Removal of heavy Metals from Tailings in Citric Acid Solution with Ferric chloride

Abstract

The release of heavy metals and other pollutants in soil became one of the most important environmental problems. A number of studies and assessments about heavily-contaminated sites indicated the possible health impacts of contaminated soil with high level of metals. To remove such kinds of pollutants, in fact, soil washing plays a role as one of the most permanent treatment alternatives to remove heavy metals from soil contamination. The studied metals in this work included six elements: Fe, As, Mn, Zn, Cu, and Pb from tailings, which have been known as a cause of soil contamination near abandoned mine sites. Furthermore, the sample observed by X-ray Diffraction (XRD) showed the high levels of quartz and metal sulfides components, which made it difficult to remove pollutants by using chemical reagents independently. Thus, the combination of citrate and ferric ion were selected to remove heavy metals from these stable forms.

Firstly, the chemical forms of six metals in contaminated soil were investigated by Tessier’s sequential extraction procedures. In that, the heavy metals include five fractions corresponding with five steps in procedures: “exchangeable”, “bound to carbonate”, “bound to Mn-Fe oxides”, “bound to organic matter and sulfide minerals”, and “residue”. However, the result comparisons between 1.0 g sample suggested by Tessier’s and 0.2 g sample in present study showed the differences in distributions. The highest portion of metal species was found in the “residual fraction” with 1.0 g sample and in the “bound to organic matter and sulfide minerals fraction” with 0.2 g sample, respectively. It illustrated that to obtain the exact information of metal distribution from sources with high metal level, the use of less sample should be chosen.

Secondly, the leaching efficiencies of heavy metals were also investigated by the combination of citrate and ferric ion. An investigation of parametric influences on metal extraction exhibited the remarkable increase the leaching efficiencies of heavy metals except As with increasing initial ferric ion concentration, agitation speed, and temperature, but decreasing pulp density and pH solution. It was found that 100 % of total Fe, Mn, Zn, Cu, and Pb concentration were extracted in solution under the conditions of 3.0 M Fe3+, 1.0 M citric acid at 50 °C of temperature, agitation speed 400 rpm, and after 3 h leaching time. The result also showed that the leaching efficiencies of heavy metals except As were enhanced with the presence of citric acid in ferric chloride solution compared to the solution without citric acid. The reducing of oxidation-reduction potential (ORP) can be considered as a main disturbing factor on As dissolution.