Experiments of arsenic (V) adsorption on birnessite: implications on arsenic cycling

Birnessite occurs as a major manganese oxide in sediments. It is characterized by a high adsorption capacity for trace elements, including arsenic. However, the effects of birnessite on arsenic cycling were less intensively investigated than that of goethite, a commonly recognized arsenic host. Therefore, this study utilizes arsenic-bearing solutions containing 0.1–50 ppm arsenic to synthesize birnessite and uses arsenic sequential extraction procedure (SEP) analysis to quantify its arsenic co-precipitation and adsorption capacity.
    SEPs showed that adsorbed/structural arsenic ratio grew from 0:100 to 60:40, with arsenic concentration increasing from 0.1–50 ppm, implying saturation of structural arsenic. SEPs quantify the adsorbed and structural As of birnessite being 0.831 and 1.308 mg/g, respectively. However, the low arsenic concentrations of < 1% in residual solutions indicate nonattainment of the maximum adsorption capacity. Subsequent adsorption experiments using higher initial arsenic concentrations reaching 250 ppm determined the maximum arsenic adsorption capacity to be 19.81 mg/g at pH 7, comparable to the values of 15.3–22.5 mg/g at pH 6.5 (Manning et al., 2002; Singh et al., 2010) and that of 3.79–15.73 mg/g for goethite. It then appears that birnessite adsorption/desorption is more effective than precipitation/dissolution in controlling arsenic cycling. However, the consideration based on the maximum adsorption capacity might overrate the controls of adsorption/desorption relative to precipitation/dissolution because the natural water generally contains less arsenic than that used for the adsorption experiments. Therefore, we conducted adsorption experiments with low arsenic concentrations of 0.5 ppm and a solid/liquid ratio of 0.01 to 0.001. The results showed that adsorption capacity rose abruptly to 1.416 mg/g in two weeks and then slowly increased to 1.523 mg/g after three months. This feature indicated incomplete filling of the adsorption sites on the surface of birnessite at low As concentration despite its large specific surface area. Another controlling factor was the abundance of birnessite in sediments. If 10% of MnO (0.05–0.15%) in the Chianan sediments in southern Taiwan occurred as birnessite, the adsorbed arsenic was calculated to be 0.0005–0.022 mg, corresponding to < 2 % of adsorbed arsenic in 1 g sediments, based on the SEP data of Yang et al. (2016). In the extreme case that all the Mn occurred as birnessite, birnessite could account for up to 25% of the adsorbed arsenic. Apparently, birnessite is not a major contributor to surface arsenic cycling. However, this inference must be evaluated by considering the adsorption and co-precipitation data from goethite and goethite proportions in sediments.