Soil science
B. Abolfazli Behrooz; S. Oustan; H. Mirseyed Hosseini; H. Etesami
Abstract
IntroductionCadmium (Cd) contamination has been a widespread concern in paddy soils because of its subsequent transfer to the food chain. Biochar amendment is proposed to stabilize Cd in the contaminated soils. However, the pristine biochar shows limited functionality towards Cd sorption in practice. ...
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IntroductionCadmium (Cd) contamination has been a widespread concern in paddy soils because of its subsequent transfer to the food chain. Biochar amendment is proposed to stabilize Cd in the contaminated soils. However, the pristine biochar shows limited functionality towards Cd sorption in practice. Recently, Mg-modified biochars have attracted much attention for their low toxicity. These biochars are coated by MgO or Mg(OH)2 precipitates during the pyrolysis process. Magnesium chloride (MgCl2)-modified biochars have been used widely in the removal of heavy metals from the aqueous solutions. However, there is little literature about their performance in soils. The present study therefore was conducted to investigate the effects of application of unmodified and MgCl2-modified rice husk biochars on the kinetics and isotherms of Cd sorption in a calyey paddy soil. Materials and MethodsThe unmodified and MgCl2-modified biochars were produced from rice husk at 600°C. Some relevant characteristics of the produced biochars (including elemental composition, pzc, pH1:10, ash content and BET surface area) were determined. Moreover, the studied soil was taken from a paddy field (0-20 cm) in the Qaemshahr region of Mazandaran province. The biochars (< 0.5 mm) were added to the soil samples at three levels (0, 3, and 5% w/w) and the amended soils were incubated at 25°C for 45 days. Then, the kinetic experiments of Cd sorption at a concentration of 375 mg Cd/L at times of 0.25, 0.5, 1, 2, 4, 8, 16, 24 and 48 hours and the isothermal experiments of Cd sorption at concentrations of 50, 100, 150, 200, 300, 350, 400, 600 and 800 mg Cd/L were performed. In both kinetic and isotherm experiments, a 0.01 M KCl solution was used as the background electrolyte. Finally, the relevant kinetic and isotherm models were fitted to the sorption data and their parameters were calculated. Results and DiscussionBiochar characterization indicated that modification with MgCl2 resulted in an increase of the O/C ratio (from 0.27 to 0.48) and pH (from 7.67 to 8.60). This modification also increased the H/C ratio (from 0.032 to 0.071) and the specific surface area (from 195.6 to 231.2 m2/g). As a result, the MgCl2-modified biochar was more hydrophilic and less carbonized than the unmodified one. Moreover, the characteristic peaks of the MgCl2-modified biochar (3700, 1428 and 500 cm-1) were present in its FTIR spectrum. The results revealed that about 74 to 89% of the Cd sorption by the soils occurred in times less than 2 hours. With MgCl2-modification, the sorption equilibration time was reduced from 48 hours to 24 hours. In contrast, the unmodified biochar had no considerable effect on the Cd sorption kinetics. Among the kinetic models, the Elovich model with lower SEE was the best to fit the Cd sorption kinetic data. The intra-particle diffusion model was not satisfactory for Cd sorption on the biochars. Freundlich model with lower SEE well described the Cd sorption isotherms. Application of 3% and 5% MgCl2-modified biochar increased the Freundlich KF parameter by 2.4 and 2.8 times as compared to the control. Moreover, the aforementioned treatments increased the heterogeneity parameter of the Freundlich model (n) from 3.48 to 6.08. The Temkin model could not reasonable fit the sorption data. In contrast, the unmodified biochar did not show any considerable effect on the Cd sorption capacity of the clayey soil used in this research. This finding means that the unmodified biochar could not improve the sorption performance of negatively charged soil clay particles. ConclusionAccording to the results obtained, it could be concluded that the Cd sorption behavior of the soil treated with unmodified rice husk biochar was similar to that of the untreated soil. Whereas, the MgCl2-modification improved both sorption rate and sorption capacity of the soil for Cd. Application of MgCl2-modified biochar improved the Cd sorption properties of a clayey soil with high intrinsic sorption ability. Thus, this may be a promising approach in remediation of Cd-contaminated paddy soils with the aim of reducing Cd mobility and availability. However, there is need to do more research to create awareness about the importance of biomass nature as well as pyrolysis temperature, the ratio of MgCl2 to biomass, the mechanism of Cd stabilization and the desorption of Cd from soils treated with MgCl2-modified biochars.
