Connecting the alkaline ‘sink’ to the atmospheric carbon ‘source’ in eutrophic water treatment

Due to human activities, large volumes of nitrogen (N) and phosphorus (P) nutrients enter waterbodies, creating eutrophication with seriously imbalanced nutrient ratios (Kharbush et al. 2023). Eutrophication seriously damages aquatic ecosystems, leading to the increase of algal biomass and potential algal toxins, reduction in light penetration and dissolved oxygen (DO), mass death of fish, and loss of biodiversity (Moal et al. 2019; Vasseghian et al. 2024). Water pollution and eutrophication have been estimated to cause economic losses of $6–16 billion and more than 3 million deaths globally every year (Zhu et al. 2024). Therefore, finding economic and efficient methods to mitigate eutrophication is the future development direction.

Typically, eutrophication is addressed by restricting the influx of both endogenous and exogenous nutrients. Measures for removing excess N and P from water include biological, chemical, and physical methods (Yin et al. 2018; Zhu et al. 2020; Fan et al. 2021). Biological methods, such as microbial remediation, biofilm technologies, constructed wetlands, and aquatic phytoremediation, are limited by the environmental adaptability of microorganisms and often require long treatment cycles (Zhou et al. 2022). Chemical methods, involving copper sulfate (CuSO₄), herbicides, and algicides, face challenges related to high costs, the generation of secondary pollution, and potential toxicity to non-target aquatic organisms, humans, livestock, and wildlife (Zhang et al. 2020). Physical methods, such as dilution, flushing, deep aeration, and sediment dredging, suffer from issues such as high energy consumption, limited treatment capacity, and unstable effectiveness (Zhang et al. 2020). Therefore, the development of innovative and more effective treatment approaches is urgently needed.

Meanwhile, carbon limitation also plays a significant role in influencing phytoplankton community structure in eutrophic waters. High dissolved inorganic carbon (DIC)/total nitrogen (TN) and DIC/total phosphorus (TP) ratios enhance photosynthesis and inhibit cyanobacteria blooms (Bao et al. 2022). Elevated CO₂ levels can alter the community composition of aquatic vegetation, promote the growth of chlorophytes, Bacillariophyta, and submerged plants and decrease the toxicity of harmful algal blooms (Lai et al. 2023).

Climate change has also become a serious global environmental problem. There is a global consensus on the need to curtail greenhouse gas emissions, particularly CO₂ emissions (Zheng et al. 2019). However, as concentrations continue to rise, simply limiting emissions is insufficient, since atmospheric CO₂ levels are expected to triple by the end of the 21st century from the current 409.95 μatm (Vilar & Molica 2020). Reducing atmospheric CO₂ and its adverse environmental impacts is critically important but challenging (Kumar et al. 2020).

Thus, CO₂ plays a key role in two major environmental challenges: eutrophic water and climate change. Studies have demonstrated significant negative CO₂ emissions in reservoir and lake systems (Wang et al. 2021). Utilizing atmospheric CO₂ to enhance carbon sequestration in eutrophic water could be an ecologically friendly and economical way to restore aquatic ecosystems, helping achieve carbon neutrality and indirectly address climate change. Aquatic plants mainly utilize DIC in water, including CO₂, ⁠, and ⁠, which are significantly influenced by pH (Tao 2017; Liu 2023). However, the precise role of pH in regulating atmospheric CO₂ absorption in eutrophic water bodies has not been explored.

In this study, we conducted experiments with ultrapure water and two types of eutrophic water (lake water and river water) to investigate the effects of pH conditions on DIC content. This study sought to address the following research questions: (1) What is the effect of different pH conditions (3.5–12.0) on the amount of CO₂ dissolved in eutrophic water? (2) Is there an ideal pH condition for eutrophic water treatment? (3) Can the amount of CO₂ (an important greenhouse gas) absorbed resulting from pH variations during the process of eutrophic water treatment be quantified?

To remove dissolved CO₂ from ultrapure water, it was boiled for 15 min and then cooled under sealed conditions. Subsequently, 1 mL of 0.1 mol/L KCl solution was added to 1 L of cooled ultrapure water to maintain the ionic concentration, resulting in an initial DIC content of 4.49 mg/L. The eutrophic water samples were sterilized using a portable pressure steam sterilizer (YX-280, Hefei Huatai Medical Equipment Factory, China) for 30 min under 0.15 MPa pressure to remove dissolved CO₂ and prevent disturbance from original organisms. After sterilization, the eutrophic water samples were filtered with non-woven cloth and cooled under sealed conditions. The initial DIC contents of the lake and river water were determined to be 14.57 and 18.94 mg/L, respectively.

From each water sample, 2,000 mL of cooled water was siphoned into separate 2,000 mL beakers. Different concentrations of H₂SO₄ (0.01, 0.05, 0.1, and 1.0 mol/L) and NaOH (0.01, 0.05, 0.1, and 1.0 mol/L) were then added dropwise to adjust the pH values. The pH was continuously monitored during the adjustment process to maintain the desired value. The initial pH range, from 3.5 to 12.0, was selected based on the carbonate equilibrium curve, with intervals of 0.5 ± 0.03 for each kind of water sample, resulting in a total of 54 treatments. All treatments were repeated three times, and mixed samples were obtained for analysis.

Once the desired pH was achieved, DO and electrical conductivity (EC) were immediately determined. The treated samples were then divided into two 1,000 mL beakers and placed in a static water bath at 25 ± 1 °C to simulate standard ambient temperature. One group of samples was used to determine pH, DO, and EC, while the other group was used to determine the contents of CO₂, ⁠, and ⁠. These measurements were conducted every 24 h for 3 days. The total content of CO₂, ⁠, and was calculated as the DIC content (⁠⁠).

It is essential to acknowledge that while NaOH served as a pH regulator in our laboratory study, its application in actual field settings requires consideration of weak alkaline agents with low corrosion and strong buffering capacity. Notably, cement has been demonstrated to effectively reduce Phosphorus content in water and regulate pH levels (Liu et al. 2020). To study the impact of pH on water quality in eutrophic systems, we investigated a eutrophic river restoration project located in Chengdu, China (Zhang et al. 2021). The river section has stone revetments built with mortar, and there is no vegetation along the banks. The water body is turbid, with a large amount of floating garbage and visible oil films on the surface. At some sewage discharge points, the water appears black-gray and emits a foul smell. In this 1.5 km long eutrophic river, a mixture of 80% ordinary Portland cement and 20% bentonite was applied at a dosage of 5 g/m³. Three monitoring sites were established in the upper, middle, and lower reaches of the 1.5 km river section, spaced approximately 0.75 km apart. The water quality was monitored monthly from December 2020 to June 2021. Key parameters monitored included pH, DO, TP, TN, ammonia nitrogen (NH₃-N), and chemical oxygen demand (COD_Mn).

The determination of pH, DO, and EC was performed using a PHB-4 Portable pH Meter, a JBP-607A Portable DO Meter, and a DDS-307A Conductivity Meter, all from Shanghai Leici Instrument Factory, China. TP, dissolved phosphorus (DP), TN, NH₃-N, and COD_Mn were determined by standard methods (APHA 2012). The CO₂ content in water was determined by a fixed method of titration, with detection values of ≥0.004 mg/L and an accuracy of ±0.5 mg/L. The and contents were determined by double mixed indicator titration, with detection values of ≥0.003 mg/L and an accuracy of ±1.2% (Liu 1981).

Statistical analyses were performed using Microsoft Excel, Origin 2018, and SPSS 27.0. A multivariate analysis of variance (MANOVA) was conducted to assess the significance of differences in DIC content across different pH treatments, time points, and water types. Pearson correlation was used to analyze linear relationships between pH levels and other indexes, while Spearman correlation was employed for non-linear relationships or data sets that did not meet normality assumptions. Statistical significance was set at P < 0.05.

After 3 days, the difference between the determined pH and initial pH (ΔpH) showed comparable trends in both ultrapure and eutrophic water (Figure 3(d)). Positive ΔpH was observed when the initial pH of ultrapure water was 3.5–6.0 and eutrophic water was 3.5–7.0. Conversely, negative ΔpH occurred for ultrapure water with an initial pH value of ≥6.5 and eutrophic water with an initial pH value of ≥7.5. The maximum ΔpH values were 0.63 (ultrapure, initial pH 5.0), 1.42 (lake, initial pH 5.0), and 1.31 (river, initial pH 5.5). The minimum ΔpH values were −2.69 (ultrapure, initial pH 10.0), −2.14 (lake, initial pH 10.5), and −2.40 (river, initial pH 10.5).

Results showed that the initial pH had minimal influence on the DO content, with no significant changes observed with increasing pH levels (P > 0.05). Similarly, the water bath time had little impact on DO content, with variations within 0.6 mg/L over 1 day. For ultrapure water, the DO contents were 7.2–7.6 mg/L (1 day), 7.4–7.7 mg/L (2 days), and 7.6–7.8 mg/L (3 days). For lake water, the DO contents were 6.0–7.8 mg/L (1 day), 6.4–7.9 mg/L (2 days), and 6.5–8.0 mg/L (3 days). For river water, the DO contents were 6.8–7.7 mg/L (1 day), 6.8–8.0 mg/L (2 days), and 6.9–8.1 mg/L (3 days). This stability can be attributed to the absence of materials in the solutions that would consume oxygen.

Before the restoration, the river's water quality parameters were as follows: TP 0.16–9.35 mg/L, TN 10.9–27.6 mg/L, NH₃-N 2.02–25.8 mg/L, and COD_Mn 7.3–36.0 mg/L. After the restoration, TP, TN, NH₃-N, and COD_Mn concentrations were reduced to 0.133, 1.156, 0.076, and 5.13 mg/L, respectively, indicating a great improvement in water quality.

In this study, a clear exponential correlation between DIC content and pH level was observed. The DIC content exceeded the initial DIC content when the initial pH value was ≥7.5. For DIC alone, higher pH levels are preferable. Notably, when the initial pH value was ≥10.5, the EC of both lake and river water significantly exceeded the original EC before treatment. Elevated EC in aquatic ecosystems can disrupt the osmotic and ionic balance of organisms, impair critical physiological processes such as respiration and osmoregulation, and adversely affect growth and reproduction (Morgan et al. 2012). EC serves as a crucial indicator of water quality, reflecting the concentration of total dissolved solids and serving as a measure of water salinity (Canedo-Arguelles et al. 2013). Salinity can impact osmotic pressure in living cells and the normal growth of organisms (Sampaio & Bianchini 2002; Geng et al. 2016; He et al. 2017). Increased salinity has been shown to reduce species richness and abundance in aquatic plants and zooplankton communities (Nielsen et al. 2003; Bielanska-Grajner & Cudak 2014). Therefore, the EC should be controlled within a certain range. Additionally, pH levels directly affect the growth and reproduction of aquatic organisms (Barth & Wilson 2010; Kim et al. 2015). Elevated pH levels can also stress aquatic organisms, especially those with narrow pH tolerance ranges. For example, fish from the alkaline parts of a lake (pH 9.90) exhibited nearly three-fold higher plasma ammonia levels compared to fish from neutral waters, highlighting a significant disruption in ammonia excretion under such conditions (Scott et al. 2005). Phytoplankton are the most important primary producers in aquatic ecosystems (Weyhenmeyer et al. 2013; Yu et al. 2015). Studies indicated that within a pH range of 7.5–9.0, the average total phytoplankton biomass in eutrophic water increases with rising pH (Tian et al. 2016). Meanwhile, according to the Environmental Quality Standard of Surface Water in China, the acceptable pH ranges from 6.0 to 9.0 (SEPA 2002). Considering these factors, maintaining a pH value of around 9.0 for eutrophic water treatment is optimal, aligning with the suitable pH level for aquatic life and maximizing CO₂ absorption into the water.

Substantial input of N and P eutrophic water generally exhibits a distinct imbalance in the proportion of nutrient elements. In this study, the initial C/N ratios of the two eutrophic water samples were 0.78 (lake) and 1.25 (river), and the C/P ratios were 7.25 (lake) and 12.65 (river), indicating significant carbon deficiency compared to the Redfield Ratio of 106:16:1 (Redfield 1960). For instance, Taihu Lake in Jiangsu Province, a typical eutrophic lake in China, has water quality indices of pH 8.2, COD_Mn 6.61 mg/L, TN 2.25 mg/L, and TP 0.133 mg/L, with a C/ N/ P ratio of 49.7:16.9:1 (Zhang et al. 2011). In wastewater treatment, industrial glucose with approximately 99% purity is commonly utilized as a carbon source additive (Wang et al. 2014), priced at around US$280/t (Wujiang Minghao Chemical Co., China 2023). Assuming the eutrophication of Taihu Lake (total volume of 4.44 × 10⁹ m³) could be alleviated by adding carbon sources, about 25,830 t of industrial glucose would be required to balance the deficient carbon proportion for US$7,232,400. In contrast, if NaOH (US$320/t) (Wujiang Minghao Chemical Co., China 2023) with a purity of about 98.5% were added to adjust the pH to 9.0, atmospheric CO₂ would naturally enter the water, fulfilling the necessary carbon addition. This approach would only require 1,517 t of NaOH, costing US$485,440, which is approximately 6.7% of the expense associated with adding industrial glucose.

Traditional techniques require aeration and the supplementation of organic carbon sources, resulting in high treatment costs (Gu et al. 2023; Wang et al. 2025). In contrast, pH adjustment enables atmospheric CO₂ to enter water bodies, promotes the growth of photosynthetic microorganisms, and strengthens the self-purification capacity of water. This approach not only eliminates the need to add carbon sources but also requires no energy consumption. Therefore, adjusting water pH to alkaline conditions is a more economical approach to increase the carbon content in water, alleviating eutrophic water pollution with lower costs and no secondary pollution.

Moreover, pH-induced CO₂ absorption not only contributes to carbon sequestration but also helps mitigate eutrophication by increasing the availability of DIC for photosynthetic organisms. This increased carbon availability promotes the growth of autotrophic organisms, which can assimilate excess nitrogen and phosphorus, further alleviating eutrophication. Nevertheless, our findings suggest that regulating the pH of eutrophic water to alkalinity may potentially reduce atmospheric CO₂ while concurrently alleviating eutrophic water pollution.

The findings of this study are based on controlled laboratory experiments and limited field conditions, which may not fully represent the complexity of natural aquatic ecosystems. To improve the generalizability and robustness of our conclusions, large-scale studies encompassing broader spatial and temporal scales and diverse environmental settings should be conducted to validate these results and account for potential variability.

In this study, biological interference was eliminated to more accurately quantify dissolved CO₂. Future studies should specifically consider the presence and impact of aquatic organisms when investigating the influence of water pH on atmospheric CO₂ dissolution. Moreover, the applicability of this approach requires additional in-depth research and supporting data to determine whether increasing the alkalinity of eutrophic waterbodies will further damage or enhance their limnological features and ecosystems. Subsequent studies should also focus on how to capture or isolate the organic matter produced by increased DIC content to ensure atmospheric CO₂ reduction. Additionally, the potential impact of excessive DIC on the water ecosystem warrants further consideration. Future research should focus on sustainable and cost-effective approaches, exploring the possibility of leveraging the power of natural ecosystems to enhance CO₂ absorption and sequestration.

The nutrient imbalance in eutrophic water can be addressed by adding carbon sources to promote the growth of aquatic organisms and alleviate eutrophication. Based on our controlled laboratory and the field test, we propose the following preliminary conclusions:

• (1) An alkaline pH effectively increases DIC content, with a significant positive correlation observed between pH levels and DIC concentrations (P < 0.05).

• (2) Considering salinity, the carbon source, and the growth of aquatic organisms, a pH level of approximately 9.0 is optimal for eutrophic water treatment under the tested conditions.

• (3) Adjusting pH levels to alkaline conditions facilitates the absorption of atmospheric CO₂ into water. At pH 9.0, lake water and river water absorbed an additional 5.06 and 5.81 mg/L of CO₂, respectively, within 3 days, compared to pH 7.0. This process provides a natural carbon source for alleviating eutrophication and contributes to the reduction of atmospheric CO₂, a major greenhouse gas.

• (4) A field test in a 1.5 km eutrophic river section confirmed positive correlations between pH and C/N (P < 0.05) and C/P (P > 0.05), indicating the potential effectiveness of alkaline pH in promoting balanced nutrient ratios.

Combining solutions to mitigate eutrophication and reduce atmospheric CO₂ provides a promising strategy for addressing major environmental challenges, including climate change and ecosystem degradation. However, this study has certain limitations, such as the exclusion of biological interference and the lack of large-scale validation. Further studies are required to validate these findings in diverse water bodies and climatic conditions and to assess the ecological impacts of sustained alkaline pH levels on aquatic biodiversity and water chemistry.

We thank Prof. Yonghong Shui and her team from Chengdu Textile College for the data test and laboratory support. We also extend our thanks to Dunlian Qiu and Prof. Iain Taylor for editing assistance.

Y.Z. performed conceptualization, methodology, and formal analysis, wrote the original draft, and reviewed and edited the manuscript. B.L. carried out conceptualization and methodology and wrote the reviewed and edited manuscript. L.L. did the formal analysis and wrote the reviewed and edited manuscript. B.C.S. wrote the reviewed and edited manuscript.

This study was financially supported by the National Natural Science Foundation of China (51879174) and the Wastewater Treatment Technology Development Program (18H0364) commissioned by Sichuan Environmental Protection and Treatment Engineering Co. Ltd.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.