OXIDATION AND ADSORPTION OF ARSENIC FROM CONTAMINATED GROUNDWATER

Abstract

Arsenic poisoning in the groundwater is one of the most critical environmental hazards for the World. Therefore, the effective and proper treatment of such contaminants in the water needs more attention to provide safe drinking water. The WHO sets the guideline of 10 g/L of arsenic in drinking water, direct long-term exposure to arsenic in drinking water beyond this value causes severe health hazards to individuals. Various research has evidenced their adverse effects after drinking arsenic-contaminated water. Originally arsenic was found naturally with mineral ores and volcanic eruptions. Then the arsenic migrates into the water bodies by eroding minerals and soils. Apart from such natural causes, various anthropogenic activities also become the prominent source of arsenic poisoning for humans and living beings. Humans can expose to arsenic by contacting the air, water, and soil that is contaminated with arsenic. Few animals, including rats, lobsters, fish, and chickens, act as arsenic sources for food chain contamination to humans. So, the current study demonstrates the arsenic sources, its speciation in the water bodies, its effects on humans, and the technologies available for removal. The primary emphasis was given to the oxidative removal processes of As(III) species using solid adsorbent under a visible light environment, followed by the complete removal of arsenic from water to produce safe drinking water. As(III) has been found in the groundwater, therefore challenging to remove from the water due to its neutral behavior. So, it has been an urgent need for the best technology that not only oxidizes As(III) but also completely removes the arsenic species from water. Groundwater majorly contains arsenite (As(III)) as well as arsenate (As(V)). Among these two, arsenite species are more carcinogenic, mobile, and lethal. It is also more difficult to remove by conventional water treatment methods. Ferromanganese slag, waste generated from steel industries, has been utilized in this study to develop an arsenic adsorbent. An adsorbent with the capacity for simultaneous oxidation of As(III) and adsorption of total arsenic species can be efficient for arsenic decontamination. The As(III) oxidation capability of the developed material is about 70±5 % based on the initial As(III) concentration. The adsorbent not only oxidizes the As(III) species but also adsorbs the arsenic species. To know the individual role of metallic oxyhydroxides of Fe, Al, and Mn in the arsenic removal process, each metal oxide/hydroxide and a mixed ternary metal oxides/oxyhydroxides were prepared by mixing Fe, Al, and Mn-salts precursors and hydrolyzed simultaneously up to an optimized pH.The formation of oxyhydroxides Fe, Al, & Mn, specifically Mn(IV) oxidation state, can quickly oxidize the As(III) species in water; simultaneously, arsenic species get adsorbed on other (Fe, Al oxides/hydroxides) adsorption sites. Hence, the maximum Langmuir adsorption capacity obtained was 34.34 mg/g and 21.22 mg/g for the As(III) and As(V). An innovative oxidative catalyst with a similar composition of Fe, Al, and Mn oxides/hydroxides has been prepared using a low-cost natural clay and industrial waste to oxidize arsenite (As(III)) and decontaminate arsenic from water. Some studies discussed in literature about combined Fe, Al, and Mn-based oxyhydroxides adsorbent to adsorb total arsenic. But, the role of each constituent and composition in oxidation and adsorption has not been discussed. Mn(IV) compound in oxidative catalyst delivers the oxidation functionality by producing either superoxide radicals or hydroxyl free radicals. The study focuses on the role of active sites created by Mn oxyhydroxides and Fe or Al oxyhydroxides. Photocatalytic oxidation of As(III) species occurs at even dark environments due to the low-energy bandgap. XANES and XPS techniques confirmed the 60% oxidation of As(III) into As(V) during adsorption. The fixed-bed column runs are the real test of an adsorbent’s capacity to remove targeted pollutants. These column runs are operated at room temperature and normal pressure with variable flow rate, different adsorbate concentrations, and height of the adsorbent bed in the column. Hence, various parameters have been evaluated by fitting the fixed bed adsorption model equations with experimental data. The adsorption capacity of the adsorbent was very high, so the adsorbent was not exhausted till 400 h, equivalent to the ~15000-bed volumes of treated water obtained when bed height was taken 10 cm. At a slower feed flow rate, the As(III) adsorption was higher due to sufficient time for its oxidative conversion into As(V). It was followed by adsorption, supported by the higher pore diffusion constant due to the readily available accessible pores in the adsorbent. Adsorption phenomena and rate of adsorption were understood by calculating the diffusivity coefficient for each adsorbent with both arsenic species. Due to the presence of Mn oxides/hydroxides, the adsorption sites were transformed and produced more active sites for the adsorption of arsenic species during the oxidation of As(III) species. Therefore, the value of diffusivity (Dp) will help to understand the arsenic adsorption behavior on the adsorbent where the reaction front moves the exterior surface to the center of the adsorbents, which refers to the total consumption of the adsorbent. The As(III) species have utilized the maximum active sites under synthetic water. In contrast, the exterior was inaccessible under the natural arsenic-contaminated water due to the low initial arsenic concentration. In the fixed column mode, the adsorbent utilization was more with arsenic-contaminated water as feed due to longer contact time. The As(V) ions followed a similar pattern under both environments. Still, they had less surface coverage than As(III) due to abundant active surface sites for As(V) species than As(III). Thus, the adsorbent is not saturated with As(V) ions and can still adsorb more As(V) ions.