Antibiotics have become a new kind of organic pollutant as they are widely used in the water environment of China. Tetracycline (TC) is a class of broad-spectrum antibiotics produced or semi-synthesized by actinomycetes. Metronidazole (MTZ) is the first generation of typical nitroimidazoles. The content of nitroimidazoles is relatively high in medical wastewater, and their ecotoxicity is worthy of attention because they are difficult to completely eliminate. In this paper, the effects of TC and MTZ on the growth, cell morphology, extracellular polymer and oxidative stress of Chlorella pyrenoidosa (C. pyrenoidosa) were studied, and the toxic interactions between TC and MTZ mixture components were analyzed. The results showed that the 96h-EC₅₀ of TC and MTZ was 8.72 mg/L and 45.125 mg/L, respectively. The toxicity of TC to C. pyrenoidosa was higher than that of MTZ, and the combined toxicity effect of TC and MTZ was synergistic after the combined action of a 1:1 toxicity ratio. In addition, the algal cells of C. pyrenoidosa died to varying degrees, the membrane permeability of algal cells was increased, the membrane was damaged, the surface of algal cells exposed to higher concentration of pollutants was wrinkled, and their morphology was changed. The extracellular polymer of C. pyrenoidosa was affected by a change in concentration. The effect of pollutants on the reactive oxygen species (ROS) level and malondialdehyde (MDA) content of C. pyrenoidosa also had an obvious dose–effect relationship. This study contributes to the assessment of the possible ecological risks to green algae due to the presence of TC and MTZ in aquatic environments.

Bisphenol A (BPA) is an important industrial chemical; bisphenol S (BPS) is a substitute for BPA. Both are frequently detected in rivers, sewage, and surface water, and have a great impact on the water environment. The effects of BPA and BPS on cell growth, chlorophyll a content, and oxidative stress of Chlorella pyrenoidosa (C. pyrenoidosa) were studied. When BPA and BPS acted alone or in combination, compared with the blank control group, the growth of C. pyrenoidosa in the experimental group showed a pattern of “low promotion and high inhibition”, and the inhibition rate reached the maximum on the 6th day. Under the combined action, the reactive oxygen species (ROS) level of C. pyrenoidosa first increased, and then decreased. In addition, the activity of superoxide dismutase (SOD) and peroxidase (POD) increased with the increase in combined concentration. In the 0.5 P treatment group, SOD and POD activity reached peak values of 29.59 U/mg∙prot and 1.35 U/mg∙prot, respectively. The combined toxicity of BPA and BPS to C. pyrenoidosa was evaluated as a synergistic effect by using toxicity unit and additive index methods. This study evaluated the effects of BPA and BPS on algae in the aquatic environment, providing some data support for their potential ecological risks.

To investigate the advantages of mixed carbon source over a single one in deep denitrification, sodium acetate, glucose and their mixture were used as carbon sources in present study. Denitrification performance, effluent pH, microbial community and carbon source cost were taken into account. With the same influent NO3–-N concentration of 50 mg/L and the same C/N ratio of 1.5, the NO3–-N removal rate with the mixed carbon source (96.53%) was slightly lower than that with sodium acetate (98.15%), but significantly higher than that with glucose (74.69%). The specific denitrification rates of the sodium acetate, glucose and sodium acetate/glucose reactor were 47.7, 29.7 and 45.4 mg N/g VSS d, respectively. The effluent pH with sodium acetate varied in the range of 9.13–9.60, exceeding the discharge standard limit of 9.0, whereas the sodium acetate/glucose reactor could keep pH in the range of 7.80–8.23. The 16S rRNA gene-based high-throughput sequencing revealed that carbon sources determined the microbial community structure and the sludge Shannon index with the mixed carbon source was the highest. Furthermore, cost estimation indicated that the mixed carbon source was the cheapest. This study is significant as it tests reasonable selection of carbon sources for deep denitrification in practice.

The transformation of organic pollutants by stabilized nano-FeS in oxic conditions is far less understood than in anoxic states. Herein, carboxymethyl cellulose-stabilized FeS (CMC-FeS) nanofluids are prepared at a CMC-to-FeS mass ratio of 1/2 and their performance of tetracycline hydrochloride (TC) degradation under oxic conditions was investigated. Here, we showed that TC could be efficiently removed by oxygenation of CMC-FeS nanofluids at neutral initial pH. We found that CMC-FeS dosages as low as 15 mg/L can achieve the TC removal efficiency as high as 99.1% at an initial TC concentration of 50 mg/L. Oxidative degradation plays a predominated role in TC removal (accounting for 58.0%), adsorption has the second importance (accounting for 37.0%), and reduction has minor impact (accounting for 4.1%) toward TC removal. Electron spin resonance assays, fluorescent detection using coumarin as a probe, and radical scavenging experiments confirm that hydroxy radicals (•OH), both in free and surface-bound forms, contribute to oxidation of TC. Humic acids brought detrimental effects on TC removal and therefore should be biologically degraded in advance. This work offers a facile and cost-effective solution to decontaminate TC in natural and engineered water bodies.