Estimation and impact of carbon dioxide capture on drinking water: Tillmans equilibrium diagram

Global or regional climate changes are significant and their origin may be natural or anthropogenic, for example the emissions of CO₂ from drinking water reservoirs (Saidi & Koschorreck 2017). Increasing numbers of studies are under way on the effects of climate changes, mainly in air, but in water few studies have been reported (Saidi & Koschorreck 2017; Chen et al. 2018). In recent years, CO₂ concentrations in water and soil that may come from the atmosphere, and the gas contained in the pores of rocks have increased significantly. This increment could be considered an important impact from climate change.

High concentrations of CO₂ cause the decrease of the pH of water. Drinking water tends to naturally capture CO₂, which raises the concentrations of some ions such as Ca²⁺, Mg²⁺, Si⁴⁺, K⁺, as well as alkalinity, and the dissolution of carbonate minerals. Elevated levels of CO₂ might dissolve some non-potable elements and compounds of host rocks into groundwater resources (Lemieux 2011). The changes of groundwater hydrochemical conditions, such as increase of total dissolved solids, decrease of pH, change of redox potential, and the desorption process of CO₂ from the soil are the consequences of elevated CO₂ concentrations (Peter et al. 2012; Cahill et al. 2013; Rillard et al. 2014; Yang et al. 2015).

Most water used for human consumption is of good quality; however, in recent decades it has been demonstrated that it is not immune to pollution (Unión Europea 2008), and to climate changes, due to the increase of CO₂ concentration in water. Current regulations do not yet consider the presence of CO₂ in drinking water and the alteration of its quality by the increase of CO₂. The estimation of free, equilibrium, and total CO₂ in water was done using Tillmans equilibrium diagrams with the modified Mojmir Mach model. With this approach it is possible to determine qualitatively and quantitatively the behavior of free, equilibrium and total CO₂ as a function of water temperature and this model can be a very useful tool for the quantification of total carbon dioxide in water for human consumption (Alvarez-Bastida et al. 2013).

The objective of this work was to determine the impact of the natural capture of free CO₂, and its transformation to H₂CO₃, HCO⁻₃ and total CO₂ by using a Tillmans equilibrium diagram obtained from the modified Mojmir Mach model, in water from wells located in urban areas or near a source of carbon dioxide where degradation of organic matter takes place. This model allows us to understand the natural processes and the effect on drinking water with respect to calcium content and considers the modification of the concentrations of Na⁺, Ca²⁺, K⁺ and Si⁴⁺, which may produce adverse effects on health. This is the most important contribution of this work because it is a simple model and useful to determine these chemical species in water; furthermore there are very few studies on this subject.

The aquifer of Toluca Valley (Figure 1) is considered one of the most overexploited in Mexico because of the high rate of groundwater extraction for urban and industrial water supply to Mexico City and Toluca City (Dávila-Hernández et al. 2014).

A two-year sampling program was developed. Ten water samples (S1–S10, in April 2015, June 2015, October 2015, November 2015, January 2016, March 2016, April 2016, June 2016, August 2016 and October 2016, respectively) were taken from three wells, W1, W2 and W3 (Figure 1), which supply water to part of the population of Toluca City.

Water was collected in polyethylene containers for physicochemical characterization. Physicochemical parameters were measured in situ: pH, ambient and water temperature, electrical conductivity and total dissolved solids by using a Hanna potentiometer model H199130. Laboratory analyses were performed to characterize water according to Standard Methods (2005).

Transient parameters were determined in situ and in the laboratory based on standardized and international techniques (Standard Methods 2005; USEPA 2009).

Na, K, Ca, Mg, and Si were determined by atomic absorption spectroscopy (Agilent model 280FS AA).

Carbon dioxide (CO₂) is a greenhouse gas that is found naturally in the atmosphere. The atmospheric concentration of CO₂ is increasing due to human activities and contributes to the global warming of the planet. CO₂ emissions arise from burning fuel and also from industrial processes. Water has the capacity to contain different concentrations of CO₂, which is mainly reflected in acidity (Artioli et al. 2012). As shown in Figure 3, the drinking water samples showed a significant variation of acidity; this behavior could be due to the variation of CO₂ concentrations. Sources of CO₂ include the atmosphere, the aerobic bacterial oxidation of the organic matter contained in the soil, which may migrate to the water, if it cannot escape freely to the atmosphere, and the gas contained in the closed pores of the rocks, which dissolves slowly by diffusion in the water.

Temperature is an important factor that influences the dissolution of the CO₂ and its transformation to carbonic acid. There is a tendency for the acidity of the water to increase with CO₂ gas dissolution (Figure 3) and, in general, the acidity decreases as the temperature increases as observed in Figures 3 and 4. The water from W3 presented the highest temperature (22.6 °C) and the lowest acidity (4.29 mg/L H₂CO₃) of the three wells; this is a common way to diminish the concentration of CO₂ in the water. The acidity of the water decreases in sample 5, because there is more formation of bicarbonates. In the present study, CO₂ gas dissolution has been observed at different water temperatures. These observations support the hypothesis that there are other effects that increase the acidity (16.78 mg/L H₂CO₃) of water and favor the dissolution of CO₂ (such as temperature, chemical composition of water, pH and thus the capture of CO₂).

The highest concentrations of free CO₂ were found in the samples taken in 2015 (samples 1–4) (6.35 mg/L), which favored the decrease in pH (6.36); this behavior agrees with studies on the effect of CO₂ in the subsoil. This species dissolves in water and tends to diminish the pH value; this makes the water acid and susceptible to dissolve other species (Yang et al. 2015). The free and equilibrium CO₂ values were calculated according to the modified Mojmir Mach method.

Figure 5 shows the difference between free and natural capture CO₂ content. Samples 6–10 show a deficiency of CO₂ due to its transformation to bicarbonate, increasing the concentration of this species in the water.

Table 1 shows the amounts of free CO₂ captured by drinking water per year, according to the flow and concentrations of total CO₂ (Equation (22)). The water with the highest concentration of CO₂ is from W2 with 189.89 tonnes of this gas per year.

In this work, different compositions of anions and cations in water were observed due to the natural capture of carbon dioxide. In 2017 El-Naas et al. proposed the modified Solvay method to desalinate water; in this process carbon dioxide is added to reduce emissions and salt concentrations in the water (El-Naas et al. 2017). The natural capture of carbon dioxide takes place according to reactions 29, 30 and 31. This is similar to the modified Solvay process; however, there are some differences in the composition of anions and cations in water. Other reactions take place involving the minerals and subsequent precipitation of carbonate and dissolution of silicate ions because of the increase of Si to about 16 mg/L (reactions 32 and 34). Precipitation of Ca or Mg decreases the carbonate ions concentration to about 14 mg/L and 10 mg/L, respectively (reaction 33) (Guyot et al. 2011).

Concentrations of Ca and Na were analyzed and it was observed that in the process of natural capture of CO₂, the main variations were in the concentrations of Ca and Na. According to Figure 6(a) the results show that as the concentration of calcium increases to 19.95 mg/L, the concentration of sodium decreases (11.18 mg/L) due to the ion exchange proposed in Equation (30). The water in W2 contains the highest concentration of Ca (9.05–19.95 mg/L) and always maintains the lowest concentration of Na (11.18–21.99 mg/L) (Figure 6(b)). Conversely, when the concentration of Ca is lowest (5.89 mg/L and 6.53 mg/L, respectively), the concentration of Na is the highest for wells 1 and 3 (23.42 mg/L and 24.12 mg/L, respectively) (Figures 6(a) and 6(c)). This figure shows that there is a natural change of the soluble ions in water according to the modified Solvay process, which is also carried out with Mg, because it is chemically similar to Ca (Yang et al. 2015) (Equation (32)).

With respect to silicon the results show that high concentrations of CO₂ cause a decrease in the pH of drinking water, which raises the concentrations of Si (reactions 32 and 34) and K, indicating that the dissolution of silicate is a key issue for the conversion of CO₂ into solid minerals (reaction 34), without dissolution of Na (Guyot et al. 2011), as shown in Figure 7. Yang et al. (2015) reported that high levels of CO₂ favor the dissolution of Ca, Mg, K and Si in sediments but do not favor the dissolution of Na. Mean community urinary levels of lithium, magnesium, strontium and silicon showed a direct correlation to the levels of exposure via the drinking water (Dawson et al. 1978).

These changes do not represent a risk to human health according to established regulations for drinking water and human consumption by WHO (2004), USEPA (2009) and Norma Oficial Mexicana (NOM-127-SSA1-2000). However it has been argued that acidic water is corrosive, erodes teeth and bones, filtering calcium and increasing heartburn, can intensify mild hypo due to metabolic acidosis, and aggravate irritable bowel syndrome, while distention of the stomach may reduce the effectiveness of stomach acid to digest those foods that require higher acidity.

This study shows a rapid and effective way to calculate CO₂ in drinking water, using the simplified modified Mojmir Mach model. Some important aspects were investigated, such as the capacity of water to contain different amounts of CO₂. Free CO₂ increases the acidity of water and this is responsible for dissolving cationic and anionic species in drinking water, including bicarbonate, silicon and potassium, and reducing the concentration of calcium. Free CO₂ in drinking water was calculated from Tillman's equilibrium diagram; it is a simple and innovative process. The capture of carbon dioxide determined was 189 tonnes per year, which has important effects on the composition of drinking water, as well as variations in pH value of more than one unit (1.1), acidity, decreased calcium, and increased silicon, which may affect the health of consumers. This method is innovative and easy to use to interpret the capture of carbon dioxide in water.

The authors thank CONACYT for the research grant awarded; UAEMex for the complementary supports granted and project 4303/2017/CI; Facultad de Química of the UAEMex; Laboratorio de Calidad del Agua CIRA; and Organismo de Agua y Saneamiento de Toluca.