纳米银在活性污泥污水处理系统中的分布及形态转化

纳米银(nanosilver，nAg)因其优越的抑菌性能成为全球应用最多的纳米材料之一。随着纳米技术的广泛应用，纳米银不可避免进入污水收集和处理系统。在污水生物处理过程中，纳米银可能发生化学形态转变，从而对污水处理微生物产生不同的抑制效应。本研究采用序批式反应器(sequencing batch reactors，SBRs)模拟活性污泥污水处理系统，连续运行50 d，在进水中分别添加1 mg·L⁻¹、10 mg·L⁻¹ nAg和0.3 mg·L⁻¹、3 mg·L⁻¹ Ag⁺，探究了Ag在活性污泥、出水中的分布以及污泥中Ag的化学形态变化。结果表明，进水中添加的Ag(nAg或Ag⁺)导致活性污泥颜色、絮体结构和Zeta电位等均发生了变化，进水中Ag含量越高，这些变化越明显；进水中分别添加1 mg·L⁻¹、10 mg·L⁻¹ nAg和3 mg·L⁻¹ Ag⁺时，运行至第20 天后，活性污泥中Ag质量浓度稳定在3.28~3.67 mg·L⁻¹，而进水中添加0.3 mg·L⁻¹ Ag⁺时，活性污泥中Ag质量浓度持续升高，运行至第50天，达(2.38±0.19) mg·L⁻¹；进水中分别添加1 mg·L⁻¹ nAg和0.3 mg·L⁻¹、3 mg·L⁻¹ Ag⁺的反应器在50 d运行期内，出水中Ag质量浓度分别低于60.0、5.0、9.0 μg·L⁻¹，而添加10 mg·L⁻¹ nAg的反应器运行20 d后，出水中Ag含量快速升高，在第50 天时达(3.96±0.16) mg·L⁻¹，这表明进水中添加的Ag主要累积在活性污泥中，活性污泥对进水中的Ag存在吸附饱和现象，超出污泥吸附阈值后，反应器出水中总Ag含量持续升高；对运行至第50 天的各反应器中活性污泥进行了X-射线衍射分析及X射线光电子能谱分析，发现进入活性污泥系统的nAg可以银的氧化物、Ag₂S和Ag⁰等形态存在于污泥中。评价nAg对污水处理微生物的毒性影响应结合nAg的化学形态转化，含有较高浓度Ag的活性污泥在处理处置过程也应评估不同形态Ag的环境影响。

The distribution and morphology transformation of nanosilver in activated sludge treatment system

Nanosilver (nAg) has been used broadly in nanotechnology enhanced consumer products because of its excellent antimicrobial properties. With wide application of nanotechnology, nAg will be inevitably released into sewage collection systems and wastewater treatment plants (WWTPs). During sewage biological treatment, the chemical form transformation of nAg will occur, which will cause different inhibitory effect on the microbial communities in WWTPs. To explore the distribution of Ag in activated sludge and effluent, and chemical form variation of Ag in sludge, the sequencing batch reactors (SBRs) were selected to simulate activated sludge treatment system for 50d running, the test experiments were conducted with addition of 1 mg·L−1 nAg (1-nAg) in influent, 10 mg·L−1 nAg (10-nAg) in influent, 0.3 mg·L−1 Ag+ (0.3-Ag+) in influent or 3 mg·L−1 Ag+ (3-Ag+) in influent, as well as the control (CK) group, respectively. The results showed that the changes in color, floc structure and Zeta potential of activated sludge occurred with the addition of nAg or Ag+ in the influent. The changes became more significant with the increase of nAg or Ag+ concentrations in the influent. The Ag concentrations in activated sludge of 1-nAg, 10- nAg and 3.0-Ag+ groups were ranged from 3.28 to 3.67 mg·L−1 after 20d SBR running. For 0.3-Ag+ group, the Ag concentration in activated sludge continued to increase and reached (2.38±0.19) mg·L−1 at the end of operation. The Ag concentrations in SBR effluent were lower than 5.0 μg·L−1, 9.0 μg·L−1 and 60.0 μg·L−1 with addition of 0.3 mg·L−1, 3 mg·L−1 Ag+ and 1 mg·L−1 nAg during 50 d operation, respectively. However, the Ag concentrations in SBR effluent with 10 mg·L−1 nAg addition increased rapidly after 20d running, and reached (3.96±0.16) mg·L−1 at the operation end. This indicates that Ag mainly accumulated in the activated sludge and the adsorption threshold for Ag occurred on it, when this threshold was exceeded, the total Ag concentration in the effluent continued to increase. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis on SBR activated sludge after 50d running indicated that nAg in activated sludge formed silver oxide, Ag2S or silver complexes. The toxic effects of nAg on microorganisms in WWTP should be evaluated in combination with the chemical transformation of nAg. The environmental impact of Ag should also be assessed in activated sludge treatment and disposal with high concentration Ag.