Novel insights into formation mechanism of organic chloramines from pre-oxidized algae-laden water: Multiple roles of dissolved organic nitrogen

A new concern raised from algal bloom: Organic chloramines in chlorination

Algal blooms have become a significant challenge in water treatment all over the world. In chlorination of drinking water, algal organic matter (AOM) leads to the formation of organic chloramines. The objectives of this review are to comprehensively summarize and discuss the up-to-date researches on AOM-derived organic chloramines and their chemical activities and toxicity, thereby drawing attention to the potentially chemical and hygienic risks of organic chloramines. The predominant algal species in water sources varied with location and season. AOM from cyanobacteria, green algae, and diatoms are composed of diverse composition. AOM-derived amino acids take a low portion of the precursors of organic chloramines. Both experimental kinetic data and quantum chemical calculation demonstrate the preferential formation of organic chloramines in the chlorination of model compounds (amino acids and peptides). Organic chloramines are persistent in water and can transform into dichloro- and trichloro-organic chloramines, unknown low-molecular-weight organic chloramines, and nitrogenous disinfection byproducts with the excess of free chlorine. The active chlorine (Cl+) in organic chloramines can lead to the formation of chlorinated phenolic compounds. Organic chloramines influence the generation and species of radicals and subsequent products in UV disinfection. Theoretical predictions and toxicological tests suggest that organic chloramines may cause oxidative or toxic pressure to bacteria or cells. Overall, organic chloramines, as one group of high-molecular-weight disinfection byproducts, have relatively long lifetimes, moderate chemical activities, and high hygienic risks to the public. Future perspectives of organic chloramines are suggested in terms of quantitative detection methods, the precursors from various predominant algal species, chemical activities of organic chloramines, and toxicity/impact.

Pre-oxidation coupled with charged covalent organic framework membranes for highly efficient removal of organic chloramines precursors in algae-containing water treatment

Organic chloramines in water would pose both chemical and microbiological risks. It is essential to remove the precursors of organic chloramine (amino acids and decomposed peptides/proteins) to limit its formation in disinfection. In our work, nanofiltration was chosen to remove organic chloramines precursors. To solve the "trade-off" effect and low rejection of small molecules in algae organic matter, we synthesized a thin film composite (TFC) nanofiltration (NF) membrane with a crumpled polyamide (PA) layer via interfacial polymerization on polyacrylonitrile (PAN) composite support loaded with covalent organic framework (COF) nanoparticles (TpPa-SO₃H). The obtained NF membrane (PA-TpPa-SO₃H/PAN) increased the permeance from 10.2 to 28.2 L m^-2 h^-1 bar^-1 and the amino acid rejection from 24% to 69% compared to the control NF membrane. The addition of TpPa-SO₃H nanoparticles decreased the thickness of PA layers, increased the hydrophilicity of the membrane, and increased the transition energy barrier for amino acids transferring through the membrane, which was identified by scanning electron microscope, contact angle test, and density functional theory computations, respectively. Finally, pre-oxidation coupled with PA-TpPa-SO₃H/PAN membrane nanofiltration on the limitation of organic chloramines formation was evaluated. We found that the combined application of KMnO₄ pre-oxidation and PA-TpPa-SO₃H/PAN membranes nanofiltration in algae-containing water treatment could minimize the formation of organic chloramines in subsequent chlorination and maintain a high flux during filtration. Our work provides an effective way for algae-containing water treatment and organic chloramines control.

Transfer organic chloramines to monochloramine using two-step chlorination: A method to inhibit N-DBPs formation in algae-containing water treatment

Organic chloramines formed in chlorination of algae-containing water are typical precursors of nitrogenous disinfection byproducts (N-DPBs). The objective to simultaneously enhance the removal efficiency of organic chloramines and control DBP formation remains a challenge. In this study, we report a two-step chlorination strategy for transferring organic chloramines to monochloramine based on the decomposition mechanisms of mono- and di-organic chloramines, which could limit organic chloramines formation and inhibit N-DBPs formation. We demonstrated that two-step chlorination could decrease the organic chloramines formation by nearly 50% than conventional one-step chlorination. Furthermore, two-step chlorination not only blocked the pathway that organic chloramines decomposed to nitriles, but also led to the conversion of organic chloramines to monochloramine. During two-step chlorination of algal organic matter, the organic chloramine transfer proportion decreased by 6.5% and the monochloramine transfer proportion increased by 17.0%. The N-DBP formation, especially haloacetonitriles (HANs), decreased significantly as organic nitrogen became inorganic nitrogen (monochloramine) in two-step chlorination. This work further clarified the process from algal organic matter to N-DBPs, which could expand our understanding of algae-derived organic chloramines removal and DBPs control.