Adsorption-desorption characteristics of arsenic on agricultural soil

Abstract

Accumulation of As on paddy field soils as a result of irrigation with As contaminated groundwater and its introduction into the food chain are major concerns. Adsorption is the principal mechanism by which As is accumulated on soils. [n this study, adsorption characteristics of arsenic on paddy field soils from three As affected areas (Munshiganj, Faridpur and Brahmanbaria) and one unaffected area (Naogaon) were assessed in laboratory batch experiments. As adsorption on paddy field soils has been found to increase with increasing oxalateextractable iron content (as well as total iron content) of soil; thus amorphous iron oxyhydroxides appear to be the principal adsorbent of As. [ron content of paddy field soil, in turn, has been found to increase with increasing dissolved Fe content in irrigation water. Thus, As adsorption onto paddy field soil depends not only on the characteristics of soil, but also on the characteristics of (e.g., iron content) of irrigation water. For some paddy field soils, very high adsorption densities, approaching 200 mg/kg, have been recorded under thc laboratory experimental conditions. [f As is present in such high concentrations in paddy field soils throughout the year, it could have significant adverse effects on growth and yield of crops. On thc other hand, As adsorption by soil may play an important role in reducing As availability to plant and its introduction into the food chain. The high adsorption capacity of soils could reduce the bioavailability of arsenic to plant and also retard its transport through the subsurface. Arsenate adsorbs much more strongly onto soil than arsenite. For same initial concentration of arsenic, arsenate adsorption is almost 3 times greater than arsenite adsorption. Phosphate was found to adsorb quite strongly on paddy field soils and, like As, its adsorption appear to increase with increasing iron content of soil. Arsenite adsorption was found to decrease with increasing phosphate concentrations due to its higher affinity for sorption sites than As; but since the batch experiments were carried out with low sorbate to sorbent ratios, the effects were apparent only when concentrations of both As and phosphate were high. Effect of pH on adsorption of both arsenate and arsenite appear to follow the same trend; adsorption as a function of pH followed a concave downward profile with the maximum adsorption occurring at pH between 4 & 6. Some recent studies suggest that significant amount of As (that is accumulated in the paddy tield soils during the irrigation season) is mobilized/released from paddy field soil in the wet scason (that immediately follow the irrigation season) during inundation with flood/rain water. Desorption from soil and reductive dissolution of iron oxyhydroxides, the principal adsorbent is the soil matrix, are thought to be responsible for the phenomenon. Results obtained from this study confirm that significant amount of As could be mobilized from paddy field soils by desorption, especially over longer time periods, and through reductive dissolution. Desorption of As from paddy field soils have been found to be a strong function of time. Under the experimental conditions, about [0% of As initially present in soil was desorbed within a period of about one month. Since paddy fields are often inundated under a significant depth of water for about three months during the wet season, As release/mobilization from soil by desorption could be significant. Significant As release from As-rich paddy field soils was recorded under reducing condition created by addition of a carbon source (glucose) and nutrients. Such

mobilization of As through reductive dissolution of iron oxyhydroxides has also been found to be a strong function of time. Both desorption and reductive dissolution of iron oxyhydroxides appears to be important processes that govern the fate of As in irrigated paddy fields. Results from this study suggest that these natural processes playa major role in preventing As concentration in soil to reach toxic levels.