Formation of disinfection by-products (DBPs), including dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), chloroform (TCM), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), chloropicrin (TCNM) and chloral hydrate (CH), from monochloramination of chironomid larvae was investigated at different contact times, pH levels and temperatures. Increased monochloramine dosage and prolonged reaction time resulted in an increase in most DBPs. Furthermore, the concentrations of TCAN, DCAN and CH initially increased over time before reaching a plateau. This is because of the hydrolysis of functional groups and the function of NH2Cl. The concentrations of DCAA, TCAA and TCM decreased as pH was increased from 5 to 10, and TCAN, DCAN and CH were not detected when the pH exceeded 8. As for TCNM, their concentration increased as the pH increased from 5 to 7, following a subsequent decrease at higher pH values. High temperatures led to higher DCAN and TCAN concentrations, while low temperatures resulted in higher TCNM and DCAA concentrations. Both high and low temperatures reduced the concentrations of TCAN and CH. Finally, concentrations of the four predominant DBPs (TCM, DCAN of DCAA and TCAA) all decreased as Cl/N ratio declined.
Since trihalomethanes (THMs) were discovered in chlorinated water in the 1970s, disinfection by-products (DBPs) have become a focus of attention in water treatment. There has been an enormous research effort directed at understanding how DBPs are formed in the chlorination or chloramination of drinking water, how these chlorination DBPs can be minimized and whether they pose a public health risk (Hrudey 2009). Many researchers usually considered the natural organic matter (NOM) to be a precursor of DBPs, and most research on DBPs is based on NOM-containing natural bodies of water (Yang et al. 2007; Bougeard et al. 2010).
The recent studies (Plummer & Edzwald 2001; Zhang et al. 2010) otherwise revealed that, besides the humic acid and organic matter, certain bacteria and algae cells with their extracellular organic matter (EOM) could also be the precursors of DBPs. Chlorination of algal cells has been reported to produce THMs and haloacetic acids (HAAs) (Plummer & Edzwald 2001). Chlorination of algal cells that are enriched in organic nitrogen generated higher concentrations of nitrogenous disinfection by-products (N-DBPs) and chloral hydrate (CH), comparable dichloroacetic acid (DCAA) concentrations but much lower concentrations of most carbonaceous disinfection by-products (C-DBPs) than chlorination of NOM (Fang et al. 2010). The complex organic substances contained in bacteria, including fatty acids, proteins and DNA, also resulted in the production of N-DBPs, such as halogenated acetonitriles (HANs). Zhang et al. (2010) additionally noted that three species of bacteria were the possible precursors of HANs, for which the bromide ions played a vital role in N-DBPs generation.
In the 1920s, chironomid larvae were detected in urban water supply systems (Alexander 1997), with per litre of water in the reservoir containing several chironomid larvae or containing up to a dozen pieces. Chironomid, from the dipteran family Chironomidae, is widely distributed in the northern hemisphere at temperate latitudes (Goslan et al. 2009). The first instar larvae are plankton, which can easily penetrate sand filters and then enter reservoirs and municipal service pipes due to their motility (Van Lieverloo et al. 2004; Cui et al. 2004). The frequent occurrence of chironomid larval densities has rapidly increased in drinking water treatment systems, causing problems to water supply (Sun et al. 2007). Chironomid larvae are large in size compared with algae and bacteria, and this therefore suggests that the contained biomass of amino acids, protein, fat and other organic matter have a higher potential to form DBPs. Although there are no indications that these organisms pose a threat to public health, their presence is still not desirable for hygiene (Van Lieverloo et al. 2004). So, it is interesting to determine how the metabolites produced by these organisms affect water safety and the production of DBPs.
How the DBPs of chironomid larvae intracellular organic matter (IOM) affect the water safety, the answer is unknown. The aim of this work was to assess the roles of chironomid larvae IOM in the formation of a number of commonly found DBPs by chloramination under various conditions. The experimental factors included the monochloramine concentration, reaction time, pH, temperature and Cl/N ratio.
Materials and methods
Chemicals and materials
All chemical solutions were prepared from reagent-grade chemicals and deionized water. A preformed monochloramine (NH2Cl) solution was freshly prepared prior to experiments by adding an aliquot of free chlorine solution to an ammonium chloride solution at a chlorine to NH4+-N molar ratio of 0.8:1 for 30 min with rapid stirring (Yang et al. 2007). Buffer solutions were prepared by adding reagent grade phosphates to deionized water. Phosphates were used for experiments performed at pH 5, 6, 7, 8, 9 and 10 (Tianjin Chemical Plant, China). A mixed standard containing calibration standards, internal standards and surrogate standards for the analyses of THM, nine HAAs (HAA9), HAN, chloral hydrate (CH) and trichloronitromethane (TCNM) were obtained from Supelco.
Culturing of chironomid larvae
Chironomid larvae, initially collected from temporary water bodies in the vicinity of waterworks in Harbin and bred in our laboratory, were cultured in an aerated 25 L glass aquaria filled with tap water. A 5 cm thick artificial sediment layer consisting of washed siliceous sand and cellulose was introduced at the bottom of the aquarium. Adult midges were confined using wooden cages covered with 1 mm mesh metal net. Aquaria were kept at a constant temperature (20 °C) and exposed to a consistent photoperiod (14 h light/10 h dark). To obtain homogenous samples (with respect to both size and age), egg masses were transferred from rearing aquaria into 2 L glass experimental tanks filled with aerated tap water. Egg masses were left for 24 h in these tanks, and non-hatched eggs were then removed. The fourth instar larvae was therefore used in the following study.
Preparation of the chironomid larvae IOM
A 1 L beaker mixed 200 ml of deionized water and five hundred active chironomid larvae of similar size. The lethal chironomid larva was using Thermostat water bath at 35 °C and 30 min. The five hundred dead larvae were placed into another beaker containing 1 L of deionized water to be broken in Ultrasonic cell disruptor. The supernatant was filtered with 0.45 μm Whatman membrane filter. IOM were taken for analysis of total organic carbon (TOC). Standard samples were prepared by diluting reagents to 4 mg/L.
The monochloramine concentration was measured by DPD/FAS titration (APHA–AWWA–WEF 1998). TOC measurements were made using the UV-persulfate oxidation method (APHA–AWWA–WEF 1998). Analyses of chloroform (TCM), CH, DCA, 1,1-DCP, 1,1,1-TCP, dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), TCNM and HAA9 were carried out with a gas chromatograph (GC) (Agilent 6890) with an electron capture detector (ECD), based on United States Environmental Protection Agency (USEPA) Method 551.1 (USEPA 1995) and USEPA Method 552.3 (USEPA 2003). The USEPA regulated formation of four THM species in drinking water. The USEPA currently sets limits for total THMs at 80 μg/L and for five haloacetic acids (HAAs) at 60 μg/L, while in the EU a 100 μg/L limit applies to total THMs (Bond et al. 2014). The column used was an HP-5 fused silica capillary column (30 mm × 0.25 mm I.D. with 0.25 mm film thickness). For HAA9 analysis, the samples were pretreated using an extraction/derivatization procedure with methyl tert-butyl ether (MTBE) and acid methanol according to USEPA method 552.3 (USEPA 2003). The injector, ECD and GC oven temperature program for compounds other than HAA9 was as follows: injector temperature: 200 °C; ECD temperature: 290 °C; and oven temperature: 35 °C for 9 min, ramp to 40 °C at 2 °C/min, hold for 8 min, ramp to 80 °C at 20 °C/min, ramp to 160 °C at 40 °C/min and hold for 4 min. The temperature program for the HAAs was as follows: injector temperature: 210 °C; ECD temperature: 290 °C; and oven temperature: 30 °C for 20 min, ramp to 40 °C at 1 °C/min, ramp to 205 °C at 20 °C/min and hold for 4 min. The THM, CH and HAN concentrations were measured by a liquid–liquid extraction procedure using MTBE and acid methanol according to USEPA Method 551.1 (USEPA 1995). The GC-ECD operating conditions were as follows: detector temperature, 290 °C; injector temperature, 200 °C; injection volume, 1 mL; and temperature program, 35 °C for 5 min, ramped to 75 °C at 10 °C/min, held for 5 min, ramped to 100 °C at 10 °C/min, and then held for 2 min.
Monochloramine experiments making chironomid larvae IOM of TOC as 4 mg/L were carried out in aluminum-foil-wrapped glass vials with Teﬂon-faced septa. The ratio of monochloramine:TOC was 5:1. All samples were buffered to pH 7.5 with phosphate buffer before chloramination to maintain the pH. All reacting samples were stored head-space free in the dark at room temperature of 20 ± 1 °C. Then all samples use termination agent to stop chloramination before analysis. The influencing multiple factors of chloramination are reaction time, monochloramine concentration, pH, temperature and Cl/N ratio, and the respective levels were as follows: reaction time (6, 12, 24, 36, 48, 72 h), monochloramine concentrations (1, 2, 4, 6, 8, 10 and 20 mg/L as Cl2), pH values (5, 6, 7, 8, 9 and 10), temperature (10, 20 and 30 °C), and Cl/N ratio (1/0, 25/1, 10/1, 5/1, 4/1, 3/1 and 2/1).
RESULTS AND DISCUSSION
Effect of the monochloramine dosage
Figure 1 shows DBP formation upon treatment with different concentrations of monochloramine at pH 7.5. Among the tested DBPs, the concentrations of DCAA and TCAA were highest while that of TCNM were lowest. Most DBPs, except TCNM and CH, monotonically increased with increasing monochloramine dosages. TCAN and DCAN production evidently increased when the monochloramine dosage was 10 mg/L, and the incremental trend of TCM concentration was very weak with the monochloramine concentration above 6 mg/L. This indicates that the free chlorine contributes to the level of HAAs increased. Alkaline hydrolysis of haloacetonitriles can form haloacetamides (and ultimately HAAs). For example, DCAN can hydrolyze to form dichloroacetamide, which can further hydrolyze to form DCAA (Bond et al. 2011). The TCNM concentration was around the level of 0.35 μg/L irrespective of the monochloramine dosage, which indicates that the reaction to form TCNM is mainly controlled by the quantities of the precursors and not by the monochloramine dosage. Phenolic amines, as well as amino acids, can act as TCNM precursors in natural waters. TCNM was formed at a yield of 53% from 3-nitrophenol, an oxidized analogue of 3-aminophenol. A rate-limiting step for HNM formation from both chlorination and chloramination of 3-aminophenol is transforming the amino group to a nitro. Ozonation–chlorination of several model amines dramatically increased the formation of TCNM relative to chlorination alone (Bond et al. 2014). Also, as to the relatively stable CH (Fang et al. 2010), the concentration increased significantly from 1 to 6 mg/L and then decreased with an increase in the monochloramine dosage. This means that the accumulation of some typical DBPs, including THMs, HAAs and HANs, could be attributed to its stable chemical structure in monochloramine solutions (Yang et al. 2007). These DBP precursors reacted with disinfectant completely at the certain higher dosage of oxidative chemical.
Effect of reaction time
Figure 2 shows the results of time-dependent formation of DBPs after the monochloramination of chironomid larvae IOM. DCAA and TCAA concentrations were the highest among the tested DBPs, followed by CH and TCAN, whereas the concentrations of TCM, DCAN and TCNM were low and at the level of several μg/L. In addition to TCNM and CH, all other DBPs monotonically accumulated with the extension of time. The concentration of TCM, DCAA and TCAA increased steadily and that of TCNM fluctuated in the range of 0.30 μg/L. More than 90% CH was generated within 24 h, and TCAN and DCAN concentrations grew relatively slowly in the first 12 h but rapidly increased after 24 h. THMs, HAAs and CH were all stable products (Fang et al. 2010), so their concentrations increased with reaction time. DCAN concentration was observed increasing could be due to the relative of DCAN concentration in monochloramine solutions (Yang et al. 2007). These experimental results showed that as the monochloramine concentrations changed, the changing trends for the DBP concentration were all similar, and this similarity is likely due to the similar reaction mechanisms.
Effect of pH
Figure 3 shows the concentrations of DBPs after 48 h treatment with monochloramine at various pH values from 5 to 10. The DCAA and TCAA concentrations decreased slightly with increasing pH, and then maintained a stable trend. These phenomena are consistent with the stabilities of these chloramines. Because monochloramine is more stable at high pH values, the reaction between monochloramine and DBPs precursors was slow; therefore, a smaller amount of DBPs was generated. The TCM concentration remained unchanged. The concentrations of TCNM increased with pH values ranging from 5 to 7, but the TCNM levels significantly decreased at a pH of 8 and then remained stable. The concentrations of CH, TCAN and DCAN decreased markedly and could not be detected at pH 8 and above. This pattern can be explained by increased hydrolysis at higher pH values, with TCAN having the fastest rates of hydrolysis, followed by DCAN. In addition, this is also related to higher pH promoting the aldehyde pathway of amino acid chlorination, which would favor TCM formation. It therefore deduced that pH played a role in the speciation of chloramines, monochloramine hydrolysis and therefore the stability of DBPs, during the monochloramination. Yang et al. (2007) showed NH2Cl was the dominant chloramine at pH 7.5 or above, and NHCl2 was dominant chloramine at pH 5 or below, in which the concentrations of DCAN, TCAN, CH and TCNM were lower with more NHCl2 than NH2Cl. Hydrolysis of monochloramine to form free chlorine was also affected by pH (Jolley & Carpenter 1983), and the hydrolysis rate was slow when pH ranged from 7.5 to 9. On the other hand, pH affected the stability of unstable DBPs. DCAN underwent base-catalyzed hydrolysis decomposition (Sun et al. 2007). The hydrolysis rates of unstable DBPs increased with increasing pH (APHA–AWW–AWEF 1998), and this resulted in much lower DCAN and TCAN concentrations at high pH.
Effect of temperature
Figure 4 shows the results of DBP formation after 48 h of monochloramination at three designated temperatures of 10, 20 and 30 °C. Formation of TCNM decreased with temperature increasing from 10 to 30 °C, while the DCAN, TCM and TCAA changed in an opposite way. The CH and TCAN have the highest concentration at 20 °C. There was no obvious decreasing trend for DCAA concentration, and the concentrations of TCM and TCAA remained almost unchanged with an increase in temperature from 10 to 20 °C but increased significantly at 30 °C. Generally speaking, temperature affects not only the formation rate but also the decomposition rate, therefore, the concentrations of DBPs at different temperatures were dependent on the balance of the DBP formation rate and the decomposition rate. The NH2Cl self-degradation reaction rate constant will be increased with rise in temperature, which results in the concentration of NH2Cl decreasing with increasing temperature. So a lower temperature is favorable to the storage of NH2Cl solution. However, the main reason, due to the reduction of DBPs, is the hydrolysis reaction. Hydrolysis reaction is an endothermic reaction, so higher temperature is beneficial to the instability of DBP hydrolysis. As shown in Figure 4, the DCAA concentration increased but that of TCAA decreased. Competitive reactions may occur between the DCAA precursor and the TCAA precursor with monochloramine, and this is why the formation of DCAA was faster than that of TCAA at higher temperatures (Glezer et al. 1999).
Effect of the Cl/N ratio
Figure 5 shows the results of four typical DBP formations after 48 h of chloramination under baseline conditions with different Cl/N mass ratios of 1/0, 25/1, 10/1, 5/1, 4/1, 3/1, 2/1. The concentrations of these four DBPs all decreased as the Cl/N ratio decreased. When the Cl/N ratio was 1/0, the concentrations of these four DBPs were respectively 8.41, 3.36, 1.35 and 1.50 times than those under the Cl/N ratio of 5/1. The larger quantity of HAAs formed at an initial Cl/N ratio of 5/1, and was partially attributable to the higher free chlorine concentration at a higher Cl/N ratio (Qi et al. 2004). The conversion of free chlorine into monochloramine decreased the formation rates of most DBPs, and the result is in agreement with the data reported by Zhang et al. (2000).
The bench-scale experiments were conducted to evaluate the DBPs of chironomid larvae IOM with monochloramine. The study showed that the disinfection by-products generated the largest amount of trihalomethanes at 7 μg/L, haloacetic acids generate a maximum amount of 20 μg/L, which all are below the detection limit. Therefore the disinfection by-products from the monochloramination of chironomid larvae do not affect the water safety. But in the reservoir, in addition to chironomid larvae, as well as NOM, algae, humus and other substances exist. They are also the precursors of disinfection by-products, which may generate a large amount of DBPs. These circumstances require it to get attention.
The results showed that most DBPs accumulated with reaction time extension and monochloramine dosage increase. CH followed an increasing and then decreasing pattern with monochloramine dosage increase. The concentrations of DCAA and TCAA were highest among all the DBPs (6–10 μg/L), and this should be given more attention. Therefore, contact reaction time should be considerably controlled in order to reduce the quantity of DBPs.
The pH affected the formation of DBPs in the different modes, where TCM, DCAN and TCNM concentrations increased initially and then remained decreasing as pH was elevated, whereas DCAA, TCAA, CH and TCAN concentrations kept on declining in the pH range of 5–8. Higher temperature enhanced TCM, DCAN and DCAA formation but weakened TCNM formation under 0.3 μg/L. The CH and TCAN concentrations increased first and then decreased.
The Cl/N is also an essential factor for chloramine disinfection. The four typical DBPs (TCM, DCAN of DCAA and TCAA) concentrations all decreased with the decline of Cl/N ratio.