Comparison of Arsenate, Lead, and Cadmium Adsorption onto Aged Biofilter Media

Abstract

Many biological water treatment plants for removal of iron and manganese from groundwater are in place for quite a long time, and thus their filters are aged—naturally coated with metal oxides and associated biomass. The particular reactivity and high adsorption capacity of these biogenic surface coatings make them potentially applicable for cost effective removal of arsenic and other heavy metals from contaminated water. However, the nature of interaction between various toxic elements and the composite materials in biological filters is not well understood. This study combines macroadsorption experiments with electron probe analysis to evaluate the adsorption properties of the biogenic surface coatings of an aged biofilter medium (BFM) for cationic lead and cadmium as well as arsenate anion. Results of batch adsorption showed that BFM has higher adsorption capacity for lead and cadmium as compared to arsenate anion. At pH 5.5, the maximum adsorption capacities of the medium for As(V), Pb(II), and Cd(II) were 17.03-, 80.77-, and 179.05-mg/g surface coatings, respectively. However, the column performance of BFM for Cd(II) was rather low. In particular, the breakthrough adsorption capacities (qb) of the BFM for As(V), Pb(II), and Cd(II) were 0.247-, 31.168-, and 4.084-mg/g surface coatings, respectively. These values represent about 1.5, 38.6, and 2.3% of the respective theoretical maximum adsorption capacities (qmax) of BFM for these metals. Data from the X-ray electron probe analysis corroborated well with that of the macroadsorption experiments. Results of this study strongly suggest that the Mn/Fe ratio and the presence of preadsorbed competing ions were two of the principal characteristics of the BFM, governing its affinity and adsorption capacity for different toxic metals.