A Soluble Humic Substance for the Simultaneous Removal of Cadmium and Arsenic from Contaminated Soils

Abstract

With abundant oxygen-containing functional groups, a humic substance (HS) has a high potential to remediate soils contaminated by heavy metals. Here, HS was first extracted from a leonardite and analyzed for its chemical compositions and spectroscopic characteristics. Then it was assessed for its ability as a washing agent to remove Cd and As from three types of soils (red soil, black soil, and fluvo-aquic soil) that were spiked with those contaminants (Cd: 40.5–49.1 mg/kg; As: 451–584 mg/kg). The operational washing conditions, including the pH and concentration of the HS, washing time and cycles, and liquid–soil ratio, were assessed for Cd and As removal efficiency. At pH 7, with an HS concentration (3672 mg C/L) higher than its critical micelle concentration and a liquid–soil ratio of 30, a single washing for 6–12 h removed 41.9 mg Cd/kg and 199.3 mg As/kg from red soil, 33.5 mg Cd/kg and 291.5 mg As/kg from black soil, and 30.4 mg Cd/kg and 325.5 mg As/kg from fluvo-aquic soil. The removal of Cd and As from the contaminated soils involved the complexation of Cd and As with the carboxyl and phenolic groups of HS. Outcomes from this research could be used to develop a tailor-made HS washing agent for the remediation of Cd- and As-contaminated soils with different properties.

Soil Quality and Heavy Metal Pollution Assessment of Iron Ore Mines in Nizna Slana (Slovakia)

“…Very Soil organic matter (SOM) is one of the main factors controlling the physical, chemical, and biological properties of soil and plays an important role in soil hygiene (immobilization of heavy metals and organic pollutants) [46]. Recent research has unequivocally confirmed the key role of organic carbon in the sorption of inorganic and organic contaminants [47], and the presence of organic substances can inhibit metal absorption from soil solution and reduce metal mobility and bioavailability [43,48,49]. In the study area, SOM values ranged from 1.4 to 9.8%, indicating that the soil humus supply is moderate to very good.…”

Humic substances and significance of their application – a review

“…The pH value of HSs was close to neutral, positively influencing the removal rate and limiting the possible destructive effect on the soil. The high removal efficiency of Cd and As from soils and the low content of other metals (Cd: 0.09, Pb: 5.32, Cu: 8.72, Cr: 6.60, As: 3.69, Ni: 8.39, Zn: 20.36 mg/kg) confirm the applicability of using HSs for the remediation of contaminated soil (Bi et. al., 2019).…”

“…It is important to adjust the washing conditions to the soil properties. Important factors are the pH value, the duration and cycles of washing and the HSs concentration (Bi, Yuan, Wei, Xiao, Feng, Meng and Wang, 2019). The content of Cd and As in soil can be a serious problem in specific places, as is the case in China.…”

“…Cd and As are toxic and carcinogenic elements -their presence in soil poses a threat to the quality of the environment and human health and has a negative effect on food. Therefore, it is so important to minimise the risk by removing these elements from the soil (Bi et. al., 2019).…”

“…Toxic elements were removed as a result of the complexation of Cd and As with the carboxyl and phenolic groups of HSs. The use of HSs as a washing agent is justified due to their efficiency, cheap availability (coal, peat, sludge) and their positive influence on the physical, chemical and biological properties of the soil (Bi et. al., 2019).…”