Challenges and Issues of Using Phosphoric Acid as an Absorption Liquid in Membrane-Based Ammonia Removal

Membrane-based ammonia removal technology has been widely applied in wastewater treatment, gas separation, and resource recovery. Selecting a suitable absorption liquid is crucial for improving ammonia removal efficiency. Common absorption liquids include sulfuric acid, nitric acid, and hydrochloric acid, which are strong acids. Phosphoric acid (H₃PO₄), a polyprotic acid, has been proposed as a potential alternative. However, experiments and practical applications have revealed several challenges, making phosphoric acid less ideal for this process. 

1. Rapid pH Fluctuation, Difficult to Control
Phosphoric acid is a polyprotic weak acid, and it dissociates in aqueous solutions as follows:
[H₃PO₄ ⇌ H⁺ + H₂PO₄⁻]
[H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻]
[HPO₄²⁻ ⇌ H⁺ + PO₄³⁻]
When phosphoric acid absorbs ammonia (NH₃), a neutralization reaction occurs, forming ammonium phosphate salts (NH₄H₂PO₄, (NH₄)₂HPO₄, etc.). However, due to the complex dissociation equilibrium of phosphoric acid, the pH fluctuates rapidly, making it difficult to maintain a stable absorption efficiency. Additionally, significant pH fluctuations can negatively affect membrane performance, making the process difficult to control.

2. Limited Absorption Capacity, Low Ammonia Removal Efficiency
Phosphoric acid is weaker than strong acids (e.g., sulfuric acid, hydrochloric acid), resulting in lower ammonia absorption capacity. Generally, stronger acids provide a greater driving force for ammonia removal. Due to its relatively low acid dissociation constants (Ka), phosphoric acid:
• Has weaker ammonia absorption capacity, making it less effective in capturing ammonia.
• Requires a higher concentration to achieve a reasonable ammonia removal rate, increasing chemical consumption and operational costs.
In contrast, strong acids such as sulfuric acid can efficiently absorb ammonia even at lower concentrations, making them more suitable for this application.

3. Unstable Reaction Products, Potential Ammonia Release
After reacting with ammonia, phosphoric acid produces ammonium phosphate salts (e.g., ammonium dihydrogen phosphate NH₄H₂PO₄, diammonium hydrogen phosphate (NH₄)₂HPO₄). However, these salts can decompose under certain conditions, leading to secondary ammonia release:
[(NH₄)₃PO₄ ⇌ NH₃ + (NH₄)₂HPO₄]
This instability reduces the long-term effectiveness of ammonia removal, making the process less reliable. Additionally, ammonium phosphate salts may further decompose under high temperatures or fluctuating pH conditions, decreasing the overall efficiency of the system.

4. Membrane Fouling and Scaling Risks
Ammonium phosphate has limited solubility in water and can precipitate under high concentration or pH conditions. This can lead to:
• Membrane surface fouling: Crystallization of ammonium phosphate on the membrane surface reduces mass transfer efficiency.
• Membrane pore blockage: Precipitated deposits may clog membrane pores, lowering operational performance and increasing cleaning and maintenance costs.

In contrast, strong acids like sulfuric acid and nitric acid do not produce similar precipitation issues, making them more favorable for industrial applications.

5. Poor Economic Viability and Practical Feasibility
The economic viability of phosphoric acid in membrane-based ammonia removal also presents challenges:
• Higher cost: Compared to common absorption liquids like sulfuric acid and nitric acid, phosphoric acid is more expensive and requires higher dosages, increasing operational costs.
• Limited regenerability: Once phosphoric acid absorbs ammonia, recovery or regeneration is difficult, whereas strong acids like sulfuric acid can be regenerated through heating or stripping processes, improving cost efficiency.
Due to these factors, phosphoric acid is generally not an economical choice for large-scale ammonia removal applications.

Conclusion
While phosphoric acid can theoretically be used in membrane-based ammonia removal, its rapid pH fluctuation, limited absorption capacity, unstable reaction products, membrane fouling risks, and poor economic viability make it less suitable for practical applications. Currently, sulfuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl) are more commonly used due to their higher efficiency and stability.
When selecting an absorption liquid, it is essential to consider chemical stability, ammonia removal performance, and long-term operational costs to optimize the overall process.to resist.