Kinetics of Chromium Reduction Associated with Varying Characteristics of Agricultural Soils

Chia-Yu Yang,Zeng-Yei Hseu,Yan-Lin Tseng

doi:10.3390/w14040570

Chia-Yu Yang, Zeng-Yei Hseu + Show 1 more

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https://doi.org/10.3390/w14040570

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Abstract

Chromium (Cr)(VI) is carcinogenic; thus, the excessive presence of Cr(VI) in soils can pose potential risks to water quality, food safety, and human health. The kinetics of Cr(VI) reduction in soils are important for assessing the fate of Cr in the environment. The present study tested physio-chemical and microbial properties in twenty-eight agricultural soils collected in Taiwan to evaluate the relationship between the reduction rate of Cr(VI) and soil properties, using 49-day incubation at 25 °C. At the beginning of incubation, 100 mg Cr(VI) kg−1 was spiked into the soils. The reduction of Cr(VI) was described by first-order kinetics at a significant level (p < 0.05) for the tested soils. The rate constant (k) of Cr(VI) reduction ranged from 0.01 to 4.21 day−1. In addition, the k value significantly increased with organic carbon (OC) and cation exchange capacity, but significantly decreased with increasing pH and dithionite-citrate-bicarbonate extractable Mn (Mnd). However, a predictive model using stepwise regression analysis indicated that the k value of the kinetics was controlled by OC, dissolved organic carbon (DOC), and Mnd, thereby identifying the complex interactions between Cr(VI) reduction and soil factors in the humid tropics.

Highlights

Chromium, which is one of the most toxic heavy metals in the environment, occurs at levels of 10 to 100 mg kg−1 in natural soils depending on different parent materials, but the concentration of Cr in soils can reach up to 4000 mg kg−1 due to human activities [1,2].The wide applications of Cr in numerous industrial activities such as electroplating, leather tanning, Cr-ore residue processing, and the weathering of Cr-rich rocks are potential sources of Cr in soils [3,4,5]
Based on the variation of exchangeable Cr(VI) in the soils during the 49-day incubation, the kinetics of Cr(VI) reduction were successfully described by first-order reaction at a significant level (p < 0.05)
The reduction rate constant k calculated by the kinetic equation ranged from 0.01 to 4.21 d− 1 in the studied soils

Summary

Introduction

Chromium, which is one of the most toxic heavy metals in the environment, occurs at levels of 10 to 100 mg kg−1 in natural soils depending on different parent materials, but the concentration of Cr in soils can reach up to 4000 mg kg−1 due to human activities [1,2].The wide applications of Cr in numerous industrial activities such as electroplating, leather tanning, Cr-ore residue processing, and the weathering of Cr-rich rocks are potential sources of Cr in soils [3,4,5]. Chromium, which is one of the most toxic heavy metals in the environment, occurs at levels of 10 to 100 mg kg−1 in natural soils depending on different parent materials, but the concentration of Cr in soils can reach up to 4000 mg kg−1 due to human activities [1,2]. Cr exists in different oxidation states, ranging from +2 to +6 naturally, while the most stable forms are Cr(III) and Cr(VI) [6]. Cr(VI), which is known to be carcinogenic, has much higher solubility and toxicity than Cr(III) in the environment [1]. The Cr released into soils can be oxidized to Cr(VI), which is leached out, causing the contamination of ground and surface water [7]. To remove Cr(VI) in the environment, the major chemical strategies include reduction, adsorption, precipitation, and ion exchange [8]

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