Toxicity of Ozonated Wastewater to HepG2 Cells: Taking Full Account of Nonvolatile, Volatile, and Inorganic Byproducts.

Wastewater ozonation forms various toxic byproducts, such as aldehydes, bromate, and organic bromine. However, there is currently no clear understanding of the overall toxicity changes in ozonated wastewater because pretreatment with solid phase extraction cannot retain inorganic bromate and volatile aldehydes, yet contributions of known ozonation byproducts to toxicity are unknown. Moreover, compared with bromate, organic bromine did not receive widespread attention. This study evaluated the toxicity of ozonated wastewater by taking aldehydes, bromate, and organic bromine into consideration. In the absence of bromide, formaldehyde contributed 96-97% cytotoxicity and 92-95% genotoxicity to HepG2 cells among the detected known byproducts, while acetaldehyde, propionaldehyde, and glyoxal had little toxicity. Both formaldehyde and dibromoacetonitrile drove toxicity among the known byproducts when bromide was present. Toxicity assays in HepG2 cells showed that when secondary effluents contained no bromide, the cytotoxicity of the nonvolatile organic fraction (NVOF) was reduced by 56-70%, and genotoxicity was completely removed after ozonation. However, the formed aldehydes (volatile organic fraction, VOF) led to increased overall toxicity. In the presence of bromide, compared with the secondary effluent, ozonation increased the cytotoxicity of the NVOFBr from 3.4-4.0 mg phenol/L to 10.3-13.9 mg phenol/L, possibly due to the formation of organic bromine. In addition, considering the toxicity of VOFBr (VOF in the presence of bromide, including aldehydes, tribromomethane, etc.), the overall cytotoxicity and genotoxicity became much higher than those of the secondary effluent. Although bromate had a limited impact on cytotoxicity and genotoxicity, it caused an increase in oxidative stress in HepG2 cells. Therefore, when taking full account of nonvolatile, volatile, and inorganic fractions, ozonation generally increases the toxicity of wastewater.

Study on synergistic effect of ozone and monochloramine on the degradation of chloromethylisothiazolinone biocide.

In this study, it was found that monochloramine induced the formation of reactive species during ozonation of chloromethylisothiazolinone (CMIT). CMIT was found recalcitrant to chloramine. However, chloramine promoted the degradation of CMIT by ozonation significantly. Hydroxyl radicals contributed most to CMIT degradation (87%) during ozone/chloramine synergistic oxidation process (SOP). The hydroxyl radical exposure during ozone/chloramine SOP was around 7.9 times higher than that of ozonation process. The hydroxyl radical yield of ozone/chloramine SOP was estimated to be 32%. The reaction mechanisms between ozone and chloramine were postulated to include the oxygen transfer reaction to form singlet oxygen, and the formation of hydroxyl radical by the insertion pathway or electron transfer pathway. Chloramine dosage and pH are essential influencing factors. The degradation of CMIT increased from 41% to 74% with increasing chloramine dosage (0-20 µM), and then decreased to 65% when chloramine dosage continually increased to 40 µM. Ozone/chloramine SOP showed better performance at acidic or neutral conditions than basic condition. Based on the intermediates identified, the degradation pathway of CMIT during ozone/chloramine SOP included the oxidation of sulfur atom and the substitution of chlorine group by hydroxyl group. The oxidation of sulfur atom induced lower toxicities of transformation products. The toxicities of hydroxylation products were lower to fish and algae, but higher to daphnia. Based on the GC-ECD results, only trichloromethane (1.94 µg/L) was detected after ozone/chloramine SOP, accounting for 0.17% (µM/µM) of the CMIT removal.

Non-volatile disinfection byproducts are far more toxic to mammalian cells than volatile byproducts.

Water is often chlorinated to protect public health, but chlorination causes harmful disinfection byproducts to form. Currently available in vitro assays generally determine non-volatile disinfection byproduct (NVDBP) toxicities because of the limitation of pretreatments used, but chemical analyses and regulations are focused on volatile disinfection byproducts (VDBPs) such as trihalomethanes. The gap of VDBP toxicities have been of concern for some time. In this study, we extracted VDBPs from two chlorinated effluent organic matters and one chlorinated natural organic matter, using a helium aeration-liquid nitrogen condensation system, and systematically assessed the VDBP and NVDBP toxicities to mammalian cells. VDBPs accounted for 10%-20% of the total organic halogen concentrations in three chlorinated water samples. VDBPs were much less cytotoxic, caused fewer DNA double-strand breaks, induced less reactive oxygen species and DNA/RNA oxidative damage marker of 8-hydroxyl(deoxy)guanosine in cells than did NVDBPs. Moreover, by collecting the VDBPs, toxicity measurement of the full range of DBPs was achieved. Cytotoxicity, reactive oxygen species and 8-hydroxyl(deoxy)guanosine levels were significantly higher for cells exposed to the mixture of VDBPs and NVDBPs than only NVDBPs, but not by large percentages (20%-30% for cytotoxicity), suggesting NVDBPs mainly contributed to the toxicity of chlorinated water. Our study suggested that future research should focus more on NVDBP toxicity and identifying toxicity drivers from NVDBPs.