalyst can be reused for i.e., provision of 10,000 m of irrigation water may require successive batches (e.g., > 50) without loss of activity (Antia 200–1000 t of catalyst pellets. 2017, 2018a, b, c). The rate of desalination commonly 1 3 Log (k ) Log (k ) Log (Rate Constant) 10 n 10 10 Log (k ) Residence Time, Days Log (k ) 10 n 10 71 Page 6 of 19 Applied Water Science (2018) 8:71 -8.0 -8.0 y = 1.5091x - 1.9346 -8.2 y = 0.0016x - 9.28 -8.2 R = 0.7047 R = 0.6637 -8.4 -8.4 -8.6 -8.6 -8.8 -8.8 -9.0 -9.0 -9.2 -9.2 -9.4 -5.0 -4.8 -4.6 -4.4 -4.2 -4.0 0 200 400 600 800 -1 -1 (a) (b) Log (Current, A g ) Pseudo-Specific Capacitance, C g 10 -8.0 -8.2 y = -0.2382Ln(x) - 7.1569 R = 0.4701 -8.4 -8.6 -8.8 -9.0 -9.2 0 500 1000 1500 2000 2500 3000 (c) -1 Capacitance, F g Fig. 5 Catalyst A: interpreted electrochemical parameters. a Pseudo-specific capacitance versus k . b Current versus k . c Capacitance versus k . n n n Raw data source trials ST1a–ST5j (50 trials Antia 2015b) -7 -1 y = 0.9755x - 14.513 -2 R = 0.2095 -3 -4 -5 -6 -7 y = -0.7013Ln(x) - 5.4641 -8 R = 0.2018 -9 -8 8.08.5 9.09.5 10.0 10.5 8.08.5 9.09.5 10.0 10.5 (b) Feed Water Salinity, g/L (a) Feed Water Salinity, g/L 0.10 Barrier 2 NaCl + S 0.08 E = - 109.9 -50 a 0.06 ΔH = -134.2 -100 E = - 24.3 0.04 -150 y = -0.0101x + 0.1504 Barrier 1 NaClO + S n 1 0.02 NaClS -200 2 1 R = 0.2095 ΔH -250 0.00 8.08.5 9.09.5 10.0 10.5 -300 (c) Feed Water Salinity, g/L (d) Reaction Co-ordinate Fig. 6 Catalyst A: interpreted thermodynamic parameters. a Equi- A desalination reaction, assuming that the principal Stage 1 reaction librium constant versus salinity. b Gibbs free energy versus salinity produces Fe(OH) and the principal Stage 2 reaction produces NaClO (trials ST1a–ST5j (50 trials Antia 2015b). c Standard potential ver- (standard enthalpy from Lide 2008). Raw source data trials ST1a– sus salinity (trials ST1a–ST5j (50 trials Antia 2015b). d Interpreted ST5j (50 trials Antia 2015b) reaction co-ordinate versus energy diagram for the generic Catalyst 1 3 Log (k ) Log (k ) ΔE , V 10 n Log (k ) 10 n o -1 ΔG , kJ M -1 Log (k ) 10 n Energy, kJ M Applied Water Science (2018) 8:71 Page 7 of 19 71 Fig. 7 Catalyst A: simplified schematic diagram illustrating the interaction of OH with the ZVI. LDH layered double hydroxide. e interactions n+ with the reduction of FeO and Fe(OH) to Fe are not shown x x -4 -4 -1 k P = 0.5 g L -5 n -5 -6 -6 -1 P = 1 g L -7 -7 -8 -8 0246 81012 0.11.0 10.0 100.0 1000.0 (a) (b) Feed Water Salinity, g/L Number of Days in the Reaction Environment -4 1000 -1 P = 1 g L -5 100 -1 P = 0.5 g L -6 10 -1 P = 0.5 g L -7 1 -1 P = 1 g L -8 0 0% 20%40% 60% 80% 100% 0% 20% 40% 60% 80% 100% (c) (d) Probability of a Lower Value Probability of a Lower Value −1 Fig. 8 Catalyst B: rate constants. a k and k versus feed water salin- reactor size 240 L, P = 0.5 g L , T = 5–25 °C, air discharge n w −1 −1 ity. b k versus time required to reduce the water salinity by 50%. rates = 0.5 L L h , air bubble–water contact surface area is 2 −1 −1 c k versus probability of a lower value. d Probability versus resi- 1 m L h , air discharge pressure 0.01 MPa; see Antia (2015b) for dence time required to achieve a 50% reduction in salinity. The pri- further operating details associated with Catalyst B mary control on k is the O saturation of the water. Trial details: n 2 increases with increasing feed water salinity (Antia 2017, generation (2018) of Type B catalysts requires < 0.02 t Fe to 2018a, b, c). A typical Bronsted relationship (Antia 2018b) partially desalinate 10,000 m of irrigation water. 0.5 2 is Log (k ) = 1.9768 (C ) − 5.8078 (n = 40; R = 40%; The commercial cost of partial desalination (for irriga- 10 a t = 0 −1 valid for salinities in the range 1–9 g NaCl L ). The current tion) using a surface-based ZVI reactor system processing 1 3 Log (Rate Constant) Log (Rate Constant) 10 10 Number of Days Log (Rate Constant) 10 71 Page 8 of 19 Applied Water Science (2018) 8:71 -4 -4 -5 -5 -6 -6 y = -0.1433Ln(x) - 6.0475 -7 -7 y = -0.255Ln(x) - 3.7183 R = 0.13 R = 0.1759 -8 -8 1001000 10000 0.0001 0.0010 0.0100 0.1000 1.0000 (a) -1 (b) -1 Pseudo Specific Capacitance, C g Current, A g -4 -4 -5 -5 -6 -6 y = 0.3938x - 2.6003 y = -0.5973x - 3.1684 -7 R = 0.4762 -7 R = 0.446 -8 -8 -9 -8 -7 -6 -5 (c) (d) - -1 -1 Log (OH Added, M L Capacitance, F g 10 ) Fig. 9 Catalyst B: interpreted electrochemical parameters. a Pseudo-specific capacitance versus k . b Current versus k . c OH added versus k n n n (OH calculated from change in pH: methodology: Ebbing and Gammon 2005) and d apparent capacitance versus k Fig. 10 Catalyst B: simplified schematic diagram illustrating the interaction of OH with the ZVI. LDH layered double hydroxide 3 −1 −3 3 −1 100 m d was initially estimated at being < $0.1 m (Antia multi-train ZVI reactor system processing 100 m d can be −1 2015b, 2016a, 2017). Subsequent trialling (2016–2018) of a expected to reduce the feed water salinity (1–9 g NaCl L ) −3 commercial-scale ZVI desalination reactor train (processing by 40–60%, for a target full cycle cost of < $0.2 m . 3 −1 0.53 m d ) has established (e.g., Antia 2018b, c) that the Moving the desalination process from a reactor environ- actual cost (excluding profit, financing, labour, facility/site ment into the aquifer (providing the saline irrigation water) −3 costs, administration, etc.) approximated to $0.02 m for has the potential to further reduce costs. −3 Type B catalyst, plus a depreciated capital cost of $0.01 m In situ placement of ZVI within aquifers [either by injec- (excluding operating costs, feed and product water storage tion or placement in permeable reactive barriers (PRBs)] costs, site cost, financing costs, insurance costs, profit, taxes, has been extensively used to decontaminate aquifers (e.g., administration costs, etc.). These trials have confirmed that a Henderson and Desmond 2007; Fu et al. 2014; Guan et al. 1 3 Log (k ) Log (k ) 10 n 10 n Log (k ) Log (k ) 10 n 10 n Applied Water Science (2018) 8:71 Page 9 of 19 71 Fig. 11 Disrupted cross-coupling catalytic cycle associated with ZVI ics are defined by von Hobe et al. (2006). Cl O reaction is from desalination resulting in the production of ClO–OCl dimers, Cl , and Kortvelysi and Gordon (2004). This cycle demonstrates the formation HO ions/radicals (demonstrated by Antia 2015b): ClO dimer kinet- of electrochemical capacitance (e.g., Wang et al. 2015) 2015) and will remove an extensive suite of cations, anions temperature (non-isothermal fluctuating within the range and microbiota from the aquifer (e.g., Antia 2014, 2016c). − 10 to 25 °C), pressure (0–0.01 MPa above atmos- Placing ZVI in an aquifer will modify the Eh and pH of the pheric pressure), operating conditions, reactor volumes surrounding groundwater (e.g., Antia 2010, 2011a, 2014, (0.2–800 L/batch), ZVI composition, ZVI particle size 2016a) and can create a requirement for an environmental (a = 44,000–77,000 nm), ZVI particle surface area, ZVI −1 impact assessment and regulatory approval (e.g., Dougherty concentration (0.5 to > 100 g L ) and ZVI treatment. ZVI and Hall 1995; Mak and Lo 2011; Albergaria et al. 2013; was held as pellets (Catalyst A), or in cartridges (Catalyst Lynch et al. 2014; Jang et al. 2014; Alvarenga et al. 2016). B) (Antia 2015b, 2017, 2018a, b, c). To date, aquifer-based ZVI environmental impact assess- Catalysts A and B were trialled on synthetic water ments have focused on freshwater aquifers. ZVI interacts containing: (i) Catalyst A: Na–Cl, Na−Cl−HCO , with NaCl to create an oxic intra-particle nano-redox envi- Na–Cl–NO –HCO and Na–K–Cl–NO –HCO , (ii) Cata- 3 3 3 3 ronment (Antia 2018a) which can facilitate the formation lyst B: Na–K–Cl–Mg–SO and Na–K–Cl–Mg–SO –HCO 4 4 3 of H Cl O species (Antia 2015b, 2016a). ZVI is known to (Antia 2015b, 2016a). The feed water also contained Ca, x y z deactivate common aquifer bacteria (e.g., Kim et al. 2010; Mg, Mn, B, Ba, Cu, Si, Sr, Zn (Antia 2016a). The synthetic Tellen et al. 2010; Barzan et al. 2014; Zabetakis et al. 2015). water was manufactured by adding NaCl to natural spring However, the oxidative redox conditions (which can develop water (Catalyst A trials) or by adding Zechstein Halite to within the intra-particle porosity during desalination) can natural spring water (Catalyst B trials). favour the growth of an extensive microbiota (e.g., Barzan pH measurements were calibrated at pH 4, 7, 10 [Equip- et al. 2014; Antia 2018a, b). The predatory iron bacterium ment manufactured/branded by Hanna Instruments Ltd. Leptothrix discophora can be present in the ZVI catalyst (Leighton Buzzard, Bedfordshire, UK), HM-Digital, Inc. during desalination, and will remove other bacterial species (Culver City, CA, USA) and Extech Instruments, Inc., from the product water (Antia 2018a). Nashua, NH, USA]; Eh measurements were calibrated to the standard hydrogen electrode using a quinhydrone cali- bration at pH of 4 and 7. Oxidation–reduction potential Data set and methodology (ORP) measurement equipment was manufactured/branded + − by Hanna, HM-Digital and Extech; direct ion (Na and Cl ) The non-isothermal ZVI desalination trials (trial identi- concentration measurements were based on ion calibration −1 fiers: ST1a–ST5j, E146a–E146q and E147 series Antia at 0.001, 0.01, 0.1 and 1.0 mol L (Catalyst B), equipment 2015b, 2016a, A–K catalysts Antia 2017, 2018a, b, c) manufactured by Bante Instruments Ltd., Shanghai, China; were used as the data base for this study. This data set salinity measurements were based on EC (Catalyst A) and −1 recorded the feed water salinity (1–20 g L ), the prod- direct ion analysis. EC measurement equipment was manu- uct water salinity, Eh, pH, electrical conductivity (EC), factured/branded by Hanna, HM-Digital and Extech. 1 3 71 Page 10 of 19 Applied Water Science (2018) 8:71 The efficiency of the desalination process can be meas- concentration increases beyond a critical level (e.g., Antia ured directly using the observed rate constant, k (Ebbing 2015b, 2016a, b). and Gammon 2005; Kent 2007; Antia 2016a), where The apparent activation energy, E (for a pseudo-first- order reaction) is derived from the slope (s) of a regression Ln NaCl ∕NaCl = Ln C ∕C = kt = k tP . (1) t=0 t=n 0 t n w line for ln (k ) (or Log (k ) vs. 1/T Ebbing and Gammon n n In this study k is defined as k /(P ) as both Catalysts n w 2005). Figure 2 schematically illustrates the relationship A and B are constructed using the same-sized ZVI parti- between the experimentally measured activation energy (E ) cles (44,000–77,000 nm). The expected surface area of the and the actual activation energy (E ). The observed activa- 2 −1 resultant catalyst is within the range 20–200 m g (Antia tion energy is: E = E − ΔH (Revell and Williamson 2013; a 2 1 2015b). The charge (C) associated with Cl removal is Antia 2016a). (Ebbing and Gammon 2005): The cross-coupled desalination catalytic reaction (Fig. 1) can be simplified into a two-stage reaction (Fig. 2), where C (Coulombs) = F 1∕M W . (2) w r The effective pseudo-specific capacitance (normalised (i) Stage 1 is NaCl + Catalyst (S ) → NaClS ; 1 1 charge) (P ) associated with desalination is (e.g., Brousse sc (ii) Stage 2 is NaClS → [Product (C) NaCl] + Catalyst et al. 2015): (S ); −1 (iii) The net reaction is NaCl + Catalyst (S ) → [Product P Cg = C∕P . (3) sc w (C) NaCl] + Catalyst (S ). The associated current, I (A) is (e.g., Sarkar et al. 2013): The observed activation energies (e.g., Antia 2016b) can be positive (Fig. 2a), or negative (Fig. 2b). I = P ∕t. (4) sc The capacitance, C , is (e.g., Kuo et al. 2007; Yagmur et al. 2013; Chen et al. 2013): Catalyst A −1 C Fg = I∕voltage. (5) Catalyst A (trial series ST1a–ST5j ZVI Antia 2015b, Voltage is defined (Shen et al. 2016) as: Voltage = (aver - 2016a): (i) composition: (Antia 2015b), (ii) particle size, age Eh (V) − initial Eh (V))/P /t. The applied working elec- a = 44,000–77,000 nm (Antia 2015b), (iii) principal cata- trochemical voltage = (average Eh (V) − initial Eh (V)). The lyst characteristics: (Fig. 3), (iv) external energy require- change in voltage during desalination is principally due to ment = none (Antia 2015b), and (v) equilibrium absorbance, the effectiveness of the cathodic sites (Shen et al. 2016). The −1 q = 0.30–0.50 g g (Antia 2015b). capacitance is a measure of external cell resistance, R , ext The key characteristics of the ST catalyst pellets (illus- where (Shen et al. 2016) trated in Antia 2015b, 2016a, 2017, 2018a, b) are (i) a rate R = Voltage∕I. (6) ext constant (k, k ) which increases with increasing feed water k increases as the amount of OH in the water increases salinity (Fig. 3a), (ii) the time required to reduce the water and as the amount of available electrochemical energy salinity by 50% increases with increasing P (Fig. 3b), (iii) increases (Antia 2015b; Wang et al. 2015). The catalyst k decreases with decreasing catalyst concentration, and there effectively operates (e.g., Antia 2014, 2015b, 2016b; Shen is a range of potential rate constants which are associated et al. 2016) with a cathodic surface, a solid electrolyte trans- with a specific catalyst concentration (Fig. 3c), and (iv) the fer surface (ion conductor) and an anodic surface. The inter- rate constant data (Fig. 3c) can be used to predict the time action of this electrochemical cell with Cl and Na ion species required [with a specific catalyst concentration (P )] for the + − results in the removal of N a and Cl ions (Antia 2015b, aquifer salinity to reduce by 50% (Fig. 3d). 2016a, 2017). 50 trials operated under identical temperature conditions −1 Desalination is a multi-stage, multi-pathway process, established equilibrium absorbance, g NaCl g Fe, q , for involving catalytic adsorption and desorption (Antia 2015a, a Type A catalyst after 70–130 d (Antia 2015b). The tri- b, 2016a, 2017, 2018a). This allows catalysts to be designed als were continued to give a total duration of 280 d. The whose rate constant (i) increases with decreasing temper- k values in Fig. 3 are based on t = 280 d. Incremental k n n ature, (ii) remains constant with changing temperature, values before the equilibrium salinity levels are reached −8 −6 (iii) increases with increasing temperature, (iv) increases are in the order of 10 –10 (Fig. 4a, b), where k = ln with increasing feed water salinity, (v) remains stable with (C /C )/tP . k varies with temperature (Fig. 4a, b). t t + 24 h w n increasing feed water salinity, (vi) decreases with increas- The apparent activation energy is calculated, from the data ing feed water salinity, (vii) increases with increasing cata- in Fig. 4, using the method described in Ebbing and Gam- lyst concentration, and (viii) decreases when the catalyst mon (2005) as: 1 3 Applied Water Science (2018) 8:71 Page 11 of 19 71 This change reflects the composite nature of the desalina- E = ln k ∕k ∕(1∕T − 1∕T )RT < T . a 1 2 1 2 1 2 (7) tion reaction (Antia 2016a). The Bronsted relationship illus- The measured apparent activation energy, E , is 0.5 trated in Fig. 6a is: Log (k) = − 8.855 (C ) − 6.3213. 10 t = 0 −1 − 109.9 kJ mol (Fig. 4b). This indicates that the transition state complex [Product (C) The principal electrochemical parameters (e.g., capaci- NaCl] (Fig. 6d) has a lower charge than the reactants and has tance) for Catalyst A are summarized in Fig. 5. The effec- a lower stability at higher ionic strengths than the reactants. tiveness of this catalyst is inversely proportional to its The observed (Fig. 6a) increase in k with decreasing ionic capacitance (Fig. 5). A relationship between the standard strength (decreasing salinity) indicates that the transition rate constant, k , and capacitance, C , has been den fi ed (Kisa n a state complex is formed by two or more ions with a differ - and Kazmierczak 1991) as: ent charge sign. 2 2 0.5 If the dominant primary reaction is associated k = RT∕n F a (c) k C . (8) n s c a with the interaction of ZVI and water (Antia 2014, The relationship between pseudo-specific capacitance 0 − − 2015b, 2016a, b), e.g., Fe + 3HO = Fe(OH) + 3e , (Eq. 3) and k (Fig. 5a) indicates that k increases with n n −1 ΔH = − 134.2 kJ mol (thermodynamic data from Lide pseudo-specific capacitance, and that the associated current −1 2008), then E = − 24.3 kJ mol (Fig. 6d). discharge associated with the ZVI catalyst (Eq. 4) decreases An interpretation of the relationship between activation as k increases (Fig. 5b). The measured residual capacitance energy, enthalpy and reaction sequencing is provided in [following desalination (Eq. 5)] associated with the ZVI Fig. 6d. Catalyst A is suitable for aquifer partial desalination catalyst (Fig. 5c) decreases with increasing k . This is inter- when the aquifer water temperature is in the range 0–90 °C preted (Antia 2015b, 2018b) as indicating that desalination (Antia 2015b). (and k ) is associated with the discharge of capacitance in n + − The primary cathodic reaction (O + 4H + 4e = 2H O) 2 2 the ZVI catalysts. (Pourbaix 1974) is a function of the availability of both O These relationships (Fig. 5) indicate, that for a specific and H . The bulk of the O entering the water will react value of C , the variation in k can be attributed to changes a n to form OH radicals and ions (i.e., 0.5O + H O = H O ; 2 2 2 2 in a and k (if it is assumed that the other parameters are s c − − H O + 2e = 2OH Pourbaix 1974). Catalyst A is present 2 2 constant). The cathodic rate constant, k , increases with the as a layered double hydroxide (LDH) and derives an O and increased availability of O (Ebbing and Gammon 2005; OH supply from four sources within the aquifer (Fig. 7): (i) Shen et al. 2016). k may increase as C decreases. Catalyst c a oxygen diffusion across the air–water interface with sub- B provides an example where k is increased by increasing n − sequent OH formation (Antia 2016a), (ii) dissolved oxy- k (Antia 2018a, b, c). c − gen within the water body with subsequent OH formation The principal thermodynamic parameters for Catalyst A (Antia 2015b), (iii) natural alkalinity within the water body (ST catalyst) are summarized in Fig. 6. At equilibrium, when (Antia 2015b), (iv) ZVI catalysed water decomposition to −1 C is between 8 and 10 g L , k decreases with increasing 0 n + − form H and OH (Antia 2014, 2016b). −1 salinity (Fig. 6a). When C is < 8 g L , k increases with 0 n All water containing ZVI shows a natural oscilla- increasing salinity (Fig. 3a). At equilibrium (Ebbing and tion between higher and lower values for both Eh and pH Gammon 2005): (Antia 2010, 2011a, 2014, 2016c). This Eh and pH oscilla- tion is associated with an oscillation in Fe valency within G = 0 =ΔG + RT ln(K). (9) the range − 2 to + 8 (Antia 2016a, 2017, 2018b). The pH The values of G are negative within the range − 4 to + − −1 oscillation reflects changes in the H :OH ion ratio in − 9 kJ mol , and decrease as C increases (Fig. 6b). Nega- −1 the water while the Eh oscillation reflects changes in the tive G values in the range 0 to − 9 kJ mol indicate that n− − − O :O :OH :O H ratio in the water (Antia 2014, 2016b, the desalination reaction will produce an equilibrium mix- 2 2 2017, 2018b). During desalination catalysis (Figs. 1, 7), the ture containing both reactants and products (Ebbing and Fe oxidation number cyclically increases, before cyclically Gammon 2005), i.e., the saline water will only be partially decreasing (Antia 2016a, 2017, 2018b). desalinated at equilibrium (e.g., Antia 2015b, 2016a, 2017). The standard potential, ΔE , is calculated as (Ebbing and Gammon 2005): Catalyst B o o ΔE =ΔG ∕− n F. (10) ΔE is related to Eh and pH as (Pourbaix 1974): Catalyst B (trial series E146 Catalyst Antia 2015b, 2016a): (i) composition: (Antia 2015b), (ii) a = 44,000–77,000 nm o i Eh =ΔE − RTm∕n F pH − RT∕n FLn(Q). (11) e e (Antia 2015b), (iii) principal operating characteris- ΔE decreases with increasing feed water salinity tics (Figs. 8, 9, 10, 11), (iv) external energy require- −3 (Fig. 6c), decreasing k (Fig. 6a) and increasing G (Fig. 6b). n ment = < 0.17 kW m (for air compression, Antia 2016a), 1 3 71 Page 12 of 19 Applied Water Science (2018) 8:71 −1 and (v) q = potentially > 1 kg NaCl g Fe (Antia 2015b, decomposes to produce Cl + ClO (Mollina and Mollina e 2 3 −1 2016a, 2018b); treatment is potentially > 52,000 m t 1987; von Hobe et al. 2005). ClO–OCl can react with ClO (Antia 2015b, 2016a); removed NaCl is concentrated in (and to produce Cl O + ClO (Zhu and Lin 2011). The catalysed 2 2 on) the ZVI and in the ZVI cartridge (Antia 2016a). decomposition of ClO produces Cl + O (Mollina and 2 2 Type B catalysts show a general trend where k increases Mollina 1987; von Hobe et al. 2005). This then initiates with increasing feed water salinity (Fig. 8a). These rate con- the coupled reaction 2O + 2OH = 2HO + O (e.g., King- 2 2 2 stants indicate (Fig. 8b–d) that a Type B catalyst (with a ston 1987). In an oxygenated environment the O will react −1 concentration of < 1 g L ) could achieve a 50% desalination with water to produce an intermediate product H O (Pour- 2 2 of a feed water. The measured rate constant can increase, as baix 1974). The H O will decompose to form 2OH (Pour- 2 2 P decreases, with some Type B catalysts, when P exceeds baix 1974). Some of the H O will react with the ClO (i.e., w w 2 2 a critical level (Antia 2018b). ClO + H O = HOCl + HO Levanov et al. 2015). 2 2 2 The electrochemical parameters associated with this cata- The disrupted oxygenated cycle (Fig. 11) dechlo- lyst (Fig. 9) demonstrate, like Catalyst A, that k increases rides the water to produce two principal prod - with decreasing pseudo-specific capacitance (Fig. 9a), ucts Cl (aq) and HO . The equilibrium relationship − − − − 2 decreasing current (Fig. 9b), decreasing OH addition to [2Cl = Cl + 2e (Eh = 1.395 + 0.295 log (Cl /(Cl ) )] 2 2 the water (Fig. 9c), and decreasing residual capacitance is independent of pH (Pourbaix 1974). The Cl product (Fig. 9d). The substantially higher values of k [relative to can react with water to form one or more of HClO, ClO , − − − Catalyst A (Fig. 3)] reflect the substantially higher values HClO , ClO , ClO and ClO (Pourbaix 1974). Their equi- 2 3 4 of k resulting from the oxygenation of the water with air. libria relationships are a function of Eh and pH (Pourbaix The desalination reaction is driven by the reaction cou- 1974). − − ple 3O + 6H O + 12e = 6H O = 12OH (Antia 2015b, HClO forms part of the pH-dependent equilibrium con- 2 2 2 2 2016a). This also allows (Fig. 10) the formation of second- tinuum (e.g., McElhatton and Marshall 2007; Hu et al. ary products. 2010; Lefrou et al. 2012; Lichtfouse et al. 2012; Sandin − − 2013) from 0.5Cl (aq) to Cl to HClO to ClO , where (i) pH = 7.49 + Log(ClO /HClO) (Pourbaix 1974), (ii) Secondary reactions Eh = 1.494 − 0.0295 pH + 0.0295 Log (HClO/Cl ) (Pour- baix 1974), and (iii) Eh = 1.494 − 0.0295 pH + 0.0295 Log − + − + − − Cl and Na ions interact with OH and H within the (ClO /Cl ) (Pourbaix 1974). water to form ion adducts and radicals of the form H Cl O , The expected change in the Eh and pH (Hasab et al. 2012; x y z Cl O , NaOH (Antia 2015b, 2016a). Their concentration in Valenzuela et al. 2013) of the intra-particle porosity in the x y the water and in ZVI varies with catalyst type and with the presence of NaCl (during desalination) is (i) a progressive operating mode selected. Their presence can allow a micro- drop in pH [relative to the situation without NaCl from flora to grow in the ZVI. This can require careful handling 11 (e.g., Antia 2010, 2011a, b) to 4–5 (e.g., Antia 2015b, of both the ZVI and the water. These factors may need to be 2016a)], and (ii) an increase in Eh from < 0.6 (e.g., Antia considered in an environment impact assessment. 2015b, 2016a) to > 1.1 V (Pourbaix 1974). The primary reactant is HClO (formed from the anodic The NaClO product entering the main water body will − + − reaction: H O + Cl = [OH–Cl] + H + 2e Pourbaix 1974). decompose (e.g., Pourbaix 1974; Falbe 1986) to form the An excess of HClO, or ClO , is generated in the inter-par- equilibr ium relationships [3NaClO = 2NaCl + NaClO ], ticle porosity when the water is saturated with air, or C O [2NaClO = NaCl + NaClO ] and [2NaClO = O + 2NaCl]. 2 2 2 (Antia 2015b). This can result in the basic cross-couple The secondary reactions associated with ZVI in fresh cycle being disrupted (Fig. 11) to produce ClO–OCl dimers water are largely benign and are associated with the (Cl O species) as an initial primary by-product (Antia removal or inactivation of microbiota (e.g., Antia 2014). 2 2 2015b). In saline water, the secondary reactions produced during III The ClO–OCl dimer (product from Fe desorption) desalination can allow microbiota to flourish. The elevated decomposes to form ClO + 0.5Cl (Figs. 10, 11). The ClO Eh nano-redox conditions (> 0.7 V) within the ZVI intra- 2 2 2 product is adsorbed by Fe (Fig. 11). This product is then particle porosity are suitable for the growth of Escherichia III desorbed from F e as 0.5Cl . The O product then inter- coli, Listeria monocytogenes, Pseudomonas aeruginosa and 2 2 acts with water to produce H O and OH. In the presence of Staphylococcus aureus (e.g., Deza et al. 2005). These spe- 2 2 I − excess O , the OH interacts with Fe to produce HO . The cies are natural constituents of many shallow aquifers (e.g., principal product (Antia 2015b) of this cycle is HO . Ridgway et al. 1990; Hossain and Anwar 2009; Feighery The primary reaction outcomes from ClO + ClO are et al. 2013; Penny et al. 2015). The sheltered intra-particle (i) Cl + O , (ii) Cl + ClO , and (iii) ClO–OCl (Mol- nano-environment will, in some aquifers, result in colonies 2 2 2 lina and Mollina 1987; von Hobe et al. 2005). ClO–OCl of these species growing within the ZVI during desalination. 1 3 Applied Water Science (2018) 8:71 Page 13 of 19 71 The Gram-positive bacteria S. aureus, which is inhibited defined by the required level of desalination, D , k and the r n 3 −1 by concentrations of NaClO above 7.5 mM, is not inhib- required abstraction rate, A (m d ). A is defined by the r r ited in water containing NaCl + NaClO , or in water con- irrigation requirements for a specific crop. The required D 3 r taining < 7.5 mM NaClO (Melvin et al. 2011). The water is defined by a cost–benefit analysis of D versus crop yield. within the ZVI can contain an extensive flora of the preda- k is defined by the selected catalyst. There is precedent for tory oxic bacterium L. discophora (Antia 2018a, b). This the placement of ZVI in aquifers, as ZVI has been widely bacterium operates by releasing acetaldehyde dehydroge- used in PRBs for > 20 years to decontaminate aquifers (e.g., nase enzyme and the associated by-product acetaldehyde Wilkin et al. 2014). into the pore waters within, and surrounding the ZVI (Antia Aquifer-specific parameters such as permeability, homo- 2018a). Therefore, appropriate biological precautions may geneity and porosity will affect both the number of wells be required during catalyst changeover, or water sampling or infiltration devices and their micro-siting (Antia 2017). from the catalyst bed. Local factors such as land ownership, land usage, aquifer UV–visible absorbance spectra associated with the ZVI geometry, aquifer distribution and regulatory constraints will nanoparticles produced during desalination (e.g., Antia also impact on the feasibility of in situ aquifer desalination. 2015b) have identified the presence of Cl (210–220 nm), The primary parameter required to undertake in situ aqui- Cl O (215 nm), Cl O (230 nm), Cl O (230–235 nm), fer desalination is: 2 6 2 2 2 4 HClO (240 nm), ClO–OCl (240–250 nm), Cl (250 nm), ClO 1∕D = NaCl ∕NaCl . − r aquifer product water (12) (270 nm), ClO (290 nm), ClO (292 nm), HO (225 nm), + − Each crop type (and variety) will have a yield decrement Na (225–230 nm), N aO (265 nm) and NaClO (294 nm) relationship with salinity. The exact relationship is a func- (Thomas and Burgess 2007; Antia 2015b, 2016a). Therefore, tion of local conditions (e.g., temperature, soil, operating appropriate chemical precautions may be required during conditions, irrigation, etc.). catalyst changeover, or water sampling from the catalyst bed. Saline water from riparian water, ground water and saline −1 drainage water from irrigated land (salinity = 0.9–9 g L ) have been used to irrigate crops (e.g., Rhodes 1984; Zaman Aquifer desalination and Ahmad 2009; Jiang et al. 2010; Wang et al. 2016). A −1 decrease in irrigation water salinity from 5 to 2.5 g NaCl L Aquifer desalination using a Type A catalyst requires a (D = 0.5) would have the potential (Antia 2015b) to increase radial treatment zone to be established around an abstrac- r (depending on the planting strategy, variety and irrigation tion well (e.g., Huang et al. 2015). The treatment zone con- strategy adopted) soybean seed yields by between 0.8 and tains a number of wells, or infiltration devices, containing −1 10.8 t ha (e.g., Khan and Khaliq 2004; Ali et al. 2013; ZVI (Fig. 12). The ZVI is held in removable open-ended AGDM 2016). cartridges, or in removable permeable containers which The average residence time, t (s), required for the water are placed in the well, or in an infiltration device (e.g., r within the ZVI-influenced desalination zone (e.g., Ebbing Antia 2015b, 2016a, 2017). The number of wells/infiltra- and Gammon 2005; Kent 2007) is: tion devices required and amount of ZVI (Z ) required are Fig. 12 Process flow diagram for the partial desalination of an aquifer using a Type A catalyst 1 3 71 Page 14 of 19 Applied Water Science (2018) 8:71 The gross area, G (m ), encompassed by the aquifer treat- t = Ln 1∕D ∕ k P . r r n w (13) ment zone is: Increasing D reduces the cost of the desalination project G = A ∕T . by reducing the average residence time required for the water (17) a V h in the reaction environment. The volume of water required The radius, r (m), of the treatment zone around the within the aquifer in the ZVI-influenced desalination zone, abstraction well V (m ), is: 0.5 r = G ∕ . (18) V = A t ∕86, 400. (14) w r r The amount of ZVI required (t) is: The land take required for ZVI desalination can be assessed by considering a hypothetical saline aquifer Z = V ∕ 1000∕P . (15) 1 w w located 2–3 m below the ground surface which is used The required gross rock aquifer volume (A, m ) is: for irrigation. The hypothetical parameters are provided in Table 1. These are integrated with the catalyst data to A = V ∕ S N . (16) V w w G Table 1 Example saline aquifer parameters Parameter Value Parameter Value Parameter Value Common parameters −1 −1 NaCl 5 g L NaCl 2.5 g L N 90% aquifer product water G Φ 50% S 70% T 1 m w h 3 −1 −3 −3 A 100 m d P (Catalyst A) 0.030 t m P (Catalyst B, 2015) 0.0005 t m r w w 1st Quartile Median 3rd Quartile -1 Catalyst A—Position in 2015 as demonstrated by batch trials (0.2 to 10L·batch )- Fig. 2-7,12. D = 0.5 or 50% desalination −10 −9 −9 k 8.425 × 10 1.47 × 10 3.95 × 10 t 317 days 182 days 68 days 3 3 3 V 31,741 m 18,192 m 6770 m 3 3 3 A 100,765 m 57,751 m 21,492 m 2 2 2 G 100,765 m 57,751 m 21,492 m r 179.1 m 135.6 m 82.7 m Z 952 t 546 t 203 t 1st Quartile Median 3rd Quartile −1 Catalyst B—Position in 2015 as demonstrated by technical scale(pilot) trials- Fig 8-11, (30–100 L d). D = 0.5 or 50% desalination −6 −6 −6 k 2.121 × 10 4.496 × 10 6.660 × 10 t 7.6 days 3.6 days 2.4 days 3 3 3 V 756 m 357 m 208 m 3 3 3 A 2400 m 1133 m 660 m 2 2 2 G 2400 m 1133 m 660 m r 27.6 m 19.0 m 15.6 m Z 0.38 t 0.18 t 0.12 t 3 −1 Air required (m d ) 9072 4284 2496 1st Quartile Median 3rd Quartile 3 −1 2 Catalyst B—Position in 2018 as demonstrated by a commercial scale reactor train (0.53 m d ) Observed D approximates to [1-(− 0.0292 C + 0.6484 C )/C )], r 0 0 0 2 −1 −1 −1 R = 76%, i.e. D = 38–50% for C < 5 g L and D = 50–61% for 5 g L < C < 9 g L . This relationship will vary with catalyst, feed water composition, and r 0 r 0 operating conditions.k varies with D n r k Function of D Function of D Function of D n r r r t 1.6 days 1.4 days 1.2 days 3 3 3 V 160 m 140 m 120 m 3 3 3 A 507 m 444 m 381 m 2 2 2 G 507 m 444 m 381 m r 12.7 m 11.9 m 11.0 m Z 0.01 t < 0.01 t < 0.01 t 3 −1 Air required (m d ) < 270 < 270 < 270 cfm cubic feet/minute. Quartile analyses are based on the probability distributions associated with k 1 3 Applied Water Science (2018) 8:71 Page 15 of 19 71 provide (Table 1) an indication of the required aquifer sizes In some regulatory environments, permits (with associ- 3 −1 required to deliver 100 m d of partially desalinated irri- ated regulatory fees) and environmental impact studies will be gation water. Four desalination strategies are considered. required to allow a specific aquifer to be partially desalinated. The first (passive) strategy places Catalyst A in a number The principal differences between conventional desalina- of infiltration devices or wells. This strategy is illustrated tion and ZVI desalination are summarized in Table 2. schematically in Fig. 12. The water is retained within the Commercial-scale trials (2016–2018) of a reactor train 3 −1 reaction zone in the aquifer for a period of 60–1200 d while (using a Type B catalyst) operating at 0.53 m d have it gradually desalinates (Fig. 12). achieved an average desalination in the range 45–55% (Antia The second (active) strategy places Catalyst B in a reac- 2017, 2018b, c) from a feed water (constructed by dissolving tor (Fig. 13) with water storage, which is located in surface- halite in natural spring water) with a salinity which varies −1 based tanks (Fig. 13). In this strategy, an abstraction well is within the range 1–9 g L (Figs. 13, 14, 15). These trials used to provide saline feed water for the reactor (Fig. 13). A (Antia 2017, 2018b, c) indicate that (i) a reactor (Figs. 13, 3 −1 3 −1 reactor containing a Type B catalyst, processing 100 m d of 14, 15) processing 100 m d will contain 19.4 kg Fe, and 3 −1 3 feed water and producing 100 m d of product water, would will require about 150 m water (including recycle water) to require 150 m of water storage (Fig. 13) and could be placed be held in storage within the reaction environment (Figs. 13, within a standard 6-m-long shipping container. This allows the 14, 15), (ii) a single catalyst charge (19.4 kg Fe) will be reactor and water storage units to be both mobile and temporary able to catalytically partially desalinate > 54,000 m of saline 3 −1 facilities. In many regulatory jurisdictions these units can be water (i.e., > 2,780,000 m t Fe). This compares with the 3 −1 employed without requiring specific regulatory consents. historical (large scale, e.g., 1000–7000 m d ) commercial The third (active) strategy places Catalyst B in a reactor ZVI municipal water treatment (Anderson process), which (Fig. 14) with water storage, which is located within an aqui- established that 1 t Fe could purify > 2,400,000 m of feed fer (Fig. 14). In most regulatory jurisdictions, this strategy water (Anon 1889), (iii) the average desalination increases, will require specific regulatory consents. as the feed water salinity increases (e.g., Figs. 13, 14, 15), −1 The fourth (active) strategy places Catalyst B in a reactor, when the feed water salinity is < 9 g L (Antia 2018b). where each reactor is located within an infiltration (recycle) Saline aquifers extend under a large number of neigh- borehole and the water storage is located within an aquifer bouring agricultural holdings. This allows Type A (Fig. 12) (Fig. 15). This strategy will require specific regulatory con- and Type B catalysts (Figs. 14, 15) to be potentially used sents and in some regulatory areas this strategy will require (by co-operatives and state/municipal authorities) to a waiver from existing regulations. This is because the partially desalinate, in situ, large aquifer volumes, e.g., water composition entering the aquifer will be different to 100,000–10,000,000 m . These partially desalinated aqui- the water composition entering the infiltration borehole(s). fers can be used for irrigation, or to provide a feed stock for Fig. 13 Process flow diagram for the partial desalination of irrigation water using a Type B catalyst 1 3 71 Page 16 of 19 Applied Water Science (2018) 8:71 Fig. 14 Process flow diagram for the partial desalination of an aquifer using a Type B catalyst and surface-based reactors Fig. 15 Process flow diagram for the partial desalination of an aquifer using a Type B catalyst and sub-surface-based reactors conventional desalination plants. This type of large-scale is a function of location, water composition, salinity reduc- aquifer desalination will be associated with a decrease in the tion required, aquifer geology and hydrology, crops selected, nitrate content of the aquifer water (Antia 2015a, b). planting strategy, land management strategy, anticipated increase in crop yield, local cost structures and commodity prices. The initial technical screening indicates that ZVI cat- Conclusions alysts could potentially be used to deliver 100 m (partially −1 −3 desalinated water) d , for a potential cost of < $0.2 m , by This study has demonstrated that it is technically feasible the in situ treatment of a saline aquifer. to use ZVI catalysts to partially desalinate a saline aquifer, The practical feasibility of using this technology using a radial treatment zone, where water is being continu- for in situ aquifer remediation will require appropriate ously removed through an abstraction well. The economics regulatory consents and will require pilot testing (e.g., 1 3 Applied Water Science (2018) 8:71 Page 17 of 19 71 Ali A, Iqbal Z, Safdar ME, Aziz AM, Asif M, Mubeen M, Noorka IR, Table 2 Comparison of conventional desalination with ZVI desalina- Rehman A (2013) Comparison of yield performance of soybean tion. Source Antia 2017, 2018a, b, c) varieties under semi-arid conditions. J Anim Plant Sci 23:828–832 Parameters Reverse osmosis ZVI desalination Alvarenga RAF, de Lins IO, de Neto JAA (2016) Evaluation of abiotic resource LCIA methods. Resources 5:13 Amount of NaCl removed (%) 50–99.9 30–70 Amarasinghe UA, Smakhtin V (2014) Global water demand projec- Amount of energy required > 0.7 0–0.1 tions: past, present and future. Report 156. International Water (kW m ) Management Institute (IWMI), Colombo Anon (1889) Purification of river water and sewage effluent and the Amount of feed water discarded as entire removal of colour from water containing peat or clay by Waste water (% of feed water) 20–80 0 means of agitation with metallic iron. Revolving Purifier Com- 3 −1 3 3 Cost for 1 m d product > $100 m < $0.2 m pany Ltd., London water Antia DDJ (1986) Kinetic method for modeling vitrinite reflectance. 3 −1 3 3 Cost for 10 m d product > $20 m < $0.1 m Geology 14:606–608 water Antia DDJ (2010) Sustainable zero-valent metal (ZVM) water treat- ment associated with diffusion, infiltration, abstraction and recir - culation. Sustainability 2:2988–3073 Antia DDJ (2011a) Modification of aquifer pore water by static diffu- 3 −1 5–1000 m d ) within an aquifer which is designed to sion using nano-zero-valent metals. Water 3:79–112 establish and test: (i) ZVI design layouts within the aquifer Antia DDJ (2011b) Hydrocarbon formation in immature sediments. Adv Pet Explor Dev 1:1–13 (including geological/hydrological data requirements), (ii) Antia DDJ (2014) Chapter 1: groundwater water remediation by static methods for placing the ZVI in the aquifer (and removing it), 0 0 0 diffusion using nano-zero valent metals [ZVM] (Fe, Cu, Al ), (iii) material and equipment requirements (including com- n+ (n+/−) n-FeH , n-Fe(OH) , n-FeOOH, n-Fe–[O H ] . In: Kharisov x x y mand and control systems), (iv) personnel requirements, (v) BI, Kharissova OV, Dias HVR (eds) Nanomaterials for environ- mental protection, 1st edn. Wiley, Inc., Hoboken, pp 3–25 desalination time frame and achievable desalination levels, Antia DDJ (2015a) Desalination of groundwater and impoundments (vi) safety codes which have to apply during installation and using nano-zero valent iron, Fe . Meteorol Hydrol Water Manag operation, (vii) environmental constraints (including energy 3:21–38 conservation), (viii) appropriate installation and operating Antia DDJ (2015b) Desalination of water using ZVI, Fe . Water 7:3671–3831 standards and codes, (ix) resources required, (x) economic Antia DDJ (2016a) ZVI (Fe ) desalination: stability of product water. constraints (including operating cost structures, adminis- Resources 5:15 trative cost structures, utility cost structures, supplies and Antia DDJ (2016b) Chapter 28: desalination of irrigation water, live- 0 0 0 equipment cost structures, capital and operating cost struc- stock water and reject brine using n-ZVM ( Fe, Cu, Al ). In: Hussain CM, Kharisov BI (eds) Advanced environmental analysis: tures, insurance cost structures) and (xi) quality of the prod- application of nanomaterials. RSC detection science series no. 10, uct water and its suitability for irrigation. 1st edn, vol 2. Royal Society of Chemistry, London, p 237–272 Antia DDJ (2016c) Chapter 84: water remediation—water remedia- Acknowledgements This study was funded by DCA Consultants Ltd. tion using nano-zero-valent metals (n-ZVM). In: Kharisov BI, Kharissova OV, Ortiz-Mendez U (eds) CRC concise encyclopedia Open Access This article is distributed under the terms of the Crea- of nanotechnology, 1st edn. CRC Press, Taylor and Francis Group, tive Commons Attribution 4.0 International License (http://creat iveco Boca Raton, pp 1103–1120 mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- Antia DDJ (2017) Provision of desalinated irrigation water by the tion, and reproduction in any medium, provided you give appropriate desalination of groundwater within a saline aquifer. Hydrology 4:1 credit to the original author(s) and the source, provide a link to the Antia DDJ (2018a) Chapter 26: irrigation water desalination using PVP Creative Commons license, and indicate if changes were made. (polyvinylpyrrolidone) coated n-Fe (ZVI, zero valent iron). In: Hussain CM, Mishra A (eds) New polymer nanocomposites for environmental remediation, 1st edn. Elsevier, Amsterdam, pp 541–600 Antia DDJ (2018b) Chapter 8: direct synthesis of air-stable metal References complexes for desalination (and water treatment). In: Kharisov BI (ed) Direct synthesis of metal complexes, 1st edn. Elsevier, Amsterdam, pp 341–367 Abuhabid AA, Ghasemi M, Mohammad AW, Rahman RA, El-Shafie Antia DDJ (2018c) Chapter 122: partial desalination of saline irri- AH (2013) Desalination of brackish water using nanofiltration: n+/− gation water using [Fe O (OH) (H O) ] . In: Martinez LMT, performance comparison of different membranes. Arab J Sci Eng. x y z 2 m Kharissova OV, Kharisov BI (eds) Handbook of ecomaterials, 1st https ://doi.org/10.1007/s1336 9-013-0616-z edn. Springer, Basel, pp 1–30 AGDM (2016) Iowa corn and soybean county yields. AG Decision Ayers RS, Westcot DW (1994) Water quality for agriculture. 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