Chromate sorption and reduction kinetics onto an aminated biosorbent

Chromium(VI) removal via reduction-sorption on bi-functional silica adsorbents

Organically-modified silica gels bearing mercaptopropyl and ethylenediaminetriacetate groups (SiO2-SH/ED3A) have been used for reduction and subsequent sequestration of Cr(VI) species. The uptake mechanism involves Cr(VI) reduction by thiol groups (SH) and further immobilization of the so-generated Cr(III) species via complexation to the ethylenediaminetriacetate moieties (ED3A). The most appropriate pH range (1-3) for complete Cr(VI) reduction-sorption by SiO2-SH/ED3A originates from the balance between full reduction of Cr(VI) by SH, requiring low pH values, and quantitative complexation of Cr(III) by ED3A, which is favored in less acidic media. Such bi-functional adsorbents are considerably more effective at removal of Cr(VI) than those simply modified with thiol groups alone. The whole reduction-sorption process was characterized by fast kinetics, thus permitting efficient use of the SiO2-SH/ED3A adsorbent in dynamic conditions (column experiments). Monitoring the amount of immobilized chromium species on the solid was achieved using X-ray fluorescence spectroscopy and UV-vis spectroscopy. Studying the influence of ionic strength and presence of heavy metals revealed few interference on Cr(VI) removal.

Bifunctionalized mesoporous silicas for Cr(VI) reduction and concomitant Cr(III) immobilization

Surfactant-templated thiol-functionalized mesoporous silica adsorbents have been prepared by cocondensation of mercaptopropyltrimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium bromide, which were then partially oxidized to get bifunctionalized materials containing both thiol and sulfonic acid moieties (MCM-41-SH/SO3H). The resulting organic-inorganic hybrid was applied to the uptake of chromium species according to a reduction-sorption mechanism involving reduction of Cr(VI) by thiol groups and immobilization of Cr(III) onto sulfonic acid moieties. These processes were strongly affected by pH, and the optimal conditions for effective chromium sequestration resulted from a compromise between pH values low enough to ensure quantitative reduction of Cr(VI) and not too low to enable Cr(III) binding to sulfonate groups, which was best achieved at pH 2-3. The effect of the solid-to-solution ratio and the relative amounts of -SH and -SO3H groups was also discussed. Even if Cr(VI) reduction by thiol groups resulted in the formation of sulfonic acid moieties, their contentwas not high enough to ensure quantitative Cr(III) immobilization, which was only attained with materials containing already some sulfonic acid groups prior to contacting Cr(VI) solutions. Redox speciation of sulfur and chromium species was analyzed by X-ray photoelectron spectroscopy (XPS) and used to support the proposed mechanism.