环境水质学国家重点实验室知识库

      混凝是保障饮用水安全、降低水质健康风险、保证后续处理工艺稳定运行的重要技术。随着水质标准不断提高，以及微污染问题越来越受关注，为实现对颗粒物和天然有机物的高效去除，强化混凝成为提升混凝效率的最有效方法之一。长久以来，铝盐在水处理工艺中被广泛使用，其在颗粒物表面或有机物附近原位水解产生的有效形态（Al13及Al(OH)3等）是影响混凝效率的关键因素。Al13
（[AlO4Al12(OH)24(H2O)12]7+，Al137+）具有高正电荷和强聚集能力，被认为是最有效的铝活性物种，但混凝过程中尤其颗粒物表面原位Al13如何检测尚不明确。原位Al13的含量受溶液pH显著影响，最佳pH范围一般为弱酸性，而天然水以及常见的含藻水源水pH通常呈中性甚至碱性。因此，如何在中性及碱性条件下强化原位Al13等铝活性物种生成并高效去除污染物是饮用水处理技术发展的难题之一。基于此，本论文开展了以下研究工作：
      创建了基于表面增强拉曼（SERS）技术的颗粒物表面原位Al13鉴定方法。通过拉曼分析检测高纯固体、溶液中的Al13发现，635 cm-1处的特征峰不易受环境中共存物质（如单体铝）的干扰，可作为有效识别Al13的主要证据，300和987 cm-1处的特征峰可作为Al13识别的辅助证据。Al13-Cln因可有效避免阴离子干扰且特征峰明显而被选为理想的Al13检测源。以Ag溶胶和金核/二氧化硅壳（Au/SiO2）结构颗粒物为SERS基质，实现了颗粒物表面Al13的鉴定，信号峰的信噪比得到明显改善。使用密度泛函理论（DFT）对Al13的几何结构进行优化和频率计算。基于计算结果，结合最小二乘拟合方法，阐明了Al13结构中化学键的振动模式与特征峰的匹配关系。
      构建了铁铝共水解强化混凝体系，即在Al盐混凝之前加入少量Fe盐进行预水解。系统考察了铁铝共水解体系在碱性pH下对模拟天然水体的净化作用。与单独Al盐体系相比，铁铝共水解体系加快了絮体的生长速率，证实Fe盐和Al盐具有协同混凝效应，铁铝比（RFe:Al）和时间间隔（TI）为关键影响因素。当RFe:Al为1:10，TI为10 s时，浊度去除率达到73.4%，显著高于单独Al体系（45.9%）。同时铁铝共水解体系可以更好地耐受高碱度水体环境，在一定碱度范围（1.0~2.0×10-3）均可实现超过70%的浊度去除率。采用新建立的SERS表征方法结合X射线光电子能谱等，进一步定性研究了铁铝共水解过程原位生成的铝形态，并通过Al-Ferron逐时络合比色法对原位铝形态进行定量解析，提出了Fe盐预水解调控颗粒物表面微环境pH，从而促进原位Al13产生的铁铝共水解机制。
      在此基础上，揭示了铁铝共水解体系与腐殖酸（HA）的形态匹配与高效去除机制。结果发现共水解体系对浊度、溶解性有机碳（DOC）、254 nm处的紫外吸收（UV254）、不同分子量HA和荧光参数（FI）的去除效果明显优于单独铝体系。铁铝共水解去除HA过程调控需要统筹考虑RFe:Al和TI的交互影响，从而实现Fe盐和Al盐最佳的协同效应，尤其TI的调控可进一步减少Fe盐的投加量。HA中三个荧光峰peak α、β及γ削减的难易程度为：peak γ > peak β > peak α，其分别代表了分子量及聚合度由高到低且携带不同官能团的有机物。深入探讨了不同铝形态与不同腐殖酸组分的匹配机制，发现AlCl3对于芳香结构HA的去除具有至关重要的作用，在保证一定量AlCl3的前提下，Al(OH)3促进了各荧光峰的去除。在铁铝共水解过程中，一方面HA与单体Al发生络合而去除；另一方面Fe预水解及与HA络合释放的H+降低了微环境pH，促进了Al盐原位水解产生Al(OH)3有效形态，进而实现了HA的高效去除。以上海某水库水为原水，在较低剂量下实现了对碱性水体（pH = 8.8）的有效处理，并有效降低残余Al浓度，对指导强化混凝工艺具有重要参考意义。

      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
coagulation.
      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.