**5. Arsenite and arsenate as oxidation and reduction species**

Wagman et al. [18] used standard free energies of formation to determine halfcell reactions for arsenate to arsenite reduction:

$$\begin{aligned} \text{(0.5)} \,\text{H}\_2\text{AsO4}^- + \text{e} &- + \text{1.5H}^+ = \text{0.5} \,\text{As} \,(\text{OH})\_3 + \text{0.5H}\_2\text{O} \,\text{p}K = -\text{10.84} \\\\ \text{(0.5)} \,\text{HAsO}\_4^{2-} + \text{e} &- + 2 \,\text{H}^+ = \text{0.5} \,\text{As} \,(\text{OH})\_3 + \text{H}\_2\text{O} \,\text{p}K = -\text{14.22} \end{aligned}$$

Using protocols from Essington [19], the predominance diagram (**Figure 1**) illustrates the relative stability regions for arsenite and arsenate species expected in the soil environment, ranging from pH 3 to pH 9. The Pourbaix diagram (predominance diagram) shows the transitional nature of As(V) as a proton donor and the reduction of As(V) to As(III). The demarcation of oxic, suboxic, and anoxic regimes was discussed in Essington and we note that arsenate largely exists in oxic to suboxic regimes [19]. Arsenite formation in anoxic soil environments is thermodynamically favored in increasingly acidic soil environments.

Arsenic reduction is mediated by the soil's microbial population, effectively supporting electron donation from suitable organic substrates. Dissimilatory arsenate-reducing bacteria can effectively reduce arsenate to arsenite by using arsenate as a terminal electron acceptor [20–22]. Xu et al. [23] demonstrated that reduction of arsenate to arsenite post root uptake, coupled with efflux from the root to the rhizosphere, also contributes to arsenate reduction. Qiao et al. [24] employed

**Figure 1.**

*Predominance diagram showing arsenic species predominance zones for given pe and pH as master variables (created by authors of this manuscript).*

anaerobic microcosms to demonstrate that humic substances facilitate arsenic reduction. Fulvic acid was more effective in reducing arsenic than humic acid, and humic acid was more effective in reducing arsenic than humin. As a carbon source, fulvic acid supported microbial activity and reduced fulvic acid acted as an electron shuttle to reduce Fe(III)-oxyhydroxides and As(V). Arsenic may be co-precipitated with Fe-oxyhydroxides, and the reductive dissolution of these Fe-oxyhydroxides may promote the release of arsenate, which may then be subsequently reduced to arsenite. Mn-oxyhydroxides have been implicated in the oxidation of arsenite to arsenate [25–29].

## **6. Groundwater irrigation as an arsenic source for Rice accumulation**

In the Indo-Gangetic Plain, Vicky-Singh et al. [30] documented arsenic concentrations of soil surface horizons and surface and groundwater resources. They reported that tube-well water ranged from 5.3 to 17.3 μg L−1 and soil horizons ranged 1.09 to 2.48 mg kg−1, with data showing that tube well water irrigation was contributing arsenic to soil. In the Mekong Delta (Vietnam), Huang et al. [31] documented multiple groundwater samples having arsenic concentration greater than 50 μg L−1 and demonstrated that As(III) was the more abundant valance state. The historical applications of As-bearing groundwater correlated with arsenic soil accumulation. Radu et al. [28] performed a batch experiment involving pyrolusite (MnO2) to show that second order kinetics, which incorporated MnO2 concentrations, described arsenite oxidation. Subsequent arsenate adsorption was appropriate described using the Langmuir equation.

Farooq et al. [32] investigated arsenic accumulation associated with irrigation and agronomic practices in the Bengal Delta. Two different fields were irrigated with different arsenic concentrations in the groundwater, with one field planted to wheat and the other field planted to rice. These authors indicated that the more concentrated As-bearing groundwater in the rice field did not increase the arsenic soil concentrations as significantly as the wheat field, which was irrigated with less concentrated As-bearing groundwater. The authors proposed and provided evidence that greater quantities of rice plant residue, with its production of organic acids, supported arsenic diffusion to deeper soil horizons. Arsenic concentrations exceeding 10 μg L−1 appear to be more frequent in the western United States, demonstrating that the local geology is important in influencing water quality [33]. In a recent review, Mohanty [2] documented the efficacy and deficiencies of technologies involving treatment of arsenic-bearing groundwater, which may be employed to improve irrigation water quality.

### **7. Adsorption of Arsenite and arsenate species**

Arsenite and arsenate species experience pH-dependent adsorption and co-precipitation with Fe-oxyhydroxides, most notably ferrihydrate (β-FeOOH), lepidocrocite (γ-FeOOH), goethite (α-FeOOH), and hematite (Fe2O3) [3, 17]. Surface protonation of goethite (pK = −9.6) permits the interface to acquire amphoteric positive charge densities sufficient to promote monodentate or bidentate arsenite adsorption [6, 29]. Arsenite and arsenate adsorption may result in both monodentate and bidentate bonding structures [3, 10, 14, 17, 22, 34–36].

The optimal pH for arsenic adsorption depends on (i) experimental protocols, and (ii) the presence of phosphate, silicic acid, naturally occurring organic acids, and other competing anions. The optimal pH for the adsorption of arsenite on

#### *An Emerging Global Understanding of Arsenic in Rice (*Oryza sativa*) and Agronomic Practices… DOI: http://dx.doi.org/10.5772/intechopen.105500*

Al- and Fe-oxyhydroxides ranges from pH 7 to 10, [14, 34–43], whereas the optimal pH for the adsorption of arsenate on Al- and Fe-oxyhydroxides varies across the pH range of 4 to 7 [17, 37, 44–46]. Cornu et al. [13] observed an arsenate adsorption pH dependency with both kaolinite and humic acid treated kaolinite. Interestingly, Cornu et al. [13] observed that arsenate adsorption onto humic acid treated kaolinite was greater than for untreated kaolinite when the electrolyte solution was a Ca(NO3)2 media, whereas arsenate adsorption was substantial decreased on humic acid treated kaolinite in NaNO3 media. Goldberg [14] investigated arsenic adsorption on Al-oxides, Fe-oxides and reference phyllosilicates (kaolinite, illite and montmorillonite). Arsenate adsorption was pH-dependent with arsenate adsorption less evident on transition to alkaline media. Arsenate adsorption decreased above pH 9 for Al-oxides, above pH 7 for Fe-oxides and pH 5 for the reference clays. Arsenite adsorption showed a maximum adsorption near pH 8 for non-crystalline aluminum oxides and exhibited little pH dependence on non-crystalline Fe-oxides [14].

Jackson and Miller [17] evaluated various concentrations of phosphate (pH 3 and 7) to extract arsenite, arsenate, dimethylarsinic acid, and monoethylarsonic acid adsorbed onto goethite and non-crystalline Fe-oxyhydroxides. Phosphate was demonstrated to displace arsenite and arsenate. Khaodhiar et al. [47] prepared iron oxide coated sand (Fe2O3) to show that arsenate adsorption was strongly adsorbed at acidic to slightly acidic pH values and adsorption decreased with increasing pH. Grafe et al. [35] investigated arsenite and arsenate adsorption on goethite and observed that arsenate adsorption decreased gradually and continuously from pH 3 to pH 11. Arsenite adsoption was shown to have a maximum adsorption at pH 9. The influence of either fulvic acid or humic acid addition resulted in a reduction in adsorption for both arsenite and arsenate.

Sulfate, carbonate, and dissolved organic matter have been shown to be relatively less effective than phosphate in displacing arsenic [37]. Using goethite as the bonding surface, Luxton et al. [34] showed that silicic acid (H4SiO4) was able to effectively displace arsenic. Swedlund and Webster [48] demonstrated that H4SiO4 may displace arsenite from ferrihydrate. Zhang and Selim [39] observed arsenic desorption by phosphate, whereas Xu et al. [43] documented arsenic phosphorus-induced desorption from crystalline and non-crystalline aluminum oxides. Smith and Naidu [46] provided data on the kinetics of arsenic desorption, illustrating the importance of studies to understand the equilibrium is rarely achieved in natural systems.

Yamamura et al. [20] amended soils with As(V) laden Fe oxyhydroxides with solution supplemented with either lactate or acetate. After 40 days, there was a greater arsenic release rate in lactate amended systems, suggesting that lactate is a suitable carbon source and both dissimilatory metal(loid) reducers and anaerobic fermenters support arsenic extraction. Razzak et al. [49] documented oxidation–reduction processes in groundwater support simultaneous release of iron and arsenic, thus demonstrating that groundwater irrigation may be an effective arsenic source.
