Chromium(VI) reduction and accumulation on the kaolinite surface in the presence of cationic soil flocculant

Katarzyna Szewczuk-Karpisz,Gracja Fijałkowska,Małgorzata Wiśniewska,Grzegorz Wójcik

doi:10.1007/s11368-020-02677-1

Abstract

PurposeHeavy metal soluble forms pose a threat to plants, soil microflora, and microfauna. To limit their toxicity and mobility, various immobilizing additives are being developed. The main aim of the study was to determine the influence of soil flocculant (cationic polyacrylamide (CtPAM)) on the hexavalent chromium ion reduction and accumulation on the kaolinite surface. In this way, the efficiency of the selected polymer conditioner in the soil remediation was determined.Materials and methodsThe adsorbed amounts of Cr(VI) and CtPAM on the kaolinite surface was determined spectrophotometrically (spectrophotometer UV/Vis) using specific indicators. The structure of electrical double layer formed on the kaolinite surface in the mixed system of cationic polyacrylamide and Cr(VI) ions was described based on the adsorptive measurements, zeta potential measurements, as well as potentiometric titration. The kaolinite aggregation in the presence of CtPAM and/or Cr(VI) ions was determined spectrophotometrically and proved using SEM microscopy. The reduction process of Cr(VI) to Cr(III) occurring in the adsorption layer was determined by diffuse reflectance spectroscopy (DRS).Results and discussionThe obtained results indicated that cationic polyacrylamide contributes to Cr(VI) reduction to Cr(III) form in the kaolinite suspension. This heavy metal ions are strongly adsorbed by the applied clay mineral, especially at pH 5 (then the Cr(VI)-adsorbed amount equals 5.42 mg/g). The CtPAM addition favors the Cr(VI) ion immobilization on the kaolinite surface. In the conditioner presence, the heavy metal-adsorbed amount is even at 7.34 mg/g. The adsorption of CtPAM and chromium(VI) ions induces changes in the kaolinite surface charge density and electrokinetic potential. What is more, both substances favors the kaolinite aggregation.ConclusionsCationic polyacrylamide may be considered a substance reducing the toxicity of chromium(VI) ions in the natural environment and decreasing their availability for soil organisms. It can improve the soil structure (by stimulating aggregation) and reduce environment pollution with heavy metals simultaneously.Graphical abstract

Highlights

Soil environment pollution with heavy metals is a threat to the proper functioning of organisms (Kilic et al 2011)
The main aim of the study was to determine the influence of soil flocculant (cationic polyacrylamide (CtPAM)) on the hexavalent chromium ion reduction and accumulation on the kaolinite surface
The structure of electrical double layer formed on the kaolinite surface in the mixed system of cationic polyacrylamide and Cr(VI) ions was described based on the adsorptive measurements, zeta potential measurements, as well as potentiometric titration

Summary

Introduction

Soil environment pollution with heavy metals is a threat to the proper functioning of organisms (Kilic et al 2011). In contrast to pollution with organic compounds (such as aromatic hydrocarbons) heavy metals are not biodegradable and remain in the soil for thousands of years. They limit the number, activity, and biodiversity of microorganisms and plants (Alkorta et al 2004). Most of heavy metals such as Cr, Pb, Cd, and Hg have toxic effects on the organisms at minimal concentrations. The chromium toxicity depends on its oxidation stage. Chromium(VI) is much more hazardous than the chromium(III) one.

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