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Heavy metal pollution has become one of the most urgent environmental issues, which also poses a potential threat to the human health. This article is suggested to review the advance on the performance of chemical coagulation process in removing heavy metal from water. Chemical coagulation process is considered to be a valid method which is determined by the hydrolyzed species of the inorganic coagulants under different raw water and coagulation conditions. And the main mechanisms of the removal of heavy metals are adsorption, complexation, and coprecipitation. Compared with the aluminum-based coagulants, the iron-based coagulants have better performance due to the use of wide pH range and large surface area of the resulting flocs. During the chemical coagulation process, the valence state of arsenic and antimony could affect the removal efficiency. Thus, the oxidants and reductants are often combined with inorganic coagulants used in this process. It is found that pH is an important factor greatly influencing the performance directly or indirectly. The complex resulting from the interaction between the inorganic/organic pollutant and inorganic coagulant may contribute to the removal of heavy metals. Overall, chemical coagulation is an effective way to control heavy metal pollution with/without other water treatment technologies. [ABSTRACT FROM AUTHOR]

PO43- and SiO32- are often used as modifier to improve stability and aggregating ability of the iron-base coagulants, however, there are few reports about their detailed comparison between the coagulation performance and mechanisms. In this study, three coagulants—polyferric phosphoric sulfate (PFPS), polysilicon ferric sulfate (PFSS), and polyferric sulfate (PFS) were synthesized; their structure and morphology were characterized by Fourier transformed infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and Scanning electron microscope (SEM). Alkali titration and Ferron species analysis were employed to investigate the hydrolysis performance and species distribution. Jar test was conducted to measure their coagulation behaviors at different dosage, pH, and temperatures in which the flocs properties were measured. The results showed that a number of new compounds were formed due to the presence of PO43- and SiO32-. Moreover, PFPS and PFSS had similar level in Fea as well as Feb. Among them, PFPS produced more multi-core iron atoms polymer and content of Feb, and the formed flocs were larger and denser. It exhibited superior coagulation performance in terms of turbidity reduction, UV254 removal and residual ferric concentration. Jar test and floc breakage/regrowth experiments indicated other than charge neutrality, the dominated mechanism involved in PFSS was the adsorption between polysilicic acid and solution particle, while PFPS was sweeping, entrapment/adsorption resulting from larger polymer colloid of Fe-P chemistry bond. [ABSTRACT FROM AUTHOR]

The drawbacks, high energy consumption, rigorous preparation condition and high impurity contents, have been well-recognized in the preparation of widely-used polyaluminum chloride via conventional methods in industrial scale. In order to overcome the drawbacks, the fresh aluminum hydroxide gels were proposed to prepare the polyaluminum chloride (PAC G ) in this study. The optimal preparation condition of PAC G was obtained by optimizing the preparation conditions including temperature, time and acid concentration. The contents of the small/middle polymeric aluminum (Al b ) in an optimized PAC G could reach 80%. During preparation, high temperature significantly reduced the preparation time but led to a poor stability. Lower rate constants of reaction between hydroxyl-Al and Ferron, k b 1 and k b 2 , were almost equal at high basicity value. It implied that the middle polymeric aluminum (Al b 2 ) was dominant in Al b . Besides, the pseudo-first-order kinetics equation was rearranged. Coagulation by PAC G2.0 was found to be the most effective for low-turbidity water treatment but limited to a narrowest optimum range of dosage compared with other PAC G . The residual aluminum concentration of the water after coagulation by the PAC G2.5 with fresh aluminum hydroxide gels was always low even overdosing. Charge neutralization was not the predominant coagulation mechanism in this treatment. [ABSTRACT FROM AUTHOR]