Treatment of Cadmium-Contaminated Water Systems Using Modified Phosphate Rock Powder: Contaminant Uptake, Adsorption Ability, and Mechanisms

By inergency On Mar 17, 2024

The morphology of heavy metal ions and the charge on the surface of heavy metal adsorbents are influenced by the pH value [57]. As Cd²⁺ more readily precipitates in a solution with a higher pH value, the pH value range of the Cd²⁺ solution in this adsorption test was set to 2–8. The adsorption of Cd²⁺ by PRP, FMPRP, CMPRP, and MMPRP increased with the increase in pH from 2, and this could have been due to the large amount of H⁺ in a low-pH environment, which protonates the surface functional groups of the MPRPs, thereby increasing the positive charge on the surface of the MPRPs and producing electrostatic repulsion with positively charged Cd²⁺, thus affecting the adsorption of Cd²⁺ by the MPRPs. Therefore, when the pH increases, H⁺ gradually decreases, and the Cd²⁺ electrostatic attraction between the MPRPs and Cd²⁺ in the solution is enhanced (see Supplementary Materials). However, when the pH value of the Cd²⁺ solution exceeded 6, the Cd²⁺ adsorption capacity of PRP, FMPRP, CMPRP, and MMPRP began to decrease, which may be related to the gradual formation of OH⁻ and Cd²⁺ into Cd(OH)₂, Cd(OH)₃⁻, and Cd(OH)₄⁻, which reduced the electrostatic attraction between the MPRPs and Cd²⁺, thereby reducing the Cd²⁺ adsorption capacity of the MPRPs [58]. The Cd²⁺ adsorption capacity of SMPRP gradually increased with the increases in initial solution pH value. This is because when the pH increases from 7 to 8, the solution contains more OH⁻, and the affinity of Cd²⁺ with the SH group is higher than that with the OH group. Therefore, SMPRP reduced the effect of pH on its ability to adsorb Cd²⁺. The Cd²⁺ adsorption capacity of SMPRP increased rapidly when the solution pH increased from 6 to 8. The results showed that, compared to the PRP and other MPRPs, SMPRP is more suitable for Cd removal in environments with a pH greater than 8. The adsorption capacities of CMPRP for Cd²⁺ were 3.32%, 6.99%, 5.09%, and 11.42% higher than those of PRP at pH 2, 4, 6, and 8, respectively. The adsorption of Cd²⁺ by FMPRP and MMPRP was 4.98% and 1.33% higher than that of PRP at pH 8, respectively, indicating that electrostatic interactions may be the key mechanism involved in Cd²⁺ adsorption by CMPRP. The Cd²⁺ adsorption capacity of the MPRPs was more easily affected than that of PRP at pH 2–6, possibly because Cd²⁺ has a smaller ionic radius. With the solution at pH 8, the Cd²⁺ adsorption capacities of PRP, FMPRP, CMPRP, and MMPRP were decreased, but the Cd²⁺ adsorption capacity of PRP was the lowest, indicating that pH had little effect on the adsorption capacity of the MPRPs, and the modification was effective. The Cd²⁺ adsorption capacities of PRP, FMPRP, CMPRP, and MMPRP were the largest at an initial pH of 6, and the Cd²⁺ removal rates were 99.34%, 97.17%, 97.97%, and 97.00%, respectively. SMPRP exhibited the highest adsorption capacity at an initial pH of 8 with 51.96% Cd²⁺ removal. There was no significant difference between the Cd²⁺ adsorption of FMPRP and MMPRP at pH 6 and 8, respectively; however, the difference between the adsorption values at pH 2 and 4 was significant, indicating that the optimal pH range for FMPRP and MMPRP in this environment is pH 6–8. The adsorption capacity of CMPRP was significantly different at pH 2, 4, 6, and 8, and its adsorption amount was the highest at pH 6, where the rate of CMPRP’s removal of Cd²⁺ was 97.97%, indicating that the applicable pH range for CMPRP in this environment is approximately pH = 6.