Achieving strong Pb-Cr complexation in Mg/Al LDHs for ultrafast chromate ions separation and chrome recovery from complex water matrices

Abstract

Layered double hydroxides with high anion exchange capacity and positive surface charge were being explored to remove metal oxo-anions. Environmental complexities such as the presence of other major interfering anions, organic matter, etc., drastically hampers their performance and limit their applicability. Based on the fundamental understanding of strong Pb-Cr complexation, in this work, Pb2+ incorporated LDH (LDH-Pb) was designed for selective chemisorption of chromate ions overcoming existing environmental limitations of LDH for chromium removal. LDH-Pb was designed by substituting Mg2+ ions of Mg/Al LDH with Pb2+ during crystallization process. Pb2+ have shown efficient incorporation at MII crystal sites of Mg/Al LDH and also sorbed in form of oxy-hydroxide on the surface. Pertaining to faster Pb-Cr complexation, strong binding, and formation of insoluble PbCrO4, LDH-Pb has shown efficient and higher removal of chromate ions than LDHs. Despite environmental complexities such as varying pH, competitive ions, organic matter, and natural matrices, efficiency of LDH-Pb remained less altered. Direct application (proof-of-concept) has been shown for extraction of chromate ions and chrome recovery from collected chromite ore processing residue leachate contaminated groundwater samples. Higher-order of removal kinetics and sorption capacity of LDH-Pb (n = 9.2, and 188.7 mg/g) further justified its candidature. Strong Pb-Cr complexation in LDH-Pb further resulted in faster and efficient continuous chromate filtration (47.3 mg/g) with adsorbent loaded syringe filters. Chrome recovery was achieved through easy acid dissolution of LDH from LDH-Pb-Cr waste (pH <3) resulting in chrome particles (monocrystalline micro rods) that have several potential industrial applications in various sectors.