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In aquatic systems, pharmaceuticals may undergo various behaviour and fate processes (Figure 2.1). These processes include (1) sorption, (2) biochemical degradation, (3) uptake and bioaccumulation by aquatic biota, (4) burial/sedimentation and subsequent resuspension and transfer along the trophic levels (Gwenzi and Chaukura). The behaviour and fate of pharmaceutical in aquatic systems depend on the physico-chemical properties of the pharmaceuticals as well as the biogeochemical conditions in aquatic systems (e.g., pH, redox potential, temperature, and salinity).

The behaviour of pharmaceuticals seems to be based on their therapeutic group, for example, antibiotic have been observed to be polar and dissolve easily in water (Wang and Wang, 2015). Nonsteroidal antiinflammatory drugs, Diclofenac, ibuprofen, and ketoprofen are in anionic form when in solution (Du et al., 2015; Williams et al., 2009). The pharmaceuticals appear in different ions when in solution ibuprofen and diclofenac form negative ions while norfloxacin form positive ions (Du et al., 2015). Carbamazepine and acetaminophen are neutral and unionized in water (Bagnis et al., 2018). This is because of acidic or basic nature of the pharmaceutical, acidic will be negative and basic will be positive ions in solution. It then will affect adsorption of these pharmaceuticals on the surfaces of charged solids with negative surfaces attracting positive ions.

To some extent wastewater treatment processes affect the behaviour of pharmaceuticals. For example, the decrease in the concentration of seven selected drugs after an upgrade of a wastewater treatment plant from 2 to 4 days solid retention to 12 days was attributed to sorption of the chemicals onto the sewage sludge (Moldovan et al., 2009). While Kay et al. (2017) also observed that low flow had lower drug concentration than medium flow. The observation also supports the sorption effect on removal of pharmaceuticals from solution in water systems. The slow speed then helps the drug to be attracted to the solid surfaces or it provided time for degradation. In another study the concentration of pharmaceuticals in water samples were higher (2.2 ng/L to 17.7 μg/L) than those found in the sediments (0.21 to35.8 ng/L) (Vazquez-Roig et al., 2011) showing that most of the pharmaceuticals in this river were more soluble than they were attracted to the solids.

Maskaoni and Zhou (2010) had observed earlier that pharmaceuticals existed in three forms in a river environment that is soluble, attached to colloids and particulate. Comparing attachment to colloids to solubility in water for five drugs they found that propranolol had 45%, sulfamethoxazole 40%, carbamazepine 22%, indomethacin 39%, and diclofenac 37% bound by colloidal particles. The similarity in percentage for sulfamethoxazole, indomethacin, and diclofenac show that there are drugs that tent to behave in a similar way when it comes to sorption. Propranolol and carbamazepine on the other hand will not belong to the same class in this case.

Sorption behaviour was observed to be consistent with charge and lipophilicity of the drug (Bagnis et al., 2018). Amitriptyline which had high lipophilicity (log k_ow 4.9) and positive charge adsorbed more than diclofenac which had log k_ow 3.6 and negative charge. Diclofenac adsorbed more than acetaminophen with log k_ow 0.3 and neutral charge. This was in agreement with what Williams et al. (2009) observed; cationic pharmaceuticals had more affinity to solid phase than the anionic such as diclofenac and ketoprofen. The charge effect then shows that most of the sorbents are negatively charged making it difficult for negative to negative attraction. Lipophilicity helps to see even how soluble these pharmaceutical are and not being soluble leads to less interaction with the surface.

Kay et al. (2017) observed that at high river flow low concentration of pharmaceuticals were obtained, which could be due to degradation of the drug enhanced by aeration at high speed. Togola and Budzinski (2007) noted that the concentration of pharmaceuticals where higher upstream than there were down which led them to conclude that it could be degradation, and yet, it could also be sorption, sedimentation, or photolysis. Aminot et al. (2018) observed that biotic degradation and not sorption to particles was the main attenuation process for pharmaceuticals in the study they carried out. While biological degradation was observed to have occurred for ibuprofen and diclofenac drugs, norfloxacin showed that most of it (72%) had been attracted to the solids that were then removed by sedimentation and carbamazepine had not been affected by the treatment (Du et al., 2015). This behaviour of carbamazepine was also noted by Zhou et al. (2011) when they observed that carbamazepine concentration were higher in the effluent of a waste water treatment than in the influent of the wastewater treatment plants studied. This was assumed to be due to its chemical stability and hydrophilic nature. Carbamazepine was also found to have a persistent behaviour during a 4-week experiment carried out by Aminot et al. (2018). The pharmaceuticals then do not behave in the same way, thus the need to have a study for individual pharmaceutical behaviour and factors that affect their fate in aquatic systems.

Pharmaceuticals have been observed to exhibit string seasonal patterns. For example, pharmaceuticals accumulate in the river and lake during winter time indicating that there is a possibility of factors such as temperature, precipitation and sunlight availability may influence the fate of pharmaceuticals in the aquatic systems (Daneshvar et al., 2010). Li et al. (2016) also observed seasonal variation in river system but they reasoned that it could be because of more use of pharmaceuticals in winter than in summer as well as temperature affecting the rate of biological degradation. Some studies showed the potential of pharmaceuticals for bioaccumulation along the trophic levels including in biota and food webs (Arnodl et al., 2013; Contardo-Jara et al., 2011; Lagesson et al., 2016; Paltiel et al., 2016). The capacity to undergo uptake and bioaccumulate in aquatic biota mays pose human health risks in the case of aquatic foods such as edible plants, fish, and crustaceans. However, data on the behaviour and fate of pharmaceuticals tend to be short-term; hence, limited knowledge exists on their environmental persistence, including in aquatic systems (Bu et al., 2016).