Environmental Levels and Degradation Behavior of Pharmaceuticals and Personal Care Products (PPCPs) in the Water Environment

Pharmaceuticals and personal care products (PPCPs) are a class of chemicals used by humans for daily life. PPCPs are closely related to people’s production and life, and are even used every day worldwide. PPCP-like compounds were first detected in treated wastewater in Kansas City, USA in 1976 (concentrations of 0.8-2μg/L⁶), and subsequently detected in various countries. The mass production and use of PPCPs have led to increasing concentrations in the environment. PPCPs can induce microorganisms to produce resistance genes because of their persistence and bioaccumulation, thus changing the structure and community of microorganisms in the ecosystem. At the same time, they are accumulated at the top of the food chain or food web^17-21, destroying the balance of the ecosystem. In addition, PPCPs also have chronic toxicity, teratogenicity and carcinogenicity. For example, sulfonamides will damage tissues and organs and cause drug resistance of pathogenic bacteria⁹. Synthetic musk interferes with the secretion of hormones and can also lead to asthma, allergies, migraines and other diseases²⁰. Long-term use will lead to liver and kidney damage and induce cancer²¹, causing irreversible damage to human health.

PPCPs are mainly accumulated in the environment through hospitals, landfills, farms, factory wastewater and domestic sewage, and enter the water environment through various pathways. After the production of PPCPs, some are used by humans, some are directly generated in the production of waste, and some are used by animals in livestock farms. The solid or liquid waste generated in the above three ways will enter the sewage treatment plant or landfill. Then through sewage, landfill leachate directly into the surface water, through further infiltration into the sediment, pore water, groundwater, ocean and other environments, in addition to the surface water through evaporation and precipitation can also return to the water environment. The above environmental behaviors will cause harm to the ecological environment, ecosystem, and humans.

PPCPs exist in surface water, groundwater, sediment, and other environmental media, but the pollution degree varies in different countries. In recent years, a large concentration of PPCPs has been detected in various water environmental media, and sulfonamides, antibiotics, ibuprofen, carbamazepine and DEET are widely distributed in the environment, among which sulfamethoxazole has the highest detection frequency and the highest concentration can reach 1080ng/L⁸. China is the world's largest consumer of drugs, with more than 20000t PPCPs used annually, which have been widely detected in surface water, groundwater, soil and sediments, among which antibiotics transmitted through water bodies are used more⁷ than others. In addition, PPCPs are also detected in water environmental media in the United States⁵⁰, Europe⁵⁷, and Africa⁹, and the study found that the concentration of PPCPs is positively correlated with the degree of economic development. In China, the highest concentration of sulfamethoxazole is detected in the sediments of the Qingpu District of Shanghai, with a concentration of 688.59ng/L⁴⁴, while the highest concentration of sulfamethoxazole in other countries is detected in groundwater of the United States, with a concentration of 1110ng/L⁵⁰. The concentration of PPCPs in pore water and seawater is relatively low, and caffeine is the most widely detected PPCP in seawater. Some compounds have been detected in rainwater because of their volatility. Atrazine has been reported in Mississippi and at the mouth of the Yangtze River^60-61. The presence of ofloxacin and ciprofloxacin has also been detected in Minnesota, USA³⁸. PPCPs in groundwater are mainly produced through the infiltration of domestic sewage, hospital and aquaculture wastewater, and compounds with greater polarity are more likely to penetrate into groundwater⁵⁹. Antibiotics such as lincomycin and erythromycin have been detected in groundwater in North America, Jianghan Plain of China^53,50 and Harbin⁵². Carbamazepine is one of the most commonly detected drugs in sediments, and it has been reported in the Haihe River and Baiyang Lake⁵⁵, with the highest concentration of 14.7ng/g, and also in the sediments of the Taihu Lake Basin⁵⁴, the concentrations of ciprofloxacin and ofloxacin are relatively high, 15.33ng/g and 18.27ng/g respectively. The ocean is considered by many to be an important sink of pollutants. Studies have found that more than 20 kinds of antibiotics with concentrations as high as μg/L have been detected in seawater⁶². Among them, caffeine has been widely detected in the Aegean and Baltic Sea. Besides caffeine, sulfamethoxazole and clarithromycin also have a high detection frequency⁵⁷. PPCPs were also detected in pore water and rainwater. The pore water samples of Baiyangdian Lake⁵⁵ mainly contain erythromycin and caffeine, but their concentrations are much lower than those of surface water in the same area. In Taihu Lake⁵⁶, the concentrations of oxytetracycline and ofloxacin are found, but the concentrations of surface water are lower than those of pore water. Therefore, the different physical and chemical conditions of environmental substrates in different study areas are considered to be the cause. There are relatively few reports of PPCPs in rainwater, and the content of PPCPS is less than 10ng/L.

PPCPs will degrade after entering water, and different degradation processes have their own degradation mechanisms. The degradation behavior of PPCPs in water mainly includes hydrolysis, photodegradation and biodegradation. Hydrolysis is an important way to eliminate or reduce the concentration of PPCPs in a water environment. Its essence is nucleophilic substitution reaction, that is, the nucleophilic group (hydroxide ion or water molecule) attacks the electrophilic group in the compound (RX), and replaces the associated strong electron-withdrawing group (X) with a negative electric tendency. For example, the hydrolysis of penicillin G and amoxicillin is the intramolecular nucleophilic attack of the side chain on the β-lactam carbonyl group, and the C-N bond is broken causing degradation. Degradation can be divided into direct photolysis and indirect photolysis processes. PPCPs with light-absorbing groups can be directly degraded by absorbing light energy. PPCPs without light-absorbing groups need to absorb photons through other substances to obtain energy, so that indirect photodegradation occurs. For example, atenolol is a degradation process that directly absorbs light energy, while acyclovir is an indirect photodegradation process by adding a catalyst. Biodegradation means that microorganisms change the chemical structure of PPCPs through a series of biochemical reactions under aerobic or anoxic conditions, and finally achieve the purpose of removal. At present, studies on the biodegradation of PPCPs mainly focus on three aspects: sewage treatment system, natural surface water and laboratory simulation system⁷⁹. For sewage treatment plants, PPCPs are mainly removed through biodegradation of secondary treatment⁸⁰.

The degradation of PPCPs is affected by various factors, among which pH and temperature are the main influencing factors. The study on hydrolysis of PPCPs mainly considers the influence of pH on PPCPs. Different pH and target compounds will have different reactions, which have certain effects on the hydrolysis rate and hydrolysis products. In addition, temperature will also affect hydrolysis. In general, the higher the temperature, the faster the hydrolysis of a compound⁶¹, because the hydrolysis process of a compound is a thermal reaction, and the activation energy mainly comes from the collision between molecules. The mechanism of photodegradation of PPCPs in water mainly lies in the molecular absorption of light energy into an excited state, which triggers various reactions⁷¹. There are many factors affecting the photodegradation of PPCPs in a water environment, mainly including pH of water and co-existing ions. It is generally believed that the higher pH in a water environment, the faster the photodegradation rate. Because many PPCP molecules contain acid-base dissociative groups, they are easily ionized in aqueous solution to produce a variety of dissociative forms, and the reason for affecting the ionization of PPCPs is the change of solution pH⁷⁴. The presence of co-existing ions can either promote or inhibit the photodegradation of pollutants. The pH and temperature of the environment will affect the absorption, growth and metabolism of nutrients by microorganisms, thus changing the growth and living state of microorganisms, and then affecting biodegradation⁸². In addition, different compounds have different sensitivity to pH and temperature in the process of biodegradation. Also, the types of degraded strains have a certain impact on degradation. In general, photodegradation and biodegradation are more common than hydrolysis. In surface water, many PPCPs have avoided the strict biodegradation environment of wastewater treatment, and photochemistry may have a greater effect than the biodegradation under sunlight, in which antibiotics are mainly photodegraded in the water environment; ibuprofen, iopromide and caffeine are more prone to biodegradation; esters and amides are the most common functional groups that are easily hydrolyzed in PPCPs⁶³, and tetracycline can undergo hydrolysis reactions due to adsorption into sediments. The factors affecting the degradation of PPCPs include pH, temperature, co-existing ions and dissolved organic matter, among which pH and temperature are the main factors affecting the degradation. Exploring the fate of PPCPs in the environment is the key to studying their distribution and environmental level, so it is necessary to analyze the degradation mode of PPCPs in a water environment to help further understand the degradation principle and behavior of PPCPs.

Future research on PPCPs should be more in-depth and detailed. More emphasis will be placed on the water environment such as rain and sea water, which has been studied less before, to make the system more complete. The current research mainly focuses on the migration, transformation and toxic effects of PPCPs, and the toxic effects of degradation products need to be studied further. It is necessary to study the behavior, migration, transformation and toxic effects of PPCPs metabolites in the water environment, so as to provide basis for water environment pollution removal. In addition, the content of PPCPs in the water environment is very low, and the testing technology and instrument requirements are relatively strict. The existing analysis technology and instrument conditions need to be continuously improved to establish a more comprehensive and systematic testing system.