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Study on Removal of Chromium and Zinc from Contaminated Soil by Washing Method

J.B. Alam
Civil and Environmental Engineering Department, Shahjalal University of Science and Technology

A. Begum
Soil Science Discipline, Khulna University, Khulna

S.M. Billah
Soil Science Discipline, Khulna University, Khulna.

M.M. Miah
Soil Science Discipline, Khulna University, Khulna.

Abstract/ Executive Summary:

Heavy metal contaminated soils can be decontaminated by the washing method. The washing method was conducted for the remediation of contaminated soil in the laboratory. Laboratory batch extraction studies were conducted with contaminated soils with Cr (VI) and Zn separately to determine the ability of extractants such as 0.12 M Na4P2O7, 0.1 M NaF, 1.0 M HCl, deionized water, and EDTA solution to remove chromium from varying soil pH (6.1 - 8.2 ), organic matter content ( 1.57% - 1.83% ) and clay content (10% - 17% ). Up to 71 % of total chromium and 61% of zinc using 0.12M Na4P2O7 could be removed from the contaminated soil.

Keywords: Soil washing, Heavy metal, EDTA, Organic matter

Location: Different places of Tannery, near Hazaribag, Dhaka.

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Heavy metal in soil, Zinc, Chromium

Objectives:

They also reported that the pH, percentage of organic matter, clay content etc affect the removal efficiency. In this paper, an attempt has been taken to remove chromium and zinc using five different solvents and a comparison study among the extractants has been discussed.

Methodology:

Laboratory studies were conducted with two types of prepared contaminated soils containing varying amounts of organic matter and clay content. The soils were spiked with K2Cr2O7 and ZnSO4 to get Cr (VI) and Zn concentrations of 500 ppm. The soils were collected from different places of Tannery, near Hazaribag, Dhaka. The pH of the soil samples was increased from 5.9 to 8.2 by adding 1N NaOH solution to prepare a sample of high pH value. The following reagents were used for washing.

Removal of chromium and zinc from contaminated soil by washing method:

• Na4P207 solution (0.12M): 3.20 gm Na4P2O7 was diluted in 100 ml distilled water. • NaF solution (0.1M): 4.20 gm NaF was diluted in 100 ml distilled water. • EDTA solution(0.05M):1.816 gm di-sodium salt of EDTA was diluted in 100 ml distilled water. • NaOH solution (1.0 M): 4.00 gm NaOH pellets were diluted in 100 ml distilled water. • HCl (1.0 M): 6.4 ml conc. HCI was taken and added in water to make the volume 100m1. • HNO3 (4.0 M): 25.6 ml conc. HNO3 was taken and added in water to make the volume 100ml.

Laboratory studies were conducted with two types of contaminated soils with varying levels of organic matter and clay content collected from the tannery. The laboratory experiments in this study were conducted with soils contaminated by Cr (VI) or Zn. Based on the behavior of chromium or zinc in soils, a feasible and effective method of in-place soil remediation has been studied by extraction of total chromium or zinc from the soil by flushing with a suitable elutant, recovery of the elutriate, and treatment of the chromium or zinc-containing elutriate as wastewater.

The carbon present in the organic matter was determined by the modified Walkley Black Method (wet digestion method). Heavy metals Cr and Zn were determined by the Spectrophotometer method (Vitale, et. al. 1997). In soil flushing experiments, 40m1 capacity glass bottles with Teflon stopper were used in soil extraction experiment. The extractants used include 0.12M Na4P2O7, 0.1M NaF, 1.0M HCI, 0.05M EDTA and deionized water. Na4P207 was selected because it can remove exchangeable, precipitated, and organic fractions of chromium; NaF was selected because it can remove both organic and coprecipitated forms of chromium; and HCI can extract all forms of metals from the soil (James, 1996). 5gm soil was removed from the bottle and extracted for total chromium determination using each extractant separately once every 24 hr. After the extract was separated from the soil by centrifugation, a portion of the extract was digested with nitric acid before analysis, according to standard methods.

Source of Information: South Asian J. Agric., 2006, 1(2): 5-7 ISSN 1991-0037

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Funding Source:

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Achievement/Findings:

Comparing the mean chromium removal efficiencies for each soil, we can see the importance of soil properties on chromium extraction. Soil sample with the lowest organic content (1.57%) among the four soils yield the highest Cr removal efficiencies, and soil sample-2 with the highest organic matter content between the two soil types (1.83%) yields the lowest Cr or Zn removal efficiencies. Soil clay content may have also influenced Cr or Zn removals achieved. Soil samples with the lowest clay content (10-11%) yielded the best Cr or Zn removals, whereas soil sample-2 with the highest clay content (15-17%) exhibited the lowest Cr or Zn removal efficiencies. The observation coincides with the findings of James et al. (1995) that the mobility of metals in soils is a function of clay content, and the extractability of metals from soil is a function of the mobility.

Knowledge Management: Journal