| Cadmium (Cd(Ⅱ)) has joined in the most toxic category of heavy metals with the greatest potential hazard to humans and the environment. Dissolved organic matter, such as humic acid (HA) and pharmaceutical and personal care products (PPCPs) are able to form compound pollutants with Cd (Ⅱ), affecting the removal of Cd (Ⅱ). Manganese oxide, a main component in the nature of minerals, has been widely used in the adsorption process of heavy metals. In present study, a nano mesoporous manganese dioxide (HMO) was synthesized by in-situ reduction of KMnO4 by H2O2 at room temperature and was used as an adsorbent to remove organic matters and Cd(Ⅱ) from aqueous solution. In the meantime, the distribution relationship between Cd(Ⅱ) and HA on HMO was also studied. The adsorption behavior and mechanism of diclofenac (DCF), cadmium (Cd(Ⅱ)) and the composite pollutant (Cd(Ⅱ)-DCF) onto HMO are systematically investigated in this study.Synergistic and antagonism effects:The adsorption processof Cd(Ⅱ), DCF and Cd(Ⅱ)-DCF onto HMO showed that the experimental equilibrium adsorption data for the three pollutants were well described by the Langmuir model and the maximum adsorption capacity for DCF increased from 8.26 to 10.83 mg/g in the absence and the presence of Cd(Ⅱ). The maximum adsorption capacity of Cd(Ⅱ) decreased from 174.74 to 154.38 mg/g in the absence and the presence of DCF. The uptake rate of DCF was obviously affected by the ionic strength, but it had tiny effect on uptake of Cd(Ⅱ) and Cd(Ⅱ)-DCF onto HMO.The anaylsisof the kinetics showed that the pseudo second-order equation could best fit for the Cd(Ⅱ) and Cd(Ⅱ)-DCF adsorption process, and the values of activation energy (Ea) showed that chemisorption mechanism dominated the process, while the physisorption mechanism was ascribed to the uptake of DCF, and the pseudo first-order equation could best fit for the DCF adsorption process. The thermodynamic parameters revealed that the process of the uptake of Cd(Ⅱ) by HMO was spontaneous and endothermic. The intraparticle diffusion process was the rate controlling step for the uptake of Cd(Ⅱ) by HMO.The solution pH had no obvious effect on values of x, but had substancial effect on values of K (logK=1.652-3.548, x=1.651-1.752). The ionic strength (IS) of solution had no directly impact on the values of K, while with the ionic strength enhancing, the values of x increased. The temperature increased with the increasing of the removal efficiency of Cd(Ⅱ), while the removal of HA decreased. The values of the distribution coefficient (Kd) of the HA or Cd(Ⅱ) on the surface of the HMO were significantly higher than that of HA-Cd (Ⅱ) complex, which means that the complexation of the HA and Cd (Ⅱ) inhibits the removal efficiency of HMO on HA and Cd(Ⅱ).The uptake of Cd(Ⅱ) was correlated with the increasing of the surface oxygen of HMO. The increased oxygen was banded together with Cd(Ⅱ) and the main speciation of Cd uptaken by HMO was CdOH+. The adsorption of DCF was due to the interaction between carboxyl groups on DCF and hydroxyl group on HMO.The synergistic and antagonistic effects corresponding to the cationic bridging of Cd(Ⅱ) between carboxyl groups on DCF and HMO and the competition of Cd(Ⅱ) and Cd(Ⅱ)-DCF for the adsorption sites were confirmed. |
PDF Full Text Request