Removal mechanisms of Cr(VI) and Cr(III) by biochar supported nanosized zero-valent iron: Synergy of adsorption, reduction and transformation

Biosorption is a technique that can be used for the removal of pollutants from waters, especially those that are not easily biodegradable such as metals and dyes. A variety of biomaterials are known to bind these pollutants, including bacteria, fungi, algae, and industrial and agricultural wastes. In this review, the biosorption abilities of bacterial biomass towards dyes and metal ions are emphasized. The properties of the cell wall constituents, such as peptidoglycan, and the role of functional groups, such as carboxyl, amine and phosphonate, are discussed on the basis of their biosorption potentials. The binding mechanisms, as well as the parameters influencing the passive uptake of pollutants, are analyzed. A detailed description of isotherm and kinetic models and the importance of mechanistic modeling are presented. A systematic comparison of literature, based on the metal/dye binding capacity of bacterial biomass under different conditions, is also provided. To enhance biosorption capacity, biomass modifications through chemical methods and genetic engineering are discussed. The problems associated with microbial biosorption are analyzed, and suitable remedies discussed. For the continuous treatment of effluents, an up-flow packed column configuration is suggested and the factors influencing its performance are discussed. The present review also highlights the necessity for the examination of biosorbents within real situations, as competition between solutes and water quality may affect the biosorption performance. Thus, this article reviews the achievements and current status of biosorption technology, and hopes to provide insights into this research frontier.

Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and envirotechnical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700 °C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".