Promotion of biochar on adsorption of cadmium and retardation on water transport in paddy soil

Abstract: The effects of biochar on the pH of soil, adsorption of cadmium, and water transport in paddy soil were studied in this paper. The topsoil (0-20 cm) and subsoil (>60-80 cm) from a paddy field in Changzhou Rice Research Institute, Jiangsu Province were tested. Biochar treatments were fine biochar (particle size range: 0.048-0.56 mm) and coarse biochar (particle size range: 0.3-0.7 mm). The soil pH was measured at a 1:1 soil/water ratio. The pH of the soil increased with an increasing amount of the applied biochar. The pH of topsoil and subsoil increased by 1.5 and 0.84, respectively, for the 9% fine biochar treatments. But the increment of soil pH decreased with an increasing amount of applied biochar, and fine biochar was more effective to increase the soil pH. The Cd adsorption isotherms of soil with different biochar treatments were tested at a 1:10 soil/solution ratio. Cadmium solutions (1, 2, 5, 10, 20 and 50 mg/L) were prepared by dissolving Cd(NO3)2·4H2O, in the background electrolyte of 0.005 M CaCl2. All solutions were adjusted to pH 6.0 with dilute HNO3. Biochar treatments of topsoil were 0, 3, 6, 9% for fine biochar, and 0, 3% for coarse biochar; biochar treatments of subsoil were 0, 3% of both for fine and coarse biochar. The suspensions were shaken for 48 hours, centrifuged. Then the Cd2+ concentration of the supernatant was determined, which were used to calculate the sorption distribution coefficient (K). Models of Freundlich, Henry, Langmuir and Temkin were used to fit the adsorption isotherms. The Freundlich and Langmuir models provided good fitting results to the isothermal adsorption process of Cd2+ for topsoil and subsoil with different biochar treatments. Also the Freundlich isotherm model was better since the correlation coefficients (R2) were all greater than 0.99. Biochar, especially fine biochar ,can effectively increase the adsorption of cadmium in soil. For the topsoil with fine biochar treatments at 0, 3, 6 and 9%, the sorption distribution coefficients (K) of the Freundlich model were 126.63, 261.66, 357.50 and 431.19 μg/g, respectively. Moreover, the decreased dimensionless exponents (N) of the Freundlich model showed an enhancive nonlinearity of adsorption. Column experiments were used to evaluate the effects of biochar on the ability of water transport in soil. The soil was packed with a bulk density of 1.30g/cm3. After being saturated, the packed columns (6.2-cm length, and 2.6-cm i.d.) were leached from top to bottom using 0.005 M CaCl2 to maintain steady flow. Then, a 100 mg/L Cd2+ solution was applied at a flow rate of 0.25 mL/min which corresponded to a pore-water velocity of about 0.1 cm/min. The volume of effluent samples was recorded by weight. Water transport was hindered if the outflow volume showed a sudden drop. The results showed that the ability of water transport decreased with the increasing amount of biochar addition. Compared with the coarse biochar, fine biochar was more inclined to hinder the water transport in soil. When fine biochar additive amount was 1%, water transport in soil column was hindered obviously. However, there were no notable effects on water transport if 3% of coarse biochar was applied. Clearly, biochar had the potential to increase the capacity to adsorb cadmium, and decrease leaching of cadmium in paddy soil. However, the impact of biochar amendments on soil was a consequence of complex chemical, physical, and biological processes. All of the factors including the quality of biochar, soils, and cropping systems should be taken into consideration when applying specific types of biochar in specific regions and for specific cropping systems.