Immobilization of Cu(II), Pb(II) and Cd(II) by the addition of rice straw derived biochar to a simulated polluted Ultisol

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Abstract

To develop new remediation methods for acidic soils polluted by heavy metals, the chemical fractions of Cu(II), Pb(II) and Cd(II) in an Ultisol with and without rice straw biochar were compared and the effect of biochar incorporation on the mobility and bioavailability of these metals was investigated. In light of the decreasing zeta potential and increasing CEC, the incorporation of biochar made the negative soil surface charge more negative. Additionally, the soil pH increased markedly after the addition of biochar. These changes in soil properties were advantageous for heavy metal immobilization in the bulk soil. The acid soluble Cu(II) and Pb(II) decreased by 19.7–100.0% and 18.8–77.0%, respectively, as the amount of biochar added increased. The descending range of acid soluble Cd(II) was 5.6–14.1%, which was much lower than that of Cu(II) and Pb(II). When 5.0 mmol/kg of these heavy metals was added, the reducible Pb(II) for treatments containing 3% and 5% biochar was 2.0 and 3.0 times higher than that of samples without biochar, while the reducible Cu(II) increased by 61.6% and 132.6% for the corresponding treatments, respectively. When 3% and 5% biochar was added, the oxidizable portion of Pb(II) increased by 1.18 and 1.94 times, respectively, while the oxidizable portion of Cu(II) increased by 8.13 and 7.16 times, respectively, primarily due to the high adsorption affinity of functional groups of biochar to Cu(II). The residual heavy metal contents were low and changed little with the incorporation of biochar.

Highlights

► The incorporation of biochar made the Ultisol surface charge more negative. ► The Ultisol pH increased markedly after addition of biochar. ► The acid soluble Cu(II) and Pb(II) decreased significantly after biochar amended. ► High adsorption affinity of functional groups on biochar to Cu(II) was found.

Introduction

Large areas of variable charge soils are distributed throughout the tropical and subtropical regions of southern China. These soils are rich in iron and aluminum oxides as a result of intensive weathering and evolution; accordingly, they usually have a low cation exchange capacity (CEC) and pH, which results in high mobility and bioavailability of heavy metals [1], [2], [3]. Therefore, these soils are more easily polluted by heavy metals than soils that are permanently negatively charged.

High concentrations of heavy metals in soils may cause long-term risks to ecosystems and humans. Heavy metals are persistent and difficult to remove or degrade once introduced into soils. Accordingly, many techniques have been developed to remediate soils polluted with heavy metals, including physical means [4], incorporation of amendments [5], electrokinetic remediation [6], [7], biological remediation [8], [9] and combined remediation technologies [10]. The importation method rapidly changes the properties of heavy metals or soils, thereby rapidly decreasing the mobility and bioavailability of metals. Accordingly, the effects of inorganic minerals and organic materials as modifiers of the mobility and bioavailability of heavy metals have been studied extensively. Zeolite [5], lime and red-mud [11] and chicken manure compost [12] have been reported to adsorb heavy metals, but have rarely been applied in practice because of their unsatisfactory effects or high cost.

Biochar has many favorable immobilization properties as a heavy metal modifier, such as a microporous structure, active functional groups, and high pH and CEC [13]. FTIR, XRD, SEM/EDS and TEM analyses have revealed that biochar has a strong adsorption affinity for heavy metals [14], [15], [16], [17]. Fourier transform infrared spectroscopy (FTIR) and equilibrium and kinetic adsorption data have shown that wheat-residue derived black carbon has high affinity toward heavy metals [18]. In addition, pyrolytic biochar has a great capacity and ability to adsorb Cu(II), Pb(II) and Cd(II) from aqueous solutions [14], [19], [20]. The adsorption of heavy metals such as Cu(II) by soils was also enhanced by the incorporation of biochar via cation exchange and complexation of Cu(II) by surface functional groups [17]. In weathered acidic soils, the ability to stabilize heavy metals, especially Cu(II) and Pb(II), has been shown to be directly correlated with the amount of oxygen functional groups on biochars [21].

Recently, the effects of biochar on the mobility and bioavailability of heavy metals have been widely reported. For example, investigations of DTPA-extractability, leachability and the extraction of pore water were conducted to clarify the impact of heavy metals on biochar amended soils (artificially polluted soil, 1:5 H2O, pH 6.38, Australia [15]; mine tailing soil, 1:2.5 H2O, pH 8.13, Italy [22], mildly acidic polluted soil, 1:2.5 H2O, pH 5.45, UK [23], polluted soil, 1:10 H2O, pH 6.2, UK [24], polluted soil, 1:10 H2O, pH 7.9, UK [25]). However, there are few studies of the chemical fractions of heavy metals in soils with low pH and variable charges that have been amended with biochar. Therefore, this study was conducted to evaluate changes in the surface properties of an Ultisol amended with biochar, and to investigate the subsequent transformation of Cu(II), Pb(II) and Cd(II) forms. The results presented herein will provide useful references for the development of new remediation methods for acidic soils polluted by heavy metals.

Section snippets

Soil and biochar

A subsoil sample of Ultisol derived from Quaternary red earth was collected from a pristine area of Liuzhou, Guangxi Province (24°19′N, 109°24′E) under natural trees. The soil sample was air-dried and ground to pass through a 60-mesh sieve. Selected properties of the investigated soil are given in Table 1. The environmental background values of Cu, Pb and Cd were negligible when compared with the added metals.

Rice straw (Oryza sativa L.) was collected from a cropland in a suburb of Nanjing,

Change of soil properties after biochar amelioration

A remarkable increase in soil CEC was observed after the soil was incubated with 3% and 5% biochar. Specifically, the value increased from 3.55 for the control to 4.90 and 6.12 cmol/kg for the 3% and 5% biochar amended treatments, respectively, which was consistent with the results of previous studies [32], [33]. The CEC is the soil negative charge at pH 7.0 and represents the adsorption capacity of soil for cations. The zeta potentials of soil particles at pH 5.0 were −23.9, −18.8 and −14.9 mV

Conclusions

Biochar was shown to effectively reduce concentrations of free Cu(II), Pb(II) and Cd(II) originating from contaminated soil systems. The active portion of those metals was retained after the soil was subjected to simulated contamination by these metals. The Ultisol CEC and zeta potential decreased, while the soil pH increased in response to the addition of biochar. Based on these changes in soil properties, biochar is useful for Cu(II) and Pb(II) immobilization in Ultisol. Additionally, the

Acknowledgments

This study was supported by the National Key Technology R&D Program of China (2012BAJ24B06).

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