Industrial lignins as efficient biosorbents for Cr(vi) water remediation: transforming a waste into an added value material

Cr(vi) represents a worldwide issue due to its carcinogenicity and toxicity, and its removal from water/waste-water is of great relevance for the protection of human health and the environment. We here present an investigation of the adsorption and reduction properties towards Cr(vi) of two different kinds of industrial lignins: a kraft softwood lignin (HMW) and a hardwood lignin (EH) obtained by an enzymatic process. Moreover, we prepared and characterized an acetylated and a phosphorylated lignin starting from HMW, along with a lignin@magnetite hybrid material. The influence of various experimental parameters on the adsorption process, such as pH, contact time, quantity of lignin, Cr(vi) concentration and ionic strength, was evaluated. The best performances can be obtained at acidic pH (pH = 2). With an initial Cr(vi) concentration of 20 mg L−1 and a contact time of 24 hours, a quantitative Cr(vi) reduction was observed, accompanied by a removal of total chromium of up to 35% when HMW was used as the biosorbent. A comparison of the adsorption profiles of the different biosorbents highlighted the higher performance of EH, endowed with a higher surface area and characterised by a maximum adsorption capacity of about 208 mg g−1, while acetylation of the hydroxyl group led to a drop in the adsorption profile. The best fitting of the adsorption isotherm by using the Langmuir model suggests that a monolayer coverage of metal ions onto the homogeneous active sites of the lignin's surface better describes the interactions occurring at the biosorbent interface. Overall, this study demonstrates that technical lignins, and particularly EH hardwood lignin, are effective and economic materials that could be successfully employed in the Cr(vi) water remediation process.