The objectives of this study were to evaluate the cadmium adsorption capacity of iron–organic associations (Fe-OM) formed by laccase-mediated modification and assess the effect of Fe-OM on the immobilization of cadmium in paddy soil. Leaf organic matter (OM) was extracted from Changshan grapefruit leaves, and then dissolved organic matter (Lac-OM) and precipitated organic matter (Lac-P) were obtained by laccase catalytic modification. Different Fe-OM associations were obtained by co-precipitation of Fe with OM, Lac-OM, and Lac-P, respectively, and the adsorption kinetics, adsorption edge, and isothermal adsorption experiments of Cd on Fe-OM were carried out. Based on the in situ generation of Fe-OM, passivation experiments on Cd-contaminated soils with a high geological background were carried out. All types of Fe-OM have a better Cd adsorption capacity than ferrihydrite (FH). The theoretical maximum adsorption capacity of the OM-FH, Lac-OM-FH, and Lac-P-FH were 2.2, 2.53, and 2.98 times higher than that of FH, respectively. The adsorption of Cd on Fe-OM is mainly chemisorption, and the -OH moieties on the Fe-OM surface form an inner-sphere complex with the Cd ions. Lac-OM-FH showed a higher Cd adsorption capacity than OM-FH, which is related to the formation of more oxygen-containing groups in the organic matter modified by laccase. The immobilization effect of Lac-OM-FH on active Cd in soil was also higher than that of OM-FH. The Lac-OM-FH formed by laccase-mediated modification has better Cd adsorption performance, which can effectively inactivate the activity of Cd in paddy soil.

This study was carried out to evaluate the efficiency and mechanism of P removal by different types of Fe-OM associations in peatlands. Humic substance (HS) and particulate organic matter (POM) were isolated from peat soils, and different types of iron-organic matter associations (Fe-HS and Fe-POM) were prepared. In addition, the natural combination of Fe³⁺ with peat was simulated. Then, isothermal adsorption experiments were carried out on the synthesized Fe-OM and iron-contained peat soils. All the adsorption data can be well fitted by the Langmuir adsorption model. The theoretical maximum adsorption capacity (Q_max) of Fe-HS associations can reach 36.90 mg/g, which is approximately two times higher than that of ferrihydrite (19.23 mg/g) and ten times higher than that of hematite (3.26 mg/g) and goethite (2.08 mg/g). The adsorption capacities of peat soils and POM were significantly enhanced by combinations with exogenous Fe³⁺. The Q_max of the original peat and iron-contained peat were 2.83 mg/g and 7.36 mg/g, respectively, and those of the original POM and Fe-POM association were 4.31 mg/g and 5.89 mg/g, respectively. The contribution of Fe-OM associations to phosphorus removal in peatlands is much higher than that of inorganic iron oxide. In addition, exogenous Fe³⁺ could also participate in other phosphorus removal processes.