Combination of interfacial reduction of hexavalent chromium and trivalent chromium immobilization on tin-functionalized hydroxyapatite materials

Sebastiano Campisi,Claudio Evangelisti,Georgeta Postole,Antonella Gervasini

doi:10.1016/j.apsusc.2020.148227

Abstract

We present an ecofriendly hydroxyapatite (HAP) material functionalized with tin (Sn/HAP) for an efficient interfacial reduction of Cr(VI) to Cr(III).Tin was deposited on HAP at different concentrations (from 0.2 to 1.2 mmol/g) using colloidal or clear acid solutions of SnCl2. The morphological and structural properties of fresh and used Sn/HAP samples were determined (N2 adsorption-desorption, transmission electron microscopy techniques, XRPD, XPS). Tests were performed under various conditions: at different pH 3–7, inert or oxidant atmosphere, sample aging (up to 15 days).Sn/HAP samples prepared using colloidal acid solutions revealed the best performances in the reduction of Cr(VI) to Cr(III), carried out under mild conditions (40 °C in acidified solution) with initial concentration of Cr(VI) in the range from 25 to 50 ppm. Best removal of Cr(VI) (10 mg/g) was obtained by using a Sn-concentration of 0.65 mmol/g with complete adsorption of the formed Cr(III) at HAP surface. This finding was associated with higher Sn-dispersion (surface Sn, 9.4 at.%) compared to samples prepared from clear solutions (surface, Sn 7.7 at.%), as evidenced by HAADF-STEM/EDX and XPS analyses. Remarkably, tin was tightly retained on the HAP surface under reaction conditions (0.7% leaching), confirming the occurrence of Cr(VI) reduction at solid-liquid interphase.

Highlights

In the environment matrices chromium typically occurs in two oxidation states: Cr(III) and Cr(VI), which differ in several chemical and physical properties
Hexavalent chromium, Cr(VI), can form different species in aqueous solutions such as hydrogen chromate (HCrO4−), chromate (CrO42−), and dichromate (Cr2O72−), whose relative distribution is a function of pH, redox potential and Cr(VI) concentration
Concerning the very weak peaks occurring at higher binding energy (581.02–590.42 eV), they may be assigned to the presence of trace amount of chromium in Cr(VI) oxidation state on the surface of spent Sn7.5/HAPcoll [63]. These results suggest that the Cr(VI) reduction process occurs at the interface between Cr(VI) and the surface of Sn/HAP; the Cr(III) formed from the redox process immediately is stabilized on the surface near a Sn-site through the strong coordination interactions with the electronrich HAP surface functionalities (Scheme 2)

Summary

Introduction

In the environment matrices chromium typically occurs in two oxidation states: Cr(III) and Cr(VI), which differ in several chemical and physical properties. Hexavalent chromium, Cr(VI), can form different species in aqueous solutions such as hydrogen chromate (HCrO4−), chromate (CrO42−), and dichromate (Cr2O72−), whose relative distribution is a function of pH, redox potential and Cr(VI) concentration. All these species have high solubility in water ( in alkaline conditions) and high mobility. Due to these properties, remarkable diffusivity of Cr(VI) through soil and aquatic environments as well as capability to cross biological membranes can occur. The latter factor is probably the basis of the well-documented adverse human health effects both at short-term and long-term exposures [1]

Methods

Results

Conclusion