Seawater desalination treatment agents are indispensable in the desalination process. However, few studies have investigated the biological toxicity of the chemical monomers used in these agents, and there is a particular lack of systematic toxicity studies. This toxicity study of a new domestically produced seawater desalination treatment agent provides a basis for risk assessment and its safe use.
The maleic acid polymer scale inhibitor examined in this study is a macromolecular polymer polymerized by maleic anhydride, sodium allylsulfonate, hydroxyethylidene diphosphate (HEDP) and acrylic acid. Maleic anhydride was found to be a skin irritant and corrosive to the eyes in New Zealand rabbits. An acute toxicity study found an oral LD50 of 400 mg/kg in rats and 465 mg/kg in mice (European Chemicals Bureau, 2016; Lewis, 2004). In a 90 day feeding trial, 100 mg/kg exposure to maleic anhydride resulted in kidney damage, and the no observed adverse effect level (NOAEL) was found to be 40 mg/kg. Respiratory exposure increased the number of peripheral white blood cells, slowed weight gain and reduced the phagocytic activity of neutrophils. No carcinogenic or teratogenic effects were observed (European Chemicals Bureau, 2016). Based on the results of experiments in which rats were fed with maleic anhydride for 2 years, the American Conference of Governmental Industrial Hygienists (ACGIH) listed it as a noncarcinogen (ACGIH, 2002). Sodium allylsulfonate is irritating to the eyes, respiratory tract and skin, and it was found to cause peripheral nerve injury in an occupational population (He & Zhang, 1985). HEDP is a low-toxicity chemical and is a commonly used water treatment agent (Franco & Ribeiro, 2020; Qingdao University of Science and Technology, 2005). The LD50 values of HEDP for mice and rats are 2.05 and 1.69 g/kg, respectively. The results of a 6 month rat feeding test showed that rats exposed to 160 and 400 mg/kg HEDP had bone and tooth growth retardation or stagnation and significantly increased urinary calcium excretion; furthermore, rickets-like bone, tooth and tissue decalcification were observed in the histopathology and X-ray examination. There were no apparent toxicity-related effects observed in rats exposed to low-dose HEDP (40 mg/kg and below) (Chen & Yang, 1983). Acrylic acid has low to moderate oral toxicity, and the oral LD50 in rats is 33.5–2500 mg/kg body weight (WHO, 1997). The results of a subchronic toxicity study showed that feeding rats with 150 and 375 mg/kg acrylic acid for 90 days resulted in gastrointestinal swelling with dyspnea, and rats in both groups died after prolonged feeding (European Chemicals Bureau, 2002). The ACGIH lists acrylic acid as a noncarcinogen (ACGIH, 2002). In a European Chemical Administration study on the genotoxicity of acrylic acid, rats were administered 0, 100, 333 or 1000 mg/kg acrylic acid by gavage, and peripheral blood was collected 6, 12 and 24 h later to test chromosome aberrations in bone marrow cells. The results showed that the mitotic activity of the rat bone marrow cells was not affected (European Chemicals Bureau, 2002). This study investigated the toxicity of a maleic acid polymer scale inhibitor. The LD50 in rats was obtained by conducting an acute oral toxicity test. The oral LD50 in male rats was 6810 mg/kg body weight, and in female rats was 9260 mg/kg body weight. The acute oral toxicity of the maleic acid polymer found in this study was significantly lower than that of the monomers found in previous studies.
The effects on rat liver and kidney were observed in a subchronic toxicity test. The liver function results showed that the AST values of the male and female rats were lower than those of the corresponding control groups. AST is an important clinical diagnostic indicator of liver function; in general, an increased AST indicates liver injury, but a decreased AST is not clinically significant (Wang X. H. & Lu, 2013). In this study, the AST values of all rats in each group were within the normal range of Wistar rats (Fan et al., 2010; Tajima, 1989). Taking into account the food consumption and body weight change results, the AST change seen in the rats may have been owing to the rats’ diet, nutrition and metabolism. Therefore, the AST results indicate that the new scale inhibitor did not negatively impact the liver function of rats (Yang & Zhang, 2022).
The UREA values of the rats in all dose groups showed a dose-dependent increase. Compared with the corresponding control group, there was no significant difference in the UREA values of the male and female rats in the low-dose group; however, the medium- and high-dose groups were significantly different from the controls. These results were in line with the principles of the NOAEL and lowest observed adverse effect level (LOAEL) (OECD, 1998). Thus, the UREA value was used as a health effect in this study. The NOAEL and LOAEL values were 1/20 LD50 and 1/10 LD50, respectively, that is, they were 340.5 and 170.25 mg/kg body weight for male rats and 436 and 231.5 mg/kg body weight for female rats.
We also found that the TBIL values of female rats in the high-dose group and of male rats in the medium- and high-dose groups were significantly lower than those of the control group. TBIL is a liver metabolite. It is an indirect bilirubin that is produced after the destruction of red blood cells; it enters the liver and is metabolized into direct bilirubin (Wang X. H. & Lu, 2013). TBIL is the sum of DBIL and IBIL. The DBIL and IBIL results indicated that the main reason for the decrease in TBIL was the decrease in IBIL. In clinical diagnosis, a decreased IBIL is often attributed to physiological reasons such as diet, anemia or other hepatobiliary diseases, but it should be judged in combination with other indicators. In this study, there was no obvious abnormality in the other hepatobiliary-related indicators. In male rats, the red blood cell analyses with obvious differences in the hematological parameters showed a dose-dependent upward trend. However, there were no obvious hematological changes in the female rats. TBIL and IBIL changed but were within the normal bilirubin range for Wistar rats. Therefore, the changes in bilirubin may have been caused by the diet of the rats. Considering the AST, food consumption and body weight results, we believe that the maleic acid polymer scale inhibitor had no significant effect on the liver function of rats. However, the subchronic experimental conditions such as the experimental duration and the diet may have affected nutrient absorption, leading to malnutrition in the rats.
We also analyzed changes in the main blood electrolytes. The blood Na levels were decreased, especially in female rats. Changes in blood electrolytes can be used to indicate the presence of disease. The decrease in blood Na may have been related to renal failure (Wang X. H. & Lu, 2013), which is consistent with the UREA results. The decreased blood Na may have also been related to gastrointestinal dysfunction. In this study, the blood Cl level in the high-dose female rats was also significantly decreased, which may also have been related to gastrointestinal dysfunction and innutrition (Wang X. H. & Lu, 2013). There were also clear changes in the blood Ca and P levels in the medium-dose and high-dose groups, showing an upward dose-dependent trend. The excessive consumption of vitamin D can promote Ca and P absorption in the small intestine, resulting in increased blood Ca and P (Wang X. H. & Lu, 2013). Taken together, the AST and blood electrolyte results suggest that long-term feeding with the macromolecular polymer may have caused gastrointestinal dysfunction in the rats.
In the hematological analysis, for female rats, only immune indicators were changed in the high-dose group, while the medium- and high-dose male rats showed changes in red blood cells, platelets and immune indicators. However, these changes were within the normal range of hematological indicators in Wistar rats of the same age, (Fan et al., 2010; Tajima, 1989); therefore, it was considered that there was no health impact.
To assess genotoxicity, both in vitro and in vivo assays were conducted in this study. After exposing the rats to the maleic acid polymer scale inhibitor, no chromosome aberrations in mouse bone marrow cells were observed. In contrast, the comet assay results showed that medium and high doses may have caused DNA damage in CHO cells. The in vivo metabolic system and the different genotoxicity testing methods can affect the determination of genotoxicity. Therefore, when evaluating safety, a combination of tests should be adopted to evaluate genotoxicity.
In addition to the two genotoxicity experiments, as part of the pre-experiment process, an Ames test was carried out in accordance with OECD Guidelines for Testing of Chemicals (No. 471, Adopted July 21, 1997). In the Ames test, countable revertant colonies were found in a dish exposed to 20 µg of the original solution of maleic acid polymer scale inhibitor, and the colony count was in the abnormal range. Repeated experiments showed background bacteria. After a 10-fold dilution, background bacteria still appeared in all dishes with revertant colonies, and the counts were in the abnormal range. The Ames test results were normal for spontaneous regression, the solvent control and the positive control group. However, the Ames test might not be suitable for assessing the genotoxicity of the new scale inhibitor because of the complexity of the macromolecular components of the maleic acid polymer scale inhibitor. Therefore, this part of the experiment is not described in detail in this paper. The genotoxicity assessment of seawater desalination treatment agents needs to consider the macromolecular structure and sample character. Appropriate research methods should to be further explored to find more suitable genotoxicity assays.