Effects of nano-Fe_3O_4 loaded acid-modified biochar on Pb~(2+) and Cd~(2+) adsorption in water

LI Jing;SHAO Xiao-hou;LIN Kai;MAO Xin-yu;QIN Heng-ji;College of Agricultural Engineering, Hohai University;Nantong Marine and Offshore Engineering Institute,Hohai University;

nano-Fe_3O_4;;HNO_3;;modified biochar;;adsorption;;Pb;;Cd

To better understand the adsorption characteristics of Pb~(2+)and Cd~(2+)in their single and binary solutions by nano-Fe_3O_4 loaded acid-modified biochar, influencing factors such as adsorbent surface properties, additive dosage, initial solution pH, adsorption time, and initial metal concentration were studied via static adsorption. Moreover, models of isothermal adsorption and dynamic adsorption were applied to analyze the relevant adsorption kinetics and mechanisms. The obtained results indicated that the specific surface area of nano-Fe_3O_4 loaded acid modified biochar was higher than that of unmodified coconut shell biochar by 221.03 m~2·g~(-1). In addition, the amount of surface functional groups such as O-H, C=O, C-O-C as well as the aromaticity were higher, while the isoelectric point of the biochar increased to5.68. The applied biochar dosage of 5 g·L~(-1) was considered to be optimal in consideration of cost-effectiveness. The adsorption efficiency was preferable when ionic solution pH was 5.0 and adsorption equilibrium was achieved at around 4 h. The adsorption process was more aligned with the pseudo-second-order kinetic model, which suggested that chemisorption consisted of rapid out-diffusion and intra-particle diffusion were mainly accounted for the adsorption of Pb~(2+)and Cd~(2+). Furthermore, Pb~(2+)and Cd~(2+)adsorption were found to be well fitted to the Langmuir and Freundlich isothermal adsorption models, respectively. The maximum adsorption capacities(Qm)of Pb~(2+)and Cd~(2+)in single solution by nano-Fe_3O_4-loaded acid modified biochar were 42.54 mg·g~(-1) and 25.79 mg·g~(-1), respectively, which were 1.87 and 2.23 times higher than those by unmodified biochar. The maximum adsorption capacities of Pb~(2+)and Cd~(2+)in composite solution were 65.16% and 54.21% of those in single solution, revealing that the phenomenon of adsorption competitiveness took place in the presence of both ions. The results indicated that a nano-Fe_3O_4 loaded modification combined with acid modification enhanced the Pb~(2+)and Cd~(2+)adsorption capacities of coconut biochar. Moreover, the adsorption capacity and competitiveness for Pb~(2+)were higher than for Cd~(2+).