Coagulation is an important technology to ensure the safety of drinking water, reduce the health risks of water quality, and ensure the normal operation of subsequent treatment processes. With the continuous improvement of water quality standards and the increasing concern of micro-pollution, the enhanced coagulation has become one of
the most feasible technologies to improve the coagulation efficiency for the removal of fine particles and natural organic matter (NOM). Al salt is a widely used flocculant, and the effective spieces produced by in situ hydrolysis of Al salt on the surface of the colloids or near the organic matters is the key factor affecting the coagulation efficiency. Al13 ([AlO4Al12(OH)24(H2O)12]7+, Al137+) has a high positive charge and strong aggregation ability, and is considered to be the most effective Al species. However, the identification of in situ Al13 on the particles surface during coagulation has not been reported. The content of in-situ Al13 has a great relationship with the pH of the solution. The optimum pH range is usually weakly acidic, while the pH of natural water and common algae-containing water is usually neutral or even alkaline. Therefore, the efficient generation of in situ Al13 for the contaminant removal under neutral or alkaline conditions is one of the challenges in drinking water treatment. Focusing on the above key issues, this study has carried out the research work as follows.
The identification of in situ Al13 on the colloid surface was realized by using surfaceenhanced Raman scattering (SERS) for the first time. The high-purity solid Al13 and Al13 in solution were detected by Raman method, and the characteristic fingerprints were determined. The characteristic peak at 635 cm-1 is not easily interfered by the common substances in the environment, and can be used as the major evidence for effective identification of Al13. While the other two peaks at 300 and 987 cm-1 could be accessorial evidences for the identification. Al13-Cln is selected as the ideal Al13 detection source because it can effectively avoid anion interference and exhibits more noticeable signals. Further, the identification of Al13 adsorbed on the surface of Ag sol and gold-core/silica-shell (Au/SiO2) colloids is confirmed by the SERS response with a significantly improved signal-to-noise ratio of the characteristic peaks. The geometry of Al137+ was optimized and its frequency was calculated using density functional theory (DFT). According to the calculation results and least squares fitting computed Raman spectra, the characteristic peaks are decomposed in detail, and the vibrational modes of each chemical bond in the Al13 7+ structure are associated with the characteristic peaks.
A combined Fe-Al process as the enhanced coagulation system was established,during which a small amount of Fe was introduced prior to Al for pre-hydrolysis. The coagulation efficiency of the combined Fe-Al process for the treatment of synthetic water under alkaline pH was systematically investigated. In contrast to the individual Al process, the combined Fe-Al coagulation accelerates the floc growth rate. Fe and Al salts have synergistic coagulation effect, and the Fe:Al mole ratio (RFe:Al) and time interval (TI) are important factors. At the RFe:Al of 1:10 and TI of 10 s, the turbidity removal of the combined Fe-Al process achieves 73%, which is significantly higher than the individual Al (45.9%). The combined Fe-Al coagulation can better tolerated the high basicity, and attains a high turbidity removal efficiency in a wider alkalinity range. The in situ Al species formed by the combined process was qualitatively studied by SERS combined with X-ray photoelectron spectroscopy, and was quantitatively determined by Al-Ferron time-by-time complexation colorimetry method. The favored local pH at the colloid-water interface governed by the hydrolysis of Fe is proposed to be responsible for the enhanced generation of in situ Al13 during the combined Fe-Al
The removal of humic acids (HA) by the combined Fe-Al coagulation system was further investigated. The removal efficiencies of turbidity, dissolved organic carbon (DOC), UV absorbance at 254 (UV254), different molecular weight HA and fluorescence index (FI) of the combined system are significantly higher than the individual Al system. To achieve the best synergy between the Fe and Al salts, the regulation of the combined process requires the overall consideration of the interaction of RFe:Al and TI. The optimization of TI can further reduce the addition of Fe salt. The reduction of FI of the three fluorescent peaks in HA follows peak γ > peak β > peak α,which represents the organic matter from high to low molecular weight and polymerization degree with different functional groups, respectively. The matching mechanism of Al speciation and humic acid components was investigated in depth. It was found that AlCl3 played a key role in the removal of aromatic structure of HA. With a certain amount of AlCl3, the Al(OH)3 promotes the decrease of fluorescence peaks. On the basis of the obtained results, the mechanism of the combined Fe-Al coagulation for enhanced HA removal is proposed. On one hand, HA is directly removed by the monomer Al of Al salt through complexation. On the other hand, the pre-hydrolysis of Fe and its complexation with HA can release H+ for the regulation of local pH,promoting the generation of Al(OH)3 by in-situ hydrolysis of Al salt and thereby the effective removal of HA. For the treatment of algae-containing source water, the combined Fe-Al coagulation can effectively reduce the residual Al concentration, and achieve the effective treatment with alkaline pH (pH = 8.8) at a lower Al dosage.