Qualitative and quantitative characterization of adsorption mechanisms for Cd2+ by silicon-rich biochar.

The adsorption characteristics of rice-husk biochar (RHB) rich in silicon (Si) for Cd2+ in solution and soil were investigated. Three biochars were produced at different pyrolytic temperatures of 300 °C(RHB300), 500 °C(RHB500) and 700 °C (RHB700). The pH effect, adsorption kinetics and isotherms were examined, and chemical analyses of Cd2+-loaded biochars were conducted by SEM-EDS, FTIR, XRD and Boehm titration. Biochars produced at higher temperature have much larger pH and surface area, resulting in greater adsorption capacities and faster adsorption kinetics. Maximum adsorption capacities calculated from Langmuir isotherm were 62.75, 77.37 and 93.50 mg/g for RHB300, RHB500 and RHB700, respectively. Cd2+ adsorption was primarily attributed to cation exchange and precipitation, which jointly contributed 59.55% (RHB300) to 76.05% (RHB700) of the total adsorption, but the mechanisms of complexation and coordination were of minor importance in total adsorption. The relationship of each mechanism with biochar's properties was further discussed. Si-containing minerals within biochar made a much larger contribution to precipitation than total adsorption, as the respective contribution proportion were 33.92% and 8.33% on average. When added to highly Cd-polluted soil, the biochars could effectively reduce the availability of Cd2+ after incubation for 35 days, and ameliorate soil acidification through the speediness of Si released into soil solutions. These demonstrate that rice husk-derived biochar, produced at high temperatures, can be suitable applied to mitigate Cd-contamination of soil and water, and the presented analyses shed light on the mechanisms underlying the adsorption by this Si-rich biochar.

Relative distribution of Cd2+ adsorption mechanisms on biochars derived from rice straw and sewage sludge.

Qualitative and quantitative characterization of Cd2+ adsorption mechanisms was performed with rice-straw and sewage-sludge biochars produced at different temperature (300-700 °C), respectively. The pH effect, adsorption kinetics and isotherms were investigated, and chemical analyses of Cd2+-loaded biochars were conducted by SEM-EDS, XRD, FTIR and Boehm titration. This demonstrated that rice-straw biochars (RSBs) have greater adsorption capacities for Cd2+ than sewage-sludge biochars (SSBs), which was mainly due to precipitation and cation exchange mechanisms, with their contribution proportion to total adsorption from 76.1% to 80.8%. While in SSBs, both mechanisms were overshadowed by coordination with π electrons mechanism accounting for 59.2%-62.9% of total adsorption, even the role of cation exchange was negligible in the adsorption mechanisms accounting for 2.3%-6.7%. The relationship of each mechanism with biochar's properties were discussed, which further deepen our understanding of adsorption on biochars. These results suggest RSBs have great potential for removing Cd2+ from aqueous solutions.

Quantitative contribution of Cd2+ adsorption mechanisms by chicken-manure-derived biochars.

This study investigated the efficiency and mechanisms of Cd2+ removal by chicken-manure biochar produced at different temperatures. Adsorption kinetics, isotherms, thermodynamic, and desorption were examined, and the biochars before and after adsorption were analyzed by SEM-EDS, FTIR, Boehm titration, and XRD. Kinetics of adsorption were better described by pseudo-second-order kinetic model than pseudo-first-order kinetic and intraparticle diffusion model under different initial Cd2+ concentrations of 20, 50, and 100 mg L-1. Equilibrium adsorption was better modeled by Freundlich and Temkin isotherm equations than Langmuir equation at different temperatures of 25, 35, and 45 °C. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption of Cd2+ at all of temperatures. Moreover, functional group complexation, precipitation, and cation exchange jointly contributed to Cd2+ adsorption on the biochars, whose relationship with the properties of biochar were also analyzed. The new precipitate as Cd5(PO4)3OH was found during the adsorption. Complexation and precipitation were predominant mechanisms for all biochars (together accounting for 92.4-98.8%), while cation exchange made a relatively minor contribution to total Cd2+ removal (accounting for 1.2-7.6%). The relative distribution of each mechanism on the biochars was determined, which deepen our understanding of the Cd2+ adsorption process. These results are useful for future practical applications of biochar to removal heavy metals from water.