A surface complexation framework for predicting water purification through metal biosorption

Biosorption has emerged as an alternative sustainable strategy for cleaning up water contaminated through industrial activities and/or natural processes. Since biomaterials contain discrete reactive sites to which adsorption takes place, the biosorption process is amenable to thermodynamic treatment using surface complexation theory, enabling the development of predictive models for complex natural or industrial mixtures. In this paper, we present such a surface complexation formalism as it relates to bacterial surfaces, which is verified using proton and single metal biosorption data plotted as a function of pH. The parameters extracted from these verification experiments are then used to predict biosorption in mixtures of metals, with excellent success. The model should be applicable to other biomaterials, such as algae, fungi and higher plants.

We have presented a surface complexation model for the adsorption of protons and metals to biomaterials, which are being adopted widely as a sustainable, affordable technology for remediation of metal contamination in natural and industrial waters. Using bacteria as a model biosorbent, we have demonstrated that surface complexation parameters calculated from single metal systems can be applied to predict metal adsorption in more complex mixtures. Importantly, the approach works for different types of metals, including lanthanides, which are often used as surrogates for some radionuclides. It remains to be demonstrated how well the model will work in real fluids, where the presence of certain ligands, particularly organic cocontaminants, may lead to the formation of ternary surface complexes for which stability constants have yet to be determined. It is worth noting that the deprotonation constants determined in our studies are similar to those determined for other single bacteria as well as bacteria consortia [12]. Moreover, recent studies suggest that similar functional groups exist on algal and fungal surfaces [6], with comparable deprotonation constants. Moreover, metal stability constants determined for the different functional groups also appear to be similar across different biomaterials for which surface complexation models have been tested. Since recent studies have shown that deprotonation constants determined for bacteria can successfully explain metal adsorption to natural organic matter [17], surface complexation models point to the existence of a universal surface chemistry for all biomaterials, and hold promise for estimating biosorptive cleanup without the need for conducting individual experiments with each biomaterial.