| Water resource is suffering heavy pollution with the development of industry and agriculture. Mercury is well-known as a highly toxic element and can cause water polluition. Some studies suggest that very low mercury concentration in natural water may also produce toxic effects when the natural water act as raw water for drinking water. The maximum mercury level in drinking water is 0.001 mg/L of standard for drinking water quality(GB5749-2006). To date, most of methods for mercury removal are used primarily for the treatment of aqueous solution that contain high mercury concentration; they may not be effective for achieving the low levels of mercury that meet the standards for drinking water quality. To solve this problem, in situ formed manganese-ferric(hydr)oxides(in situ Fe-Mn) and in situ formed manganese(hydr)oxides combined with poly-aluminum chloride(PAC)(in situ Mn Ox-PAC) are used to remove trace mercury removal. The experimental process is based on the enhanced coagulation. Some impact factor on mercury removal was studied, and the mechanism of mercury removal was studied via several methods such as FTIR and XPS analysis.The efficiency and mechanism of trace mercury(Hg(II)) removal by in situ Fe-Mn were investigated by reacting KMn O4 with Fe(II) in simulated solutions. The impact of coagulant dosage, p H, and temperature on mercury removal was studied. The performance of in situ Fe-Mn was also compared with polyaluminium chloride(PAC), in situ Fe(III) and iron(III) chloride(Fe Cl3) coagulation processes, since these metal coagulants are frequently applied in practice. Experimental results showed that in situ Fe-Mn more effectively removed mercury compared with other coagulants. The highly efficient removal for mercury by in situ Fe-Mn may be attributed to effective adsorption and precipitate of in situ Fe-Mn, producing the effective system of adsorption-flocculation-precipitation process for Hg-Fe-Mn mixture solutions. The Cl- and p H values affect the mercury species and further affect mercury removal. Fourier transform infrared spectroscopy demonstrated that in situ Fe-Mn contained hydroxyl groups as the surface active sites, while X-ray photoelectron spectroscopy(XPS) measurements revealed that Mn O2 or Mn OOH and Fe OOH were the dominant species in the precipitates. XPS analysis indicated that an Hg-Fe-Mn mixture was formed in the precipitates, suggesting that mercury was removed from solutions via transfer from the liquid phase to solid phase. These results indicated that the primary mercury removal mechanisms in in situ Fe-Mn were surface complexation and flocculation-precipitation processes.The in situ Mn O2 played an important role in the process of mercury removal by in situ Fe-Mn. Therefore, the experiments were designed to study mercury removal by in situ Mn O2 alone and poor removal was achieved. This proved the case in the negative that in situ Fe-Mn as a whole for high mercury removal is important. However, a new phenomenon was observed. With the molar ratio of Mn/Na2S2O3 decreasing, the mercury removal increased. The reason may be different properties of in situ Mn O2 produced from different molar ratio of Mn/Na2S2O3. Therefore, a new technology(in situ Mn O2-PAC) was designed to study the mercury removal by in situ Mn O2, and the in situ Mn O2 was produced as the molar ratio of Mn/Na2S2O3 was 1.5.Removal of trace mercury from aqueous solution by in situ Mn O2-PAC was investigated. The efficiency of trace mercury removal was evaluated under the experimental conditions of reaction time, Mn dosage, p H, and temperature. The results clearly demonstrated that in situ Mn O2-PAC was effective for trace mercury removal from aqueous solution. The mercury removal increased with time increasing or Mn dosage increasing or p H valued decreasing or temperature increasing. Based on the experimental results and mercury species analysis, the reason for mercury removal by in situ Mn O2-PAC(the molar ratio of Mn/Na2S2O3 of 1.5) had been preliminarily discussed. The presence of S2O32- was the primary factor, rather than other possible chemical SO42-, SO32- and Mn2+ produced from the process. In addition, the mercury removal increased with the increasing of SO42- or PO43- or Si O42- or HA concentration. The amount of mercury removed(removal amount) is used to evaluate the mercury removal efficient by in situ Mn O2-PAC. The removal amount increased with ionic strength increasing or p H values decreasing.To further discuss the mechanism for enhancement of mercury removal with S2O32- by in situ Mn O2-PAC, the experiments for effect of S2O32- on mercury removal were designed. The different molar ratio of Na2S2O3/Mn(Mn is the in situ Mn O2 dosage) and the changes in Mn dosage, reaction time, p H values and ionic strength were used to study the effect of thiosulfate on mercury removal by in situ Mn O2-PAC(in situ Mn O2 alone). The Mn dosage was kept constant in the process if no special state. The results indicated that the presence of thiosulfate clearly improved removal of mercury from solution, and that increases in the ionic strength enhanced removal in a certain range of thiosulfate concentration. According to the analysis of redox of thiosulfate, zeta potentials and aggregations, S2O32- can reduce some Mn O2 to Mn2+ and some Mn2+ can be adsorbed on the surface of in situ Mn O2 to enhance the aggregation of small particles, resulting in decrease in absorbance of mixture solution. At the macro level, the mercury removal increased with absorbance decreasing. The analysis of mercury species indicated that [Hg(S2O3)2]2- is the main species in the presence of thiosulfate. Thus, the ternary complexes(≡S–OMn–[Hg(S2O3)2]2-) or large aggregations(Mn Ox–O–Mn2+[Hg(S2O3)2]2-) may be formed due to surface complexation or electrostatic attraction. Consequently, the mercury in liquid phase was easily transferred to the solid phase. At the micro level, FTIR and XPS analysis revealed that sulfur from thiosulfate and mercury concomitantly increased with the molar ratio of Na2S2O3/Mn increasing. This further indicated that mercury as S-Hg complexes was adsorbed on the surface of in situ Mn O2.In order to evaluate mercury removal under more realistic conditions, natural surface water spiked with mercury as the object was investigated. The results indicated that satisfactory removal efficiency of mercury was also observed following in situ Fe-Mn(in situ Mn O2-PAC) in natural waters, and strong enhancement of mercury removal with thiosulfate by in situ Fe-Mn or in situ Mn O2-PAC. Under optimum test conditions, the residual Hg was less than 1.0 μg/L which meet the standards for drinking water quality. The residual metal(Mn, Fe and Al) and turbidity can meet the standards for drinking water quality. The analysis on mercury removal, dosage of chemicals, process and energy consumption demonstrated that in situ Fe-Mn or in situ Mn O2-PAC had their own advantages and disadvantages, and thus the choice of methods in practical engineering had to carry out technical and economical comparison. |
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