Highly Efficient Removal of Lead/Cadmium by Phosphoric Acid-Modified Hydrochar Prepared from Fresh Banana Peels: Adsorption Mechanisms and Environmental Application

In this work, a phosphoric acid (H 3 PO 4 )-modified hydrochar (BPH200) was prepared at a low temperature (200 °C) in an air atmosphere with fresh banana peels as the raw material. The Cd 2+ and Pb 2+ adsorption behaviors and mechanisms of BPH200 were explored. As the temperature rose, co-hydrothermal carbonization of the banana peels and H 3 PO 4 enhanced the transformation of phosphorus (P) species. More orthophosphate and metaphosphate were found in BPH200 than in banana peel hydrochar (BP) without modification. The adsorption kinetics for Cd 2+ and Pb 2+ followed the pseudo-second-order model. The Redlich-Peterson model best fit the experimental results of the adsorption isotherm, with maximum adsorption capacities of 84.25 and 237.90 mg·g –1 for Cd 2+ and Pb 2+, respectively. H 3 PO 4 promoted Cd 2+ and Pb 2+ adsorption by forming precipitates, which, respectively, accounted for 32.75 and 41.12% of the total adsorption onto BPH200. In addition, the cation-exchange capacities of BPH200 with Cd 2+ and Pb 2+ were weakened compared with those of BP. However, complexation with these two ions strengthened, accounting for 26.68 and 32.81%, respectively, of the total adsorption capacity. This indicated that the adsorption of Cd 2+ and Pb 2+ onto BPH200 was dominated by precipitation with minerals and complexation with oxygen-containing functional groups. The removal rates of Cd 2+ and Pb 2+ by BPH200 from different water bodies were more than 99.95 and 99.97%, respectively. The addition of BPH200 also decreased the amounts of bioavailable Cd 2+ and Pb 2+ in the soil, resulting in relatively high immobilization rates of Cd 2+ (67.13%) and Pb 2+ (70.07%).