The development of low-cost and efficient new mineral adsorbents has been a hot topic in recent years. In this study, Friedel＇s salt （FS：3CaO·A12O3 ·CaCl2 ·10H2O）, a hexagonal layered inorganic absorbent, was synthesized to remove Cd2＋ from water. The adsorption process was simulated by Langmuir and Freundlich models. The adsorption mechanism was further analyzed with TEM, XRD, FT-IR analysis and monitoring of metal cations released and solution pH variation. The results indicated the adsorbent FS had an outstanding ability for Cd（Ⅱ） adsorption. The maximum adsorption capacity of the FS for Cd（Ⅱ） removal can reach up to 671.14 mg/g. The nearly equal numbers of Cd2＋ adsorbed and Ca2＋ released demonstrated that ion-exchange （both surface and inner） of the FS for Cd（Ⅱ） played an important role during the adsorption process. Furthermore, the surface of the FS after adsorption was microscopically disintegrated while the inner lamellar structure was almost unchanged. The behavior of Cd（Ⅱ） adsorption by FS was significantly affected by surface reactions. The mechanisms of Cd2＋ adsorption by the FS mainly included surface complexation and surface precipitation. In the present study, the adsorption process was fitted better by the Langmuir isotherm model （R2 = 0.9999） than the Freundlich isotherm model （R2 = 0.8122）. Finally, due to the high capacity for ion-exchange on the FS surface, FS is a promising layered inorganic adsorbent for the removal of Cd（Ⅱ） from water.
Juanjuan ZhangHe ZhaoHongbin CaoHeping LiZhibao Li
Whether a cationic organic polymer can remove more total cyanide （TCN） than a non-ionic organic polymer during the same flocculation system has not been reported previously. In this study, the effects of organic polymers with different charge density on the removal mechanisms of TCN in coking wastewater are investigated by polyferric sulfate （PFS） with a cationic organic polymer （PFS-C） or a non-ionic polymer （PFS-N）. The coagulation experiments results show that residual concentrations of TCN （Fe（CN）6^3-） after PFS-C flocculation （TCN 〈 0.2 mg/L） are much lower than that after PFS-N precipitation. This can be attributed to the different TCN removal mechanisms of the individual organic polymers. To investigate the roles of organic polymers, physical and structural characteristics of the floes are analyzed by FT-IR, XPS, TEM and XRD. Owing to the presence of N＋ in PFS-C, Fe（CN）3- and negative flocs （Fe（CN）63- adsorbed on ferric hydroxides） can be removed via charge neutralization and electrostatic patch flocculation by the cationic organic polymer. However, non-ionic N in PFS-N barely reacts with cyanides through sweeping or bridging, which indicates that the non-ionic polymer has little influence on TCN removal.
In this paper, composite coagulants （PFS, PFSC05, PFSC1 and PFSC5）, prepared by mixing polyferric sulfate （PFS） and cationic polyelectrolyte （CP） coagulants with different weight percent （Wv） of CP （Wp = 0%, 0.5%, 1% and 5%, respectively）, were adopted to treat cyanide-containing wastewater. PFSC5 exhibited superior coagulation performances at optimal conditions： the removal of total cyanide （TCN） and chemical oxygen demand （COD） was 95%-97% and 50%-55%, respectively. The effects of CP on the properties and structure of flocs were investigated by laser diffraction instrument and small-angle laser light scattering （SALLS）, respectively. The results show that the flocs of PFSC5 have higher growth rate, higher strength factor and lower recovery factor than other flocs. They are also much denser and more uniform owing to the higher fractal dimension （DO and less microflocs （10-100μm）. Furthermore, the dense structure of the PFSC5 flocs can be restored after shear and is more resistant to hydraulic conditions. Particularly, detailed morphology evolution of the flocs was in-situ detected by on-line particle imaging. Due to strong ionic strength in wastewater, the CP in PFSC5 plays a significant role of adsorption, while the main mechanism of CP is electrostatic patch aggregation during the PFSC05 systems.
