Arsenic (As) is the most hazardous material on the list, while being only the 20th most prevalent element in the Earth’s crust [1]. Because of its extreme toxicity, it is a major source of concern in both terrestrial and aquatic settings. In aquifer systems, rock–water interactions are the primary source of As release and groundwater quality degradation. As is usually found in two states in natural water bodies: trivalent arsenic (As³⁺, Arsenite) and pentavalent arsenic (As⁵⁺, Arsenate), both of which are inorganic and very poisonous [2]. Arsenite is more poisonous than arsenate in terms of toxicity. The amount of As released from minerals into groundwater depends on the kind of mineral, pH, and redox conditions [3] as well as the presence of other ions that facilitate As desorption from secondary minerals [4]. Natural occurrences of elevated As levels in unconsolidated sediment aquifers have been documented [5]. Anomalies of As in groundwater can be increased or decreased as a result of human activity and climatic variability. Aquifer management or recharging the aquifer [6] with uncontaminated water can lead to a reduction in contamination. On the other hand, significant groundwater withdrawal [7] or surface contaminated water reaching the aquifer can pollute the water via As pollution [8].

2. Mobilization of Arsenic

Mobility of As is mostly influenced by processes occurring at mineral surfaces, including precipitation, dissolution, adsorption, and desorption. These reactions are governed by geochemical variables such pH, Eh, ionic composition, and mineral type ^[9][56]. As is effectively immobilized via sorption under neutral pH and oxic circumstances, or co-precipitates with metal oxides ^[10][13]. These processes involve surface complexation reactions and the creation of certain inner sphere complexes. The mobility of metal oxides is increased by dissolution at low pH and lowered redox potential. Sulfide mineral formation regulates As concentrations under highly reducing circumstances ^[11][57]. The sorption of As is enhanced by dissolved calcium, while competition for sorption sites reduces As sorption in the presence of anions such as phosphate and bicarbonate ^[12][58]. The mobility of As may also be influenced by dissolved organic matter, whose concentrations typically vary from 1 to 20 mg/L in fresh streams and can be greater in wetlands. Fulvic or humic acids effectively prevent As from adhering to iron oxides, alumina, quartz, or kaolinite by forming stable complexes with the mineral surface ^[13][59]. A geochemical trigger of some kind that releases As from the solid phase of the aquifer into the groundwater is the first of two main processes that appear to be involved in the creation of high-As groundwaters on a regional scale. Second, the released As is still present in the groundwater and should not be removed by flushing. Numerous geochemical causes may be present. Sulphide ores begin to oxidize in mining and mineralized areas when oxygen or other oxidizing factors are introduced. This may occur after a decrease in the water table, a modification of the hydrogeological regime, or mining excavation. The most significant catalyst for As desorption or dissolution from oxide minerals, notably iron oxides, appears to be in the majority of arsenic-affected aquifers. Since adsorption reactions are surface reactions, this process’ initial adaptation to environmental changes is likely to be relatively quick [5]. Because of the natural weathering and dissolution of its parent minerals, As is frequently found in relatively high amounts in sediments and groundwater. Microorganisms can also mediate a variety of processes, including oxidation-reduction and methylation, that regulate the As concentration in these materials. However, the movement of As between the various mineral forms, solid phases, and organic matter can be achieved by purely chemical mechanisms. One of the more significant mineral components of soils and sediments is iron(III) oxide, which can be found as intergrain cements, particle coatings with a complex and diverse composition, and discrete amorphous or crystalline phases. Fe(III) oxides are formed as secondary weathering products ^[14][60]. It is becoming more and more obvious that bacteria are crucial for the speciation and mobilization of As. According to ^[15][61] Lloyd and Oremland, 2006 ^[16][62] Oremland and Stolz, 2005, microorganisms can play a substantial role as catalysts in the reactions of arsenite oxidation, arsenate respiration, methylation, and volatilization. The microbial reactions serve as detoxification processes or sources of energy. Many chemoautotrophs use CO₂ as their only carbon source and oxygen, nitrate, or ferric iron as their terminal electron acceptors to oxidize As(III). Some heterotrophs can also use organic carbon to oxidize As(III) ^[17][63]. On the other hand, numerous prokaryotes, such as Bacillus selentireducens and Bacillus arsenicoselantis, are able to respire As(V) or oxidize As(III) ^[18][64]. Additionally, other redox processes brought on by microbes might cause the release of As indirectly. As(V) is produced when, for instance, the dissimilatory Fe-reducing bacteria Shewanella alga lowers Fe(III) to Fe(II) in scorodite ^[19][65]. A significant step in the mobilization of As in a variety of subsurface environments appears to be the microbial oxidation of organic materials in combination with the reduction of Fe(III) to Fe(II). According to ^[20][66] Fredrickson et al., 1998, both organic matter and reduced inorganic species, such as Fe²⁺, Mn²⁺, S²⁻, or H₂, can act as reducing agents in microbial metabolism. The lacustrine sediments are said to be home to the two most extensively researched genera of Fe(III) reducers, Geobacter and Shewanella ^[21][22][67,68]. Anaerobic metal-reducing bacteria may be crucial in the mobilization of As in the groundwater; the penetration of labile dissolved organic carbon (DOC) into shallow aquifers by irrigation pumping may facilitate the considerable reduction of Fe oxyhydroxides, leading to higher As concentrations in groundwaters, according to ^[11][57] Harvey et al., 2002 and ^[23][69] Islam et al., 2004. The simultaneous mobilization of As and Fe(II) from aquifer sediments strongly suggests that their mobilization was caused by the reduction of Fe oxyhydroxides by the increased activity of indigenous bacteria present in the sediments; this phenomenon also sheds light on the mobilization mechanism of As in groundwater. Organic material from the surface that seeps into the shallow aquifer may encourage the activity of microbial communities, thereby leading to a reduction of Fe oxyhydroxides and As release. As is primarily found in three phases in aquifers: silicate phases, organic matter and sulfide phases, and iron and manganese oxide phases. It is believed that a three-stage process led to the emergence of these phases ^[24][70]. According to British Geological Survey researchers, the high concentration of As in groundwater is caused by hydrogeochemical processes ^[25][12]. They hypothesized the possibility of two separate geochemical causes. First, under dry environments, rapid evaporation rates and mineral weathering may result in high pH levels (>8.5). At such pH levels, either As desorption from the binding mineral surfaces or a reduction in the amount of adsorption may occur. Second, under severely reducing conditions, iron and manganese oxides may dissolve and release As that has been adsorbed under pH values that are neutral or nearly neutral. Generally, fertilizers contain phosphate, which competes with As for adsorption on the soil; as a result the As is displaced and enters the groundwater system. Increased irrigation and fertilizer use has mobilized phosphate from fertilizers into shallow aquifers, causing As to mobilize via anion exchange onto the surfaces of reactive mineral surfaces ^[24][70]. As an alternative process for the genesis of high-As groundwater, a new theory is based on the displacement of As by dissolved bicarbonate. As is leached into groundwater by the carbonation of As sulphide in the presence of bicarbonate, which results from the weathering and dissolution of sedimentary carbonates. The physical characteristics of the sediments, such as the grain size of clay minerals, have a role to play in controlling the distribution and mobility of As, and other altered mineral such as scorodite may also contribute As into groundwater ^[26][71]. Due to the advent of reductive conditions during sediment burial and diagenesis, it is usually believed that microbial processes regulate As release by dissolving As-bearing Fe-oxides, with the presence and type of organic matter ^[27][72] and the presence of SO₄²⁻ also playing an important role. According to groundwater geochemical analyses, similar processes were proposed as the main cause of As release in Hungary ^[28][73]. Varsányi and Kovács’ (2006) ^[29][74] investigations into the mineralogy and aqueous geochemistry of SE Hungary’s sediments revealed a correlation between As and both extractable organic matter and Fe-oxides. However, they hypothesized that two processes—(i) dissolution of As-bearing Fe-minerals (as previously proposed) occurring in low pH (7.6) in areas of groundwater recharge and (ii) high concentrations of organic ligands promoting mobilization in areas of groundwater discharge with longer residence times—were responsible for As release ^[29][74]. According to a recent study in Eastern Croatia, the spatial distribution of As in groundwater is related to the geological, geomorphological, and hydrogeological development of the alluvial basin. Reductive dissolution of iron oxides, desorption of As from clay minerals and/or iron oxides, and competition for sorption sites with organic matter may be the main mechanisms regulating As mobilization ^[30][75].