Fate and removal of emerging contaminants during chlorination in drinking water

Abstract

The prevalence of emerging contaminants (ECs) in drinking water is among the emerging challenges for the water industry. The problem is more complex in the developing world, where the water and sanitation facilities are underdeveloped, and the consumption of pharmaceuticals is relatively higher due to fewer healthcare facilities. Consequently, the occurrence of ECs in surface water is well reported in the literature. The presence of ECs in surface water therefore makes it necessary to investigate their removal in conventional drinking water treatment plants (DWTPs). A class of ECs known as antiretrovirals (ARVs) is commonly consumed on the African continent due to the prevalence of acquired immunodeficiency syndrome (AIDS). The chlorination process in DWTPs can have some potential for the removal of these ARVs. However, investigations on the interaction between the chlorination process and ARV removal are scarce. Therefore, the objective of this study was to investigate the removal of five ECs from conventional DWTPs. The ECs selected for the investigation were Nevirapine and Efavirenz (antiretroviral drugs), Atenolol (beta blocker), Sulfamethoxazole (antibiotic), and Carbamazepine (antiepileptic drug). Further, the chlorination process was in depth investigated to understand its kinetics, the effects of operational parameters, and the formation of disinfection by-products during the removal of two ARVs (Nevirapine and Efavirenz). To identify and measure the ECs, Ultra-High-Performance Liquid-Mass Spectrometry (uHPLC-MS) was used. In addition, laboratory studies were conducted to determine the effect of operational parameters on the removal of selected antiretroviral (Nevirapine and Efavirenz) and the formation of disinfection by products during chlorination. During optimization, the solid-phase extraction conditions for all five ECs were achieved at a pH of 3, with an average recovery rate of 64% between all selected ECs. In ascending order, the average EC concentrations detected from the influents were: Sulfamethoxazole (114.37 ng/L), Carbamazepine (118.69 ng/L), Efavirenz (156.12 ng/L), Nevirapine (164.06 ng/L), and Atenolol (197.47 ng/L), respectively. Atenolol exhibited the highest concentration levels among all the ECs in the influent Nevirapine however demonstrated the highest risk quotient (RQmax) values after post-chlorination, particularly in toddlers. The treated effluent showed a significant reduction in the amount of EC detected which was below detection and quantification level. The average removal efficiencies of ECs between the raw influent to the treated effluent where as follows: Sulfamethoxazole (87.17%), Nevirapine (85.32%), Carbamazepine (79.94%), Atenolol (76.99%), and Efavirenz (70.89%). Notably, Sulfamethoxazole exhibited significantly higher degradation and removal rates in all three DWTPs compared to the other four ECs. Using laboratory (batch) experiments, the second phase of the study examined the interaction of Nevirapine and Efavirenz with chlorination process with a prime focus on the effect of operational parameters (pH, temperature, chlorine dosage and compound concentration) and kinetics. The maximum removal of Nevirapine (97%) and Efavirenz (90%), was observed at pH=7.5 and temperature 25oC and chlorine concentration of 3 mg/L. It was further observed that Efavirenz was removed better at basic pH than acidic (37% removal at pH 5.5 versus 68% at pH 8). A threefold increase in temperature from 10oC to 30oC increased the removal of Nevirapine by 42% and Efavirenz by 39%. Higher chlorine dosages of 3 mg/L and 5 mg/L showed efficient removal of both compounds (90 - 97%). The maximum values of pseudo second order rate constant (Kapp) of Nevirapine and Efavirenz were 109.67 x 10 -2M-1 . s-1 and 95.47 x 10 -2 M-1 . s-1 at optimum conditions. The estimated the hydraulic residence time (HRT) for both ARVs was within the practical limits of 1-2 hours, considering a continuous stirred-tank reactor configuration and a chlorine dose of 2 mg/L. Further, the study investigated the formation of disinfection by-products (DBPs), specifically trihalomethanes (THMs), namely chloroform (CHCl3), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3), during chlorination of the two ARVs (Nevirapine and Efavirenz). Notably, among the two drugs, Efavirenz degradation produced the highest formation of THM observed in CHCl3 (63.49 µg/L) followed by CHBr2Cl, (25.83 µg/L) and CHBr3 (11.94 µg/L). This occurred under specific conditions, including a temperature of 25 ºC, a reaction time of 6 hours, a pH of 7.5, and a residual chlorine concentration of 0.1429 μM. Furthermore, the study revealed interesting insights into the kinetics of trihalomethane formation. The highest rate Kapp for trihalomethanes was observed in CHBr2Cl, with a remarkable value of 7.722 x 10-4 M-1 ·s-1 , under the following conditions:100 µg/L efavirenz, at 25°C, chlorine concentration of 0.1426 µM, and a pH of 7.5. Conversely, the lowest Kapp value for trihalomethanes was found for CHBr3, which exhibited a Kapp value of 9.33 x 10-4 M-1 ·s -1 under the same conditions. Importantly, the investigation discovered that the Kapp for CHCl3 and CHBr2Cl formation during the degradation of Efavirenz were higher compared to those observed during the degradation of Nevirapine. This research study addressed the knowledge gap regarding EC pollution in South African drinking water by conducting a risk assessment and investigating the occurrence and removal efficiencies of specific ECs in DWTPs in KwaZulu-Natal, South Africa. The findings highlight the need for tailored approaches considering the specific characteristics and sources of ECs in the country, as complete adoption of EC management practices from developed countries may only partially mitigate EC pollution in South Africa. There is also a vital need to investigate the DPBs in an up-scale environment to assess the prevalence of DBPs in different water matrices.