Desorption of Arsenic and Its Mobilization in the Subsurface Environment

In nature, arsenic (As) is ubiquitous in the subsurface sediment of the Holocene alluvial aquifers of the Ganges delta basin. Sorption and desorption of this As onto the mineral surfaces of the sediment matrix mostly control its mobility. This paper intends to depict the subsurface fate of arsenic by exploring its desorption characteristics observed after reacting with the leaching agents.

Different adsorption phases of Arsenic present in the sediment were identified by sequential extraction test. A seven days' leaching test using HCO3 as the leaching agent was accomplished. To estimate the leaching potentiality of the aquifer sediment, linear sediment-water partition co-efficient, Kd was calculated based on laboratory and field data.

Presence of Fe minerals in a relatively large amount detected by the XRF and SEM-EDX represents it as the key adsorbent of As in the subsurface sediments.

Equilibrium linear partition coefficient Kd for As appeared to have inverse relationship with groundwater Fe. Besides Fe, pH was also appeared to have adverse influence on this Kd. For a smaller field Kd (0.12 l/g), the significant amount of Fe (8.4 mg/l) was found to remain in dissolved form. Subsequently, adsorbed As from the Fe mineral surfaces might also get released in the pore water.

Significant leached As (24.5 ìg/l) noticed after seven day's experiment confirmed that HCO3 might act as the efficient desorbing agent. Bicarbonate was found to affect inversely on laboratory Kd (As) reflecting its stronger effect on leaching rather than sorption.

Finally, adsorbed arsenic in the subsurface sediment after getting leached due to the successive desorption mainly from the Fe minerals by the shared effects of the competing ligands in a reductive environment where soil-water pH acts as a typical promoter, starts to mobilize in the sediment-liquid interface guided by the pore-water velocity.