Dinoflagellate requires a lower temperature and blooms frequently in the spring and autumn compared to regular cyanobacteria. The outbreak of dinoflagellate bloom will also lead to the death of some aquatic organisms. However, research on freshwater dinoflagellates is still lacking due to the challenges posed by classification and culture in laboratory. The removal effect and mechanism of Peridinium umbonatum (P. umbonatum, a typical dinoflagellate) were investigated using solar/chlorine in this study. The effect of simulated solar alone on the removal of algae was negligible, and chlorine alone had only a slight effect in removing algae. However, solar/chlorine showed a better removal efficiency with shoulder length reduction factor and kmax enhancement factor of 2.80 and 3.8, respectively, indicating a shorter latency period and faster inactivation rate for solar/chlorine compared to solar and chlorine alone. The removal efficiency of algae gradually increased with the chlorine dosage, but it dropped as the cell density grew. When the experimental temperature was raised to 30 °C, algal removal efficiency significantly increased, as the temperature was unsuitable for the survival of P. umbonatum. Attacks on cell membranes by chlorine and hydroxyl radicals (•OH) produced by solar/chlorine led to a decrease in cell membrane integrity, leading to a rise in intracellular reactive oxygen species and an inhibition of photosynthetic and antioxidant systems. Cell regeneration was not observed in either the chlorine or solar/chlorine systems due to severe cell damage or cysts formation. In addition, natural solar radiation was demonstrated to have the same enhancing effect as simulated solar radiation. However, the algal removal efficiency of solar/chlorine in real water was reduced compared to 119 medium, mainly due to background material in the real water substrate that consumed the oxidant or acted as shading agents.

Amoebae are widespread in water and serve as environment vectors for pathogens, which may threaten public health. This study evaluated the inactivation of amoeba spores and their intraspore bacteria by solar/chlorine. Dictyostelium discoideum and Burkholderia agricolaris B1qs70 were selected as model amoebae and intraspore bacteria, respectively. Compared to solar irradiation and chlorine, solar/chlorine enhanced the inactivation of amoeba spores and intraspore bacteria, with 5.1 and 5.2-log reduction at 20 min, respectively. The enhancement was similar in real drinking water by solar/chlorine under natural sunlight. However, the spore inactivation decreased to 2.97-log by 20 min solar/chlorine under oxygen-free condition, indicating that ozone played a crucial role in the spore inactivation, as also confirmed by the scavenging test using tert‑butanol to scavenge the ground-state atomic oxygen (O(3P)) as a ozone precursor. Moreover, solar/chlorine induced the shape destruction and structural collapse of amoeba spores by scanning electron microscopy. As for intraspore bacteria, their inactivation was likely ascribed to endogenous reactive oxygen species. As pH increased from 5.0 to 9.0, the inactivation of amoeba spores decreased, whereas that of intraspore bacteria was similar at pH 5.0 and 6.5 during solar/chlorine treatment. This study first reports the efficient inactivation of amoeba spores and their intraspore pathogenic bacteria by solar/chlorine in drinking water.

Solar photolysis of free chlorine (solar/chlorine) in bromide-containing water occurs under various scenarios, such as chlorinated reservoirs and outdoor swimming pools, and the formation of chlorate and bromate is an important issue in the system. We reported unexpected trends for the formation of chlorate and bromate in the solar/chlorine system. Excess chlorine inhibited the formation of bromate, i.e., increasing chlorine dosages from 50 to 100 μM reduced the bromate yield from 6.4 to 1.2 μM in solar/chlorine at 50 μM bromide and pH 7. The yield of bromate in solar/chlorine at 100 μM chlorine and 50 μM bromide in 240 min was 18.8% of that at 50 μM bromine only. The underlying mechanism was that HOCl can react with bromite (BrO2-) to form HOClOBrO-, whose multi-step transformation finally formed chlorate as the major product and bromate as the minor product. This reaction overwhelmed the oxidation of bromite to form bromate by reactive species, such as •OH, BrO• and ozone. On the other hand, the presence of bromide greatly enhanced the formation of chlorate. Increasing bromide concentrations from 0 to 50 μM enhanced the chlorate yields from 2.2 to 7.0 μM at 100 μM chlorine. The absorbance of bromine was higher than that of chlorine, thus the photolysis of bromine formed higher levels of bromite at higher bromide concentrations. Then, bromite rapidly reacted with HOCl to form HOClOBrO- and it further transformed to chlorate. Additionally, 1 mg L-1 NOM had a negligible effect on bromate yields in solar/chlorine at 50 μM bromide, 100 μM chlorine and pH 7. This study demonstrated a new pathway of chlorate and bromate formation in the solar/chlorine system with bromide.

Microcystin-LR (MC-LR) is the most widely distributed and harmful variant toxins released by cyanobacteria, which poses potential threaten to people and aquatic animals when entering natural water. In our research, solar/chlorine process was comprehensively investigated to degrade and detoxify MC-LR. Under the chlorine concentration of 1.0 mg L-1, MC-LR (1.0 μM) was decreased by 96.7%, 26%, and 9% by solar/chlorine process, chlorination, and solar irradiation respectively. Quenching experiments confirmed that reactive chlorine species (RCS) and hydroxyl radical (HO) were the predominant reactive species in solar/chlorine process at neutral condition, and ozone was generated because of the participation of triplet-state oxygen (O(3P)). The respective contributions of each reactive species were calculated with the order as: RCS, HO, ozone, and solar irradiation. The presence of HCO3- and natural organic matter in water inhibited the degradation efficiency of MC-LR. Moreover, the transformation products of MC-LR generated during the solar/chlorine process were identified and a possible pathway was proposed. The hepatotoxicity of MC-LR and its transformation products was compared using protein phosphatase 2A. Our experimental results revealed that the concentration and hepatotoxicity of MC-LR both significantly decreased, and most products were not hepatoxic. Overall, the solar/chlorine process is a promising alternative technology to degrade MC-LR during eutrophication.

This work investigated the feasibility and mechanisms of solar/chlorine process in the removal of a kind of emerging contaminants, lipid regulators (gemfibrozil (GFRZ), benzafibrate (BZF), and clofibric acid (CA)), in simulated and real waters. These lipid regulators could be effectively removed by solar/chlorine treatment, and their corresponding pseudo-first-order rate constants (k\') increased with increasing chlorine dosage. The degradation of GFRZ and BZF was primarily ascribed to reactive chlorine species (RCS) and ozone, while that of CA was mainly attributable to hydroxyl radical (HO) and ozone. As pH rose from 5.0 to 8.4, kozone\' of GFRZ and BZF increased, while kHO\' decreased. However, kRCS\' of GFRZ increased by 130%, while that of BZF decreased by 43.3%. These changes resulted in slight changes in the overall k\'s with increasing pH. k\'s of GFRZ, BZF, and CA by solar/chorine treatment were inhibited by natural organic matter (NOM) while the presence of bromide enhanced the degradation of GFRZ by solar/chlorine process. The degradation of lipid regulators was still effective in a secondary wastewater effluent sample and a sand-filtered water sample, although that was inhibited due to the dissolve organic matter (DOM) contained in real waters. The acute toxicity during the degradation of GFRZ by solar/chlorine treatment was comparable to that by treatment with chlorine alone. This study demonstrated that RCS played an important role in the degradation of micropollutants by the solar/chlorine treatment and the feasibility of solar/chlorine process in the application for the degradation of organic compounds in real waters.

The solar/chlorine process produces multiple reactive species by solar photolysis of chlorine, which can be used as an energy-efficient technology for water treatment. This study investigated the effects of pH and dissolved oxygen (DO) on the degradation of pharmaceuticals and personal care products (PPCPs) and on the formation of disinfection byproducts (DBPs) in the solar/chlorine system. The degradation of 24 structurally diverse PPCPs was enhanced appreciably in the solar/chlorine system compared to solar irradiation and dark chlorination. The reactive species in the solar/chlorine system were identified to be hydroxyl radicals (HO), reactive chlorine species (RCS, i.e., Cl and ClO) and ozone. With increasing pH from 6 to 8, the steady-state concentrations of HO and Cl decreased from 1.23 × 10-14 M to 4.79 × 10-15 M and from 9.80 × 10-16 M to 4.31 × 10-16 M, respectively, whereas that of ClO increased from 5.30 × 10-14 M to 2.68 × 10-13 M and the exposure of ozone increased from 0.44 μM min to 1.01 μM min in 90 min. Accordingly, the removal efficiencies of 6 PPCPs decreased and 11 PPCPs increased. The decreased removal of PPCPs with increasing pH was due to the decrease in HO and Cl, while the increased removal was attributed to the increased ClO and ozone. The presence of DO enhanced the degradation of most PPCPs, indicating the role of ozone on the degradation. The formation of total organic chlorine (TOCl) and known DBPs was enhanced by 60.7% and 159.4%, respectively, in the solar/chlorine system compared to chlorination in a simulated drinking water containing 2.5 mg L-1 natural organic matter (NOM). As the pH rose from 6 to 8, TOCl formation decreased by 16.2%, while that of known DBPs increased by 58.6% in solar/chlorine. The absence of DO slightly suppressed the formation of TOCl and known DBPs. This study illustrated the significant role of RCS in the solar/chlorine system, which enhanced the degradation of micropollutants but increased the formation of chlorinated DBPs.