Metal sorption studies biased by filtration of insoluble metal oxides and hydroxides

Abstract

Toxic metals in the environment are often remediated using sorption techniques, particularly in aquatic and drinking water systems. However, a review of over 30 published sorption studies in the past two years alone revealed that the use of filtration to separate sorbed from unsorbed metals do not take into account metal hydroxide and oxide formation, and thus likely produce erroneous results. We quantified the effect of filtration on the removal of metal oxide/hydroxides from solution using a 0.45 μm filter as a function of pH, initial metal concentration and ionic strength for As, Be, Cd, Cu, Cr, Pb and Zn. We found that even when the initial metal concentration was as low as 0.1 mg/L, up to 93% of metals in solution were removed and up to 100% removal was observed when the initial metal concentration was 5 mg/L at a pH of 7. If this was unaccounted for, precipitated metal oxide/hydroxide removed via filtration will be inaccurately attributed to metal sorption. Additionally, we demonstrate that speciation modeling can underestimate the pH at which insoluble metal species form and therefore can only be used to approximate metal precipitation, especially in complex matrices. Overestimating the sorption capacity of sorbent materials has major implications if these sorbents are used for the purification of drinking water or other vital environmental remediation efforts. We recommend sorption studies using filtration prepare the appropriate matrix-matched control samples to quantify potential metal oxide/hydroxide formation.