Impacts of PAC modified clay applied in the control of cyanobacteria bloom and left in water on water environment

利用改性粘土治理蓝藻水华堆积的湖泊近岸区域以及发生水华的养殖水体成为应急治理蓝藻的重要措施，然而负载有毒藻体的改性粘土沉积水体可能引发的安全性风险尚缺乏研究.选取PAC改性粘土作为研究对象，通过模拟实验，研究改性粘土处理蓝藻水华后沉积水体对负载的藻细胞结构、水体营养盐与藻毒素的释放与削减以及对其他水体功能性细菌（硝化、反硝化细菌）的影响.结果表明，PAC改性粘土的施用对低水华水体蓝藻细胞的去除率为96.04%±0.99%，高水华水体与低水华水体施用的去除率之间差异不显著，PAC改性粘土的施用能够在较长的时间内有效地控制水体中的蓝藻浓度.透射电镜结果表明，PAC改性粘土沉降蓝藻第4 d后，蓝藻细胞膜出现了一定程度的不完整，细胞内规则的类囊体片层结构出现了实质性损伤.随着粘土负载蓝藻细胞的损伤，水体中的总可溶性氮浓度迅速上升，但总可溶性磷浓度仍可在较长一段时间内维持在较低水平.PAC改性粘土施用后，水体的总细菌数与空白对照组趋于一致，但硝化、反硝化细菌数却呈显著下降趋势.PAC改性粘土施用下的藻毒素释放风险主要集中在高水华水体.高水华水体中，PAC改性粘土施用导致藻毒素MC-LR和MC-RR加速释放，其最高峰值分别达到空白对照组的1.69±0.09和2.04±0.09倍，但水体MC-LR浓度达到安全限（<1 μg/L）的时间明显比空白对照组早8 d.此外，PAC改性粘土的施用并没有导致水体中Al³⁺和Cl^-浓度超标.;It has become an important measure for emergency treatment of cyanobacteria by using modified clays to control algal blooms in coastal regions and aquaculture water.However, there is less research focusing on the security risks led by toxic algae which were attached on the modified clay and left in the water. In this paper, PAC modified clay was selected for simulation experiments.The effects of applying the modified clay on the structure of cyanobacteria attached on the PAC modified clay, release and reduction of nutrients and microcystins, and the amounts of other functional bacteria (nitrifying-denitrifying bacteria) were investigated. The results showed that algae removal rate of modified clay applied in the slightly blooming water was 96.04%±0.99%. There was not a significant difference of algae removal rate inslightly blooming water and severely blooming water. Application of the modified clay effectively controlled the concentration of cyanobacteria in water for a long time. Transmission electron microscopy experiments revealed that cyanobacterial cell membrane became incomplete and the regular lamella structure was damaged after the cyanobacteria being settled with modified clay for 4 days. With the damage of cyanobacteria attached on PAC modified clay, the content of dissoloved total nitrogen in water increased rapidly, while the content of dissoloved total phosphorus could be maintained at a low concentration for a long time. With the application of PAC modified clay in water, the total bacteria number in the water was in agreement with that of the blank control, while the number of nitrifying-denitrifying bacteria decreased significantly. The releasing risks of microcystins after loading the modified clay mainly happened in the severely blooming water. The application of PAC modified clays in severely blooming water resulted in an accelerating release of MC-LR and MC-RR. The peak value of MC-LR and MC-RR amounts were 1.69±0.09 times and 2.04±0.09 times larger than that in control case,respectively. But the time of MC-LR amount reaching to the safety limit(<1 μg/L)was shortened for about 8 days, compared to the control case. In addition, the application of PAC modified clay in water did not lead to excessive concentrations of Al³⁺ and Cl^-.