Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments – A review

Abstract

The spread of contaminants in soil can be hindered by the soil stabilization technique. Contaminant immobilizing amendments decrease trace element leaching and their bioavailability by inducing various sorption processes: adsorption to mineral surfaces, formation of stable complexes with organic ligands, surface precipitation and ion exchange. Precipitation as salts and co-precipitation can also contribute to reducing contaminant mobility. The technique can be used in in situ and ex situ applications to reclaim and re-vegetate industrially devastated areas and mine-spoils, improve soil quality and reduce contaminant mobility by stabilizing agents and a beneficial use of industrial by-products. This study is an overview of data published during the last five years on the immobilization of one metalloid, As, and four heavy metals, Cr, Cu, Pb and Zn, in soils. The most extensively studied amendments for As immobilization are Fe containing materials. The immobilization of As occurs through adsorption on Fe oxides by replacing the surface hydroxyl groups with the As ions, as well as by the formation of amorphous Fe(III) arsenates and/or insoluble secondary oxidation minerals. Cr stabilization mainly deals with Cr reduction from its toxic and mobile hexavalent form Cr(VI) to stable in natural environments Cr(III). The reduction is accelerated in soil by the presence of organic matter and divalent iron. Clays, carbonates, phosphates and Fe oxides were the common amendments tested for Cu immobilization. The suggested mechanisms of Cu retention were precipitation of Cu carbonates and oxy-hydroxides, ion exchange and formation of ternary cation–anion complexes on the surface of Fe and Al oxy-hydroxides. Most of the studies on Pb stabilization were performed using various phosphorus-containing amendments, which reduce the Pb mobility by ionic exchange and precipitation of pyromorphite-type minerals. Zn can be successfully immobilized in soil by phosphorus amendments and clays.

Introduction

The concentration of trace elements in the Earth’s crust is constant and their enrichment in one place causes depletion in others. In the soil remediation context, trace elements cannot be destroyed like organic contaminants but only be relocated from one place, e.g. contaminated site, to another, e.g. landfill. The high cost of traditional soil remediation techniques (excavation and landfilling) and limited resources allocated to remediate contaminated sites prompted the development of alternative techniques that are cost-effective and less disruptive to the environment such as soil stabilization. For the comparison of soil stabilization with other commonly used soil remediation techniques, refer to Mulligan et al. (2001).

In this review, the term stabilization refers to the chemical stabilization of trace elements induced by immobilizing soil additives (amendments). The paper is an overview of data published during 2000–2005 on immobilization of one metalloid, As, and four heavy metals, Cr, Cu, Pb and Zn, in soils. These elements are of environmental relevance as they are common soil contaminants. As, Cu, Cr, and Zn can be found all together in soil contaminated with a wood impregnation chemical, chromated copper arsenate (CCA), while Pb is a contaminant usually related to the mining industry.

The choice of amendments depends on the type of soil contaminants; therefore, the main question to be answered by this review is: what amendments have been used lately to stabilize soil contaminated with one of the above named elements or their combinations? Particular attention is given to As due to its high toxicity to humans.