Adsorption and Desorption of Arsenate in Different Soils and Gold Mining Substrates of Minas Gerais State, Brazil

Abstract

Arsenic (As) availability in natural environment is related to the element’s adsorption and desorption processes in soils. Total As is better related to available As in temperate soils than in tropical soils. In tropical soils, total As is not very significant in terms of availability, therefore justifying the necessity for studies into As dynamics. Knowledge of As dynamics in soil as well as development of new analytical methodologies involving tropical soils are insufficient and necessary for future mitigation projects. The objectives of this study were: (1) To adjust methodologies which may assist in understanding arsenate dynamics in tropical soils and substrates; (2) To evaluate the adsorption and desorption of arsenate in soils and substrate samples, and to find a minimum value of arsenate available in soil which is lethal to sorghum plants. Samples of three soils from Minas Gerais State (YL, RYL, and CS) and two sulfide substrates of gold mining (B1 and B2) were used in the assays. All the material was physically and chemically characterized. Remaining As (As-rem) and remaining P (P-rem) of each material, along with MACP and MACAs (using the Langmuir isotherms), were obtained. After agitation to obtain MACP and MACAs, arsenate was extracted by anionic resin and Mehlich-III to evaluate arsenate desorption of the material retained on the filter paper. Subsequently, arsenate desorption curves for the different materials were obtained, and arsenate availability was determined through a bioassay with sorghum plants. Samples of soils and substrate B1 were incubated with six levels of As doses. Plants were grown under greenhouse conditions for 30 days. The plants were then harvested, dried and weighed. Available As in the soils and substrate was determined by Mehlich-III. As-rem level decreased from YL (sandy) to RYL (clayey) soil samples, which always showed lower values than P-rem. Among the soils and substrates evaluated, RYL showed the highest MACAs and MACP, followed by CS, YL and Bl. The results were in accordance with the values observed for As-rem and P-rem and confirm the idea that the ability of the assayed materials to remove As from the soil/substrate solution is higher than the ability to remove P. On the other hand, the binding energy (a) between soil/substrate and As is weaker than the binding energy of P. Given the fact that the studied soils present a real ability to remove As from the solution, only a small part of As would be unavailable considering MACAs as a reference. As-Mehlich-III values were higher than As-resin for substrate Bl. Mehlich-III seemed to be more appropriate to extract labile forms of arsenate in substrate B1 as well as in the soils. Available As by Mehlich-III (26.9 mg/dm3) was considered a reference of As LCL to sorghum plants. CC50 was sensitive to the buffering capacity of each soil, showing values varying from 1.34 mg/dm3 As (clay soil with lower As-rem) to 12.31 mg/dm3 As (sandy soil with higher As-rem). The adaptation of the As-rem and MACAs methodologies was satisfactory and of great value in the study of adsorption, desorption and As availability for soils and mining substrate. Mehlich-III was also satisfactory to estimate available As and was sensitive to soil buffering capacity. Nevertheless, resin can also be used as an alternative. MACAs varied among soils and was higher than MACp. However, As showed higher lability than P. Using Mehlich-III, we determined the value corresponding to CC50 that showed a good reference of toxicity to available As. The environmental implications of the As behavior are quite serious. Beside the fact that arsenate is removed very fast from the soil solution, an anthropogenic input of the element, being part of the soil quantity factor, may remain in a reversible form for a long time. As may therefore return to the soil solution and becomes available to plants, animals and the entire environment. Considering that CC50 is the maximum contents of available As the environment can tolerate to allow some vegetal biomass production, the maximum capacity of As immobilization in each soil is reduced when compared to the soils’ MACAs values. Therefore, the maximum and safe values of reference to be used in the evaluation of incidental discharge of the element in soils must be reduced.