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In this work, a new strategy by in situ combining slag-based geopolymer microsphere (SGS) with dithiocarbamate (DTC) was employed to synthesize in situ the composite adsorbent of the SGS and DTC (SGS-DTC). The differences of material properties, performance, and mechanisms in adsorption and immobilization of toxic Cd(II) in water between the SGS and SGS-DTC were investigated. For the static adsorption, the adsorption rate of SGS-DTC(1.5h) was faster than that of SGS(6h); the Cd(II) adsorption capacity of the SGS-DTC (211.2 mg/g) was almost twice that of the SGS (116.7 mg/g), corresponding to the removal rate of the SGS-DTC (99.75%) was nearly twice that of the SGS (53.2%). For the dynamic adsorption, the adsorption capacity of the SGS-DTC is higher than that of the SGS in the Cd(II) solution prepared by the deionized water or the river water from Yongjiang, Nanning, Guangxi, China. The mechanism of the difference of the Cd(II) adsorption between SGS and SGS-DTC was analyzed in dept. [Display omitted] In this study, slag-based geopolymer microspheres (SGS) were combined with dithiocarbamate (DTC) to synthesize the composite adsorbent of SGS and DTC in situ (SGS-DTC). Synthesis was carried out with optimal dosages of 10 mL of EDA, 1.0 g of SGS, and 20 mL of CS 2. The differences in material properties, performance, and mechanisms in the adsorption and immobilization of toxic Cd(II) in water between SGS and SGS-DTC were investigated. SGS-DTC showed better adsorption performance than SGS, irrespective of adsorbent dosage, pH, original content, and contact duration. Although after the Cd(II) adsorption, the immobilization performance at a different pH was better in the SGS-DTC than in the SGS, the immobilization performance was unaffected by changes in the other factors. For static adsorption, the adsorption rate of SGS-DTC (1.5 h) was faster than that of SGS (6 h); the Cd(II) adsorption capacity of SGS-DTC (211.2 mg/g) was almost twice that of SGS (116.7 mg/g), and correspondingly, the removal rate of SGS-DTC (99.75%) was nearly twice that of SGS (53.2%). For dynamic adsorption, the adsorption capacity of SGS-DTC was 389.78 mg/g, which is considerably higher than that of SGS (293.38 mg/g) in the Cd(II) solution prepared with deionized water. Furthermore, the adsorption capacity of the SGS-DTC was 299.26 mg/g, which is significantly higher than that of SGS (150.03 mg/g) in the Cd(II) solution prepared by the river water from Yongjiang, Nanning, Guangxi, China. One reason is that DTC was able to activate Si-O-Si without adsorption performance within SGS, thereby improving its adsorption and purification properties significantly. The other reason is that, after anchoring DTC on SGS, the specific surface area varied from 34.05–146.47 m2/g, the morphology was smooth-leaf-like, the pore volume was 0.13–0.20 cm3/g, and the pore size in SGS, was 14.75–5.60 nm. The high potential of SGS-DTC in removing and immobilizing heavy metal materials in wastewater is demonstrated in the results. [ABSTRACT FROM AUTHOR]