Halloysite-zero-valent iron nanocomposites for removal of Pb(II)/Cd(II) and As(V)/Cr(VI): Competitive effects, regeneration possibilities and mechanisms

Abstract

The work aims at exploring competitive adsorption of Pb/Cd and As/Cr by halloysite-containing sample doped with zero-valent iron (Fe0) particles both in powdered and granulated form. The halloysite-Fe0 (10 and 25 wt% loading) nanocomposites were synthesized by reduction of Fe(III) with sodium borohydride (NaBH4). The XRD peak at 2.03 Å confirmed deposition of Fe0 particles exclusively on the mineral surface. The SEM and STEM observations showed Fe0 chain-like agglomerates build of individual nanoparticles with ∼20−60 nm size. The competitive adsorption experiments indicated that the halloysite-Fe0 composites had higher affinity toward Pb(II) as compared to Cd(II). In the case of As(V) and Cr(VI) the selectivity was not evident. The adsorption–desorption studies showed that the composites undergo efficient regeneration after NaBH4 treatment which enables their reuse with high removal capacity. The studied composites also showed high chemical stability, with minor release of Al, Si and Fe during treatment. The adsorption mechanisms studied by XPS revealed Pb(II) and Cd(II) removal by chemisorption on the iron hydroxide shell covering the Fe0 core. Furthermore the XPS confirmed redox reactions as one of the adsorption mechanism involved in As(V) and in particular Cr(VI) removal. The anions were reduced which was accompanied by Fe oxidation. Most importantly, the investigations in a dynamic fix bed column system revealed high potential for Pb/Cd and As/Cr removal by granular form of the halloysite-Fe0 nanocomposite. This increases the material attractiveness for application in wastewater remediation.