Electrolytic recovery of low-grade nickel from Ni-rich soil

Date

2016

Abstract

Heavy metal contamination of the soil is a serious threat to human health, plants, animals and microorganisms if left untreated if left untreated in the soil matrix. Hence, remediation is deemed necessary for soils with heavy metal concentration exceeding the threshold limits for plant and soil toxicity. In this study, removal of nickel from contaminated soil by soil washing using chelating agents such as chitosan and ethylenediaminetetraacetic acid (EDTA) was explored. The extent of heavy metal contamination in Sta. Cruz, Zambales [Philippines] was initially evaluated by analyzing twelve soil samples collected from various agricultural sites (i.e. fishponds, rice fields). Metal profiling by X-Ray Fluorescence (XRF) showed that the dominant metal in all soil samples is iron with concentrations ranging from 272,000 to 404,000 mg/kg in rice field soil samples and 76,000 to 101,000 mg/kg in fishpond soil samples, followed by chromium (7,700 to 10, 000 mg/kg in rice fields and 3,000 to 3,700 mg/kg in fishponds) and nickel (5,900 to 7,700 mg/kg in rice fields and 2,700 to 3,400 mg/kg in fishponds). All of these heavy metals far exceed their allowable concentration in the soil (1,000 mg/kg for Fe, 150 mg/kg for Cr and 100 mg/kg for Ni); hence, the soil in this area needs remediation to prevent further environmental degradation and improve soil productivity. Among the twelve soil samples, the soil with t he highest nickel concentration of about 7,658 mg Ni/kg soil (RF1 R1) was selected for further characterization and soil washing experiments. Physico-chemical characterization of RF1 R1 was done to determine soil pH (7.63), particle size distribution (25% w/w sand, 36% w/w clay), cation exchange capacity (25.47 meq/kg soil), organic matter (1% w/w), and organic carbon (0.378% w/w). Using sequential extraction, nickel distribution in different soil fractions – exchangeable (3.56% w/w), reducible (7.44% w/w), organics (7.70% w/w), and residual (81.31% w/w)was determined. Soil washing experiments for both chitosan and EDTA was initially done by evaluating the factors (solution pH, chelating agent concentration and soil loading) based on their effect on nickel removal from the soil by implementing a two-level factorial experimental design. This was followed by numerical optimization using Design Expert v7.0.0 to determine the soil washing conditions that maximizes the amount of nickel removed from the soil. For chitosan, only solution pH was found to have significant effect on nickel removal based on ANOVA results at alpha = 0.05. An increasing trend in nickel removal was observed as solution pH was decreased. Optimum soil washing conditions was predicted using One Factor Design Design Expert v7.0.0 to be at pH 0.5, 0.1 g/L chitosan concentration and 0.10 g/ml soil loading. At this condition, the predicted amount of nickel which can be removed from the soil is approximately equal to 2,336 +- 19.47 mg Ni/kg soil. For EDTA, on the other hand, all three (3) main factors (solution pH, EDTA concentration and soil loading) as well as their interactions were found to have significant effects on nickel removal during soil washing. Generally, decreasing the solution pH and soil loading and increasing EDTA concentration increased nickel removal. Optimization using Box-Behnken experimental design resulted to a quadratic log10 model which was used to predict the highest nickel removal of about 1160.86 +- 176.49 mg Ni/kg soil at the optimum conditions of pH 3,0.1375 M EDTA, and 0.05 g/mL soil loading. Using these optimum conditions, the preliminary cost of the soil washing technology was calculated to be about Php 1.95 per mg of Ni removed for chitosan and Php 1.52 per mg of Ni removed for EDTA.

Language

English

Document Type

Article

Pages /Collation

53 leaves

En – AGROVOC descriptors

SOIL; NICKEL; SOIL TOXICITY; CHELATING AGENTS; CHITOSAN; SOIL FERTILITY; HEAVY METALS; POLLUTION

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