FQs themselves, their synthesis by-products, and the metabolites of bacteria are raising increasing concern regarding their ecological risks . Such compounds exhibit toxicity and non-biodegradability and are expected to form complexes with metal ions and other unknown organic compounds, representing hazardous environmental and health impacts. The resistance capacity of microorganisms will also be inhibited . Constructed wetlands have been used internationally for over two decades in a variety of aquatic enhancement projects . CWs are an important and effective measure used to purify wastewater that can effectively remove antibiotics and pollutants . In the existing research, most of the studies on antibiotics in constructed wetlands address the effects of high concentrations on the removal of antibiotics, the generation and removal of resistance genes, etc . The effect of antibiotics in the natural environment on the purification of pollutants in constructed wetlands and the influence mechanism are still not known. Moreover, temperature is the key factor affecting the water purification effect of constructed wetlands . The removal rate of selenium by cattail floating systems is 93–100% at 35 ◦C in summer and 51–100% at 5 ◦C in winter . Low-temperature stress has two distinct components: chilling and freezing. Usually, chilling is defined as occurring at temperatures that are lower than the normal growing temperature for a given species but higher than 0 ◦C, and freezing is defined as occurring at temperatures less than 0 ◦C.However, ebb and flow trays the potential threat and impact of antibiotics with respect to the environment cannot be ignored.
Whether different concentrations of antibiotics have an effect on pollutant removal under different temperature conditions and the difference in the influence mechanism are worth investigating. In this study, a hydroponic culture experiment was designed, and the reactors were placed in an artificial climate chamber at 4–8 ◦C and in a greenhouse at 25–28 ◦C . Different concentrations of LOFL were put into the reactors at two temperature conditions. The concentration of 0–10.0 µg/L may appear in the environment, and 100 µg/L is the concentration in emergencies, which can strengthen the impact of LOFL on the entire system . The objectives of the present study were to evaluate the long-term effects of different LOFL levels on pollutant removal in CWs at low and normal temperatures; to clarify the changes in the bacterial community structure of CWs after long-term exposure to LOFL at low and normal temperatures; and to explore the correlation analysis of environmental factors and predict the functional changes at low and normal temperatures. The experiment was conducted in an open field at the China Research Academy of Environmental Sciences , Beijing, China . Plants were hydroponically cultured in white plastic buckets. Each incubator, 191 mm bottom diameter × 214 mm top diameter × 210 mm height, was filled with 4.5 L culture medium . Plants were fixed in planting baskets and incubated with tap water for 2 months in a greenhouse before the start of the experiment. Hydroponic culture was used in this experiment, and the devices were placed in an artificial climate room or greenhouse . Plants with similar growth were selected. Four Iris pseudacorus L. specimens were planted in each incubator and fixed with a guardrail. Specimens were cultivated for 81 days, from May 31 to August 20. Temperature was the main factor affecting the degradation of pollutants, and the concentration of LOFL was the secondary factor. The removal rate of pollutants increased with increasing temperature and contact time and decreased with increasing LOFL concentration. The removal rate of NH4+ was greatly affected by the environmental conditions , while the removal rates of TN and TP were less affected by environmental factors, and the removal rate of CODCr was the least affected. Normal-temperature conditions are more beneficial than low-temperature conditions to the degradation of pollutants.
Under the two temperature conditions, there was a significant difference in the removal rate of pollution indices between the control group and the experimental groups . The pollutant removal rate of the control group was lower than those of the experimental groups, which indicated that the plants played an important role in the system. At normal temperature, the removal rate of TN was the highest at 0 µg/L LOFL, with a value of 82.90%. Any concentration of LOFL has a certain inhibitory effect on the removal rate of TN. The degree of inhibition of TN removal at a concentration of 0.3 µg/L was lower than that at 0.1–0.2 µg/L, which indicated that the former concentration promoted the degradation of TN compared with the effect of 0.1–0.2 µg/L. Under the condition of low temperature, a LOFL concentration of 0.1–0.5 µg/L promoted the removal of TN by the system. When the concentration of LOFL was 0.2 µg/L, the promoting effect was the most significant, and the removal rate was 73.28%. The degradation of TN was inhibited when the concentration of LOFL was more than 1 µg/L, and when the concentration of LOFL was 100 µg/L, the removal rate decreased to 66.91%. There was a significant difference in ammonia nitrogen removal efficiency between 0 and 0.5 µg/L LOFL and 100 µg/L . Moreover, the difference between 10 µg/L and 100 µg/L was significant. At normal temperature, with increasing LOFL, the removal rate of ammonia nitrogen showed a continuous inverted “V” shape; the tip of the “V” appeared at 0, 0.3, and 10 µg/L, and the removal rates were 85.10%, 87.40% and 77.90%, respectively. The removal rate of ammonia nitrogen was the highest at a LOFL concentration of 0.3 µg/L, and the degradation of ammonia nitrogen was inhibited at other concentrations. When the LOFL concentration was 100 µg/L, the removal rate decreased to 59.99%. LOFL concentrations of 0.1–0.5 µg/L can promote the degradation of ammonia nitrogen, and the maximum removal rate was 45.03% at low temperature. When the concentration of LOFL was higher than 1 µg/L, it showed an inhibitory effect, and the removal rate was 28.90–30.18%. This result showed that a high concentration of LOFL has a great effect on ammonia nitrogen and inhibits the degradation of ammonia nitrogen. Overall, the removal rate of ammonia nitrogen at low temperature was 17.67–40.96% lower than that at normal temperature. The removal rate of CODCr at normal temperature was 2.06–27.9% higher than that at low temperature.
Under normal-temperature conditions, LOFL at concentrations of 0.1–0.5 µg/L promoted the removal of CODCr, and the promoting effect of 0.2 µg/L was the largest: the removal rate of CODCr increased from 54.63% to 64.78% compared with that of the control group. When the concentration of LOFL was higher than that of 1 µg/L, it inhibited the degradation of CODCr. At 100 µg/L, the removal rate of CODCr was 50.64%. At low temperature, LOFL concentrations of 0.1–100 µg/L promoted the degradation of CODCr. Under the conditions of low and normal temperature, the removal rate of TP in the control group increased from 71.28% to 86.34% and from 83.49% to 92.19%, respectively. Plants played a key role in the removal of total phosphorus. Under normal-temperature conditions, concentrations of 0.1–0.5 µg/L can promote the removal of TP. The maximum removal rate appeared at 0.3 µg/L, and the removal rate was 95.79%. When the concentration of LOFL was higher than 1 µg/L, the removal rate of total phosphorus began to be inhibited. When the concentration of LOFL increased to 100 µg/L, the removal rate of TP decreased to 86.98%. At low temperature, concentrations of 0.1–1 µg/L can promote TP, and when the concentration of LOFL was higher than 1 µg/L, the removal efficiency was obviously inhibited. When the concentration of LOFL increased to 100 µg/L, the removal rate of TP was 80.20%. In this study, the average removal rate of NH4+ at normal temperature was 12.84–55.89% higher than that at low temperature. The average removal rate of TN was 14.71–40.96% higher than that at low temperature. The removal rate of TP was 5.22–12.21% higher than that at low temperature. Studies by relevant scholars have shown that the removal efficiencies of ammonia nitrogen , total nitrogen and total phosphorus at low temperatures were reduced by 15%, 45% and 16%, respectively, compared to those observed at the optimal temperature . Those experimental results are similar to those of this study. At the same concentration of LOFL, the removal rate of pollutants in the system at room temperature was significantly higher than that at low temperature. In this study, the purification of pollutants was mainly due to plant absorption and bacterial degradation . Plants play an important role in hydroponic wastewater treatment systems and directly affect the quality of wastewater . The nutrient absorption mechanism for plant utilization involves plant extraction, 4×8 flood tray plant transformation, plant filtration and plant degradation . Through these processes, plants release exudates from their roots, which can stabilize, fix and bind organic pollutants, thereby reducing biodiversity . This process can further improve water treatment and potentially minimize the discharge of nutrients absorbed by plants.
Existing studies have shown that hydroponic systems can not only purify nutrients but also remove pollutants such as antibiotics . TN removal in hydroponic culture systems is mainly realized by physical precipitation, nitrification/denitrification and plant absorption. However, plant tissue may release and decompose pollutants . Therefore, the mechanism by which plants absorb pollutants and decompose plant tissues is a dynamic process . This study does not conduct an in-depth analysis of this problem. In the low-temperature environment, the removal rate of pollutants in the system was reduced mainly for two reasons. First, plant growth, development, and productivity are negatively affected by low temperatures . Two direct effects at the molecular level would be caused by the reduction in temperature. Enzyme activity and membrane flexibility are reduced with decreasing temperature . When plants suffer from cold temperature, although the photosynthetic light reaction is relatively stable, the activity of dark reactive enzymes is reduced. Therefore, photoinhibition of photosystem I and sometimes II occurs . Membrane elasticity also decreases with decreasing temperature. This decrease leads to membrane damage, which leads to increased electrolyte leakage . The supply of oxygen to Iris pseudacorus L. is restricted at low temperature, and the root activity decreases. These processes lead to the narrowing of voids connected with substrates and air, which affects radial oxygen loss. As a result, reoxygenation is reduced, and the assimilation of nitrogen pollutants is decreased . Therefore, in the low-temperature environment, the nutrient absorption and transformation ability of the plants decreased, resulting in a reduction in the removal rate. The second reason is that the bacterial activity in the system decreased at low temperature. The pollutant transformation processes and mass transfer in CWs were restrained when bacterial activity was affected by low temperatures. Therefore, the efficiency of the ecosystem function limits the removal efficiency of CWs .
The activity of anaerobic ammonia oxidation decreases at 10 ◦C, resulting in the deterioration of nitrogen removal performance . The bacterial activity and metabolic rate in the CW systems were reduced at low temperatures. This reduction impedes heterotrophic bacteria from decomposing organic pollutants . The nitrification reaction efficiency drops rapidly at temperatures of 20–30 ◦C, and the reaction almost stops at 5 ◦C . Denitrification is the most effective way to denitrify CWs, and the reaction rate decreases at 15 ◦C . With increasing temperature in CWs, denitrification and dissimilatory nitrate reduction to ammonium increased. After 81 days of culture, compared with the values at 6 days of culture, ace and sobs were larger, and Chao, coverage, Shannon and Simpson were smaller. The higher the concentration of LOFL is, the smaller ace and sobs are and the larger Chao, coverage, Shannon and Simpson are. The ACE and Chao indices indicate bacterial community richness, while the Shannon and Simpson indices reflect bacterial community diversity . Sobs is the actual observation of richness. Therefore, compared with the results at low temperature, the bacterial community richness was higher and the diversity of bacterial communities was lower at normal temperature.This difference might be because the microorganisms were more active in the high-temperature environment and the dominant flora inhibited the reproduction of the inferior flora. In the low-temperature environment, most of the bacteria were inhibited.