Biosorption-mediated reduction of Cr(VI) using heterotrophically-grown Chlorella vulgaris: Active sites and ionic strength effect

•A heterotrophically-grown biomass of Chlorella vulgaris was used for Cr(VI) removal.•Titration modeling evidenced hydroxyl, carboxyl, phenolic and amino groups.•IR spectra confirmed titration assignment and evidenced the presence of sulfonates.•ANOVA denoted significant positive effect of ionic strength on reduced Cr.•A model for Cr-sites interactions in biosorption–bioreduction process was proposed.

In this work experimental results of potentiometric titration and Cr removal at different ionic strength using a heterotrophically-grown biomass of Chlorella vulgaris were reported for the first time in the literature. Biomass cell wall was characterized by potentiometric titrations and IR spectra. Titration modeling denoted the presence of four kinds of sites with acid–base properties: hydroxyl, carboxylic, phenolic and amino groups. IR spectra confirmed such assignment and also showed the presence of sulfonates. Kinetic tests of Cr(VI) removal were performed according to a full factorial design with two factors (Cr(VI) concentration and ionic strength) at two levels: all tests can be represented by pseudo-first and pseudo-second order models. Analysis of variance showed significant negative effects of ionic strength and of ionic strength–time interaction on sorbed Cr, and positive effect of ionic strength on reduced Cr. These findings along with characterization results were elaborated according to a conceptual model for Cr(VI) biosorption-mediated reduction: protonated hydroxyl and amino groups can be involved in Cr(VI) sorption, hydroxyl groups can reduce sorbed Cr(VI) giving Cr(III) and new carboxyl groups. Cr(III) can be bound by labile electrostatic interactions to sulfonates. Competition with Na+ determined the release of Cr(III) and favored further Cr(VI) reduction.