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 (H3PO4)-modified hydrochar (BPH200) was prepared at a low temperature (200 °C) in an air atmosphere with fresh banana peels as the raw material. The Cd2+ and Pb2+ adsorption behaviors and mechanisms of BPH200 were explored. As the temperature rose, co-hydrothermal carbonization of the banana peels and H3PO4 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 Cd2+ and Pb2+ 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 Cd2+ and Pb2+, respectively. H3PO4 promoted Cd2+ and Pb2+ 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 Cd2+ and Pb2+ 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 Cd2+ and Pb2+ onto BPH200 was dominated by precipitation with minerals and complexation with oxygen-containing functional groups. The removal rates of Cd2+ and Pb2+ 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 Cd2+ and Pb2+ in the soil, resulting in relatively high immobilization rates of Cd2+ (67.13%) and Pb2+ (70.07%).