Molecular investigation on changing behaviors of natural organic matter by coagulation with non-targeting screen using high-resolution mass spectrometry

Recognition of screening out hierarchical toxic contaminants tuned by quantified pseudo-components from complex engineering co-combustion

Co-combustion of industrial and municipal solid wastes has emerged as the most promising disposal technology, yet its effect on unknown contaminants generation remains rarely revealed due to waste complexity. Hence, six batches of large-scale engineering experiments were designed in an incinerator of 650 t/d, which overcame the inauthenticity and deviation of laboratory tests. 953-1772 non-targeted compounds were screened in fly ash. Targeting the impact of co-combustion, a pseudo-component matrix model was innovatively integrated to quantitatively extract nine components from complex wastes grouped into biomass and plastic. Thus, the influence was evaluated across eight dimensions, covering molecular characteristics and toxicity. The effect of co-combustion with biomass pseudo-components was insignificant. However, co-combustion with high ratios of plastic pseudo-components induced higher potential risks, significantly promoting the formation of unsaturated hydrocarbons, highly unsaturated compounds (DBE≥15), and cyclic compounds by 19 %- 49 %, 17 %- 31 %, and 7 %- 27 %, respectively. Especially, blending with high ratios of PET plastic pseudo-components produced more species of contaminants. Unique 2 Level I toxicants, bromomethyl benzene and benzofuran-2-carbaldehyde, as well as 4 Level II toxicants, were locked, receiving no concern in previous combustion. The results highlighted risks during high proportion plastics co-combustion, which can help pollution reduction by tuning source wastes to enable healthy co-combustion.

Feasibility of intimately coupled CaO-catalytic-ozonation and bio-contact oxidation reactor for heavy metal and color removal and deep mineralization of refractory organics in actual coking wastewater

Considering the challenges for both single and traditional two-stage treatments, advanced oxidation and biodegradation, in the treatment of actual coking wastewater, an intimately coupled catalytic ozonation and biodegradation (ICOB) reactor was developed. In this study, ICOB treatment significantly enhanced the removal of Cu2+, Fe3+, and color by 39 %, 45 %, and 52 %, respectively, outperforming biodegradation. Catalytic ozonation effectively breaking down unsaturated organic substances and high-molecular-weight dissolved organic matter into smaller, more biodegradable molecules. Compared with biodegradation, the ICOB system significantly increased the abundances of Pseudomonas, Sphingopyxis, and Brevundimonas by ∼ 96 %, ∼67 %, and ∼ 85 %, respectively. These microorganisms, possessing genes for degrading phenol, aromatic compounds, polycyclic aromatics, and sulfur metabolism, further enhanced the mineralization of intermediates. Consequently, the ICOB system outperformed biodegradation and catalytic ozonation treatments, exhibiting chemical oxygen demand removal rate of ∼ 58 % and toxicity reduction of ∼ 47 %. Overall, the ICOB treatment showcases promise for practical engineering applications in coking wastewater treatment.

Effect of surface functional groups of polystyrene micro/nano plastics on the release of NOM from flocs during the aging process

Currently, the hidden risk of microplastics in the coagulation process has attracted much attention. However, previous studies aimed at improving the removal efficiency of microplastics and ignored the importance of interactions between microplastics and natural organic matter (NOM). This study investigated how polystyrene micro/nano particles impact the release of NOM during the aging of flocs formed by aluminum-based coagulants Al13 and AlCl3. The results elucidated that nano-particles with small particle sizes and agglomerative states are more likely to interact with coagulants. After 7 years of floc aging, the DOC content of the nano system decreased by more than 40%, while the micron system did not change significantly. During coagulation, the benzene rings in polystyrene particles form complexes with electrophilic aluminum ions through π-bonding, creating new Al-O bonds. NOM tends to adsorb at micro/nano plastic interfaces due to hydrophobic interactions and conformational entropy. In the aging process, the structure of PS-Al13 or PS-AlCl3 flocs and the functional groups on the surface of micro/nano plastics control the absorption and release of organic matter through hydrophobic, van der Waals forces, hydration, and polymer bridging. In the system with the addition of nano plastics, several DBPs such as TCAA, DCAA, TBM, DBCM and nitrosamines were reduced by more than 50%. The reaction order of different morphological structures and surface functional groups of microplastics to Al13 and AlCl3 systems is aromatic C-H > C-OH > C-O > NH2 > aromatic CC > aliphatic C-H and C-O>H-CO> NH2 >C-OH> aliphatic C-H. The results provided a new sight to explore the effect of micro/nano plastics on the release of NOM during flocs aging.

Unraveling the characteristics of dissolved organic matter removed by aluminum species based on FT-ICR MS analysis

Given the complexity of dissolved organic matter (DOM) and its interactions with coagulant chemicals, the mechanisms of DOM removal by aluminum (Al) coagulants remains a significant unknown. In this study, six test waters containing DOM with molecular weight (MW, <1 kDa, 1-10 kDa and >10 kDa) and hydrophobicity (hydrophilic, transphilic and hydrophobic) were prepared and coagulated with Al0, Al13 and Al30. The molecular-level characteristics of DOM molecules that were removed or resistant to removal by Al species were analyzed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that at the molecular level, saturated and reduced tannins and lignin-like compounds containing abundant carboxyl groups exhibited higher coagulation efficiency. Unsaturated and oxidized lipids, protein-like, and carbohydrates compounds were relatively resistant to Al coagulation due to their higher polarity and lower content of carboxyl groups. Al13 removed molecules across a wider range of molecular weights than Al0 and Al30, thus the DOC removal efficiency of Al13 was the highest. This study furthers the understanding of interactions between Al species and DOM, and provides scientific insights on the operation of water treatment plants to improve control of DOM.

Compositional and structural identification of organic matter contributing to high residual soluble aluminum after coagulation

Understanding the complexation of aluminum (Al) with dissolved organic matter (DOM) is of great significance for the control of residual Al in drinking water after treatment. Here, we used high-resolution and accurate mass measurements to identify the composition and structure of DOM contributing to the formation of soluble organically-bound Al during coagulation at near neutral pH (pH 7.50). The results showed that the organic compounds contributing to soluble organically-bound Al were primarily phenolic compounds and aliphatic compounds. Among them, phenolic compounds with a sulfonic acid group could greatly enhance the hydrolysis of polymeric Al and the formation of high concentrations of monomeric/oligomeric Al-DOM complexes. These organic molecules had a mass-to-charge ratio concentrated below 350. Based on the assumption that oxygen-containing functional groups providing unsaturation in the molecular structure were carboxyl groups, it was inferred that the maximum number of carboxyl groups in phenolic compounds and aliphatic compounds was concentrated between 1-2 and 2-4, respectively. The presence of these molecules was responsible for soluble organically-bound Al accounting for over 80 % of the total soluble Al in the supernatant after coagulation in this study. These findings deepen the understanding of the complexation of Al with DOM. In drinking water treatment plants, the combination of coagulation with processes that can remove such characteristic organics is beneficial for controlling residual Al.

Formation and transformation of pre-chlorination-formed disinfection byproducts in drinking water treatment process

As pre-chlorination is increasingly adopted in drinking water treatment plant (DWTP), an attractive question emerged: how the disinfection by-products that formed during pre-chlorination (preformed DBPs) would be transformed in the drinking water treatment process? This study investigated the DBP formation kinetics and molecular characteristics in chlorinated source water, DBP transformation and removal in practical DWTP. It was found that the formation of trihalomethanes (THMs) followed pseudo first-order kinetic model and the intensified Br- exposure facilitated the transformation of TCM into TBM. As Br- concentration shifted from 0.5 mg L-1 to 2.0 mg L-1, the predicted maximum yield of TBM was doubled to 53.7 μg L-1 with the increase of formation rate constant (k-value) from 0.249 h-1 to 0.336 h-1. Besides known DBPs, the molecular-scale investigation unveiled that the preformed unknown Cl-DBPs were a cluster of unsaturated aromatic DBPs ((DBE-O)/Cwa = 0.16, AImod, wa = 0.36) with high H/C (H/Cwa = 1.25). Pre-ozonation exhibited a preferential removal pattern towards condensed aromatic preformed Cl-DBPs with high H/C (AImod ≥ 0.67, H/C > 1.2 and O/C < 0.3). However, the removal of Cl-DBPs in coagulation-clarification process was limited with 56 more unknown Cl-DBP formulas identified. O3-biological activated carbon process exhibited effective removal of preformed DBPs featured with low MW (carbon number ≤ 13), high unsaturation (DBE ≥ 7), condensed aromaticity (AImod ≥ 0.67), and higher H/C (H/C > 1.6). When the pre-chlorination process is adopted, the removal of preformed DBPs during the conventional treatment process is limited, while advanced treatment process can effectively remove these preformed DBPs.

Effects of dissolved organic matter removal and molecular transformation in different water treatment processes on formation of disinfection byproducts

Alterations in molecular composition of dissolved organic matter (DOM) during water treatments can influence the composition and toxicity of disinfection by-products (DBPs) in subsequent chlorination disinfection process. In this study, the impacts of DOM composition after various water treatment techniques (coagulation, adsorption, nanofiltration, biological aerated filter (BAF), and their integrated processes) on the generation mechanisms of DBPs were comprehensively explored by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in combination with GC-MS and LC-MS analysis. The results indicated that coagulation preferentially removed unsaturated (low H/C) and oxidized (high O/C) compounds, whereas adsorption was prone to remove the reduced (low O/C) component that was more reactive with chlorine, leading to lower yields (μg DBP/mg DOC) of trihalomethanes (THMs) and haloacetic acids (HAAs) during subsequent chlorination. The coagulation-adsorption technique exhibited a relatively high removal of both known and unknown DBPs, demonstrating that coagulation and adsorption were complementary for DOM removal at the molecular level. Nanofiltration selectively removed molecules with relatively high O/C, however, those with very low O/C that were more reactive with chlorine could pass through the nanofiltration membrane, resulting in the highest yields of THMs and HAAs. Although BAF was inefficient in removing DBPs precursors, it could convert molecules with low degree of oxidation and unsaturation into highly oxidized and unsaturated ones, thereby significantly enhancing the removal of DBPs precursors in the subsequent coagulation-adsorption process. These findings are instrumental in developing and selecting more effective techniques to minimize the formation of DBPs in water treatment.

Impact of primary treatment methods on sludge characteristics and digestibility, and wastewater treatment plant-wide economics

Biogas production from anaerobic sludge digestion plays a central role for wastewater treatment plants to become more energy-efficient or even energy-neutral. Dedicated configurations have been developed to maximize the diversion of soluble and suspended organic matter to sludge streams for energy production through anaerobic digestion, such as A-stage treatment or chemically enhanced primary treatment (CEPT) instead of primary clarifiers. Still, it remains to be investigated to what extent these different treatment steps affect the sludge characteristics and digestibility, which may also impact the economic feasibility of the integrated systems. In this study, a detailed characterization has been performed for sludge obtained from primary clarification (primary sludge), A-stage treatment (A-sludge) and CEPT. The characteristics of all sludges differed significantly from each other. The organic compounds in primary sludge consisted mainly of 40% of carbohydrates, 23% of lipids, and 21% of proteins. A-sludge was characterized by a high amount of proteins (40%) and a moderate amount of carbohydrates (23%), and lipids (16%), while in CEPT sludge, organic compounds were mainly 26% of proteins, 18% of carbohydrates, 18% of lignin, and 12% of lipids. The highest methane yield was obtained from anaerobic digestion of primary sludge (347 ± 16 mL CH₄/g VS) and A-sludge (333 ± 6 mL CH₄/g VS), while it was lower for CEPT sludge (245 ± 5 mL CH₄/g VS). Furthermore, an economic evaluation has been carried out for the three systems, considering energy consumption and recovery, as well as effluent quality and chemical costs. Energy consumption of A-stage was the highest among the three configurations due to aeration energy demand, while CEPT had the highest operational costs due to chemical use. Energy surplus was the highest by the use of CEPT, resulting from the highest fraction of recovered organic matter. By considering the effluent quality of the three systems, CEPT had the highest benefits, followed by A-stage. Integration of CEPT or A-stage, instead of primary clarification in existing wastewater treatment plants, would potentially improve the effluent quality and energy recovery.

Molecular insights towards changing behaviors of organic matter in a full-scale water treatment plant using FTICR-MS

The changing behavior of organic matter in a full-scale water treatment process was characterized based on the three-dimensional excitation-emission matrix (3D-EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Polyaluminum chloride (PAC) as a coagulant can help to effectively remove soluble microbial by-products-like and aromatic protein-like substances during coagulation and sedimentation, corresponding to tannin and coagulated aromatic regions. The leakage of soluble microbial products during sand filtration resulted in an increase in the intensity of biomass-like regions. Nitrogen-containing compounds have higher weighted average value of double bond equivalents (DBE_w) and the modified aromaticity index (AI_mod-w) than nitrogen-free compounds. Water treatment can preferentially remove unsaturated nitrogen-containing compounds with more O atoms and higher-oxidation-state carbon. The dissolved organic carbon (DOC) and UV₂₅₄ were not correlated well with changes in nitrogen-containing compounds due to the preferential removal of nitrogen-containing compounds. This study revealed the specificity of organic matter removal during water treatment, and it was helpful in optimizing treatment processes for various raw water to ensure water quality.

Fractionation, molecular composition, and biological effects of organic matter in bio-stabilization sludge with implication to land utilization

Bioactive organic compounds (BOCs) contained in bio-stabilized products of waste activated sludge (WAS) have attracted considerable attention, as they can enhance the fertilizing effect of WAS in land applications. This study investigated the molecular composition and plant-growth-promoting mechanisms of various BOCs in the bio-stabilized products of WAS. After stepwise fractionation, aerobic composting sludge (ACS) and anaerobic digestion sludge (ADS) were chemically fractioned into five subcomponents, namely dissolved organic matter (DOM) (C1), weakly interacted organic matter (OM) (C2), metal-bonded OM (C3), NaOH-extracted OM (C4), and strongly interacted OM (C5), in sequence. The results showed that fatty acids and carboxylic acid (CAs) present in ACS C2 promoted plant growth and enhanced the ability of plants against stresses by upregulating pathways related to "carbohydrate metabolism," "lipid metabolism," "amino acid metabolism," and "phenylpropanoid biosynthesis." However, in ACS C4, plenty of amino acids could promote plant growth via upregulating "carbohydrate metabolism" and "amino acid metabolism" pathways. As an important precursor, aromatic amino acids inside ACS C4 also stimulated the production of indoleacetic acids. In ADS C1, amino sugar and phytohormone were the major BOCs causing the up-regulation of "carbohydrate metabolism" and AAA catabolism in "amino acid metabolism" pathways. CAs enriched in ADS C2 stimulated plant growth through "amino acid metabolism" pathway. In summary, alkali extraction can recycle a large proportion of BOCs with low environmental risk from the bio-stabilization products of WAS. The results from this study provide scientific guidance for safe and value-added resource utilization of bio-stabilization products of WAS in land applications.

Molecular-level characterization of natural organic matter in the reactive electrochemical ceramic membrane system for drinking water treatment using FT-ICR MS

Applications of electrochemical advanced oxidation processes are rising in drinking water treatment for effective mitigation of refractory organic compounds. This study explored the fate of natural organic matter (NOM) (lake water and standard NOM (SRNOM solution)) at molecular-level in the reactive electrochemical membrane (REM) system utilizing Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Fluorescence spectroscopy showed above 90 % removal of the humic-like component in both lake water and SRNOM solution in 10 min of REM operation compared to 70-80 % removal of the fulvic-like component after 30 min. REM-based treatment effectively eliminated (>70 %) the disinfection byproduct precursors. The lake water, sharing ~70 % of similar compounds with SRNOM, displayed a different propensity toward electrochemical oxidation, and its finished water was characterized with relatively lower double-bond equivalent (DBE), nominal oxidation state of carbon (NOSC), and aromaticity compared to that of SRNOM. The chloride ions in the water matrix of lake water impacted the electrochemical oxidation and generated significantly different transformation products than SRNOM solution. The heteroatoms (N and S) containing compounds (CHON and CHOS) were preferentially degraded in lake water; however, CHOS compounds were removed fewer in SRNOM. The electrosorption and electrochemical oxidation on the REM surface were the significant contributors for NOM removal. The newly formed compounds were mostly retained on the REM surface and fewer were released in finished water. This study is believed to help understand the fate of NOM in real source drinking water during electrochemical treatment.

Molecular transformation of dissolved organic matter and the formation of disinfection byproducts in full-scale surface water treatment processes

Dissolved organic matters (DOM) have important effects on the performance of surface water treatment processes and may convert into disinfection by-products (DBPs) during disinfection. In this work, the transformation of DOM and the chlorinated DBPs (Cl-DBPs) formation in two different full-scale surface water treatment processes (process 1: prechlorination-coagulation-precipitation-filtration; process 2: coagulation-precipitation-post-disinfection-filtration) were comparatively investigated at molecular scale. The results showed that coagulation preferentially removed unsaturated (H/C < 1.0 and DBE > 17) and oxidized (O/C > 0.5) compounds containing more carboxyl groups. Therefore, prechlorination produced more Cl-DBPs with H/C < 1.0 and O/C > 0.5 than post-disinfection. However, the algal in the influent produced many reduced molecules (O/C < 0.5) without prechlorination, and these compounds were more reactive with disinfectants. Sand filtration was ineffective in DOM removal, while microorganisms in the filter produced high molecular weight (MW) substances that were involved in the Cl-DBPs formation, causing the generation of higher MW Cl-DBPs under post-disinfection. Furthermore, the CHO molecules with high O atom number and the CHON molecules containing one N atom were the main Cl-DBPs precursors in both surface water treatment processes. In consideration of the putative Cl-DBPs precursors and their reaction pathways, the precursors with higher unsaturation degree and aromaticity were prone to produce Cl-DBPs through addition reactions, while that with higher saturation degree tended to form Cl-DBPs through substitution reactions. These findings are useful to optimize the treatment processes to ensure the safety of water quality.

Molecular insights into the transformation of refractory organic matter in landfill leachate nanofiltration concentrates during a flocculation and O3/H2O2 treatment

During leachate treatment, molecular information regarding the completely removed, partially removed, less-reactive, increased, and produced parts of dissolved organic matter (DOM) remains unknown. This study applied ESI FT-ICR MS to investigate the transformation characteristics of leachate nanofiltration concentrate (NFC) DOM during a combined flocculation-O₃/H₂O₂ process. The NFC contained 5069 compounds in four main classes (CHO, CHON, CHOS, and CHONS compounds). The DOM number decreased to 4489 during flocculation and to 2903 after the O₃/H₂O₂ process. During flocculation, the completely and partially removed DOM was mainly low-oxygen unsaturated and phenolic compounds. Saturated DOM was produced and remained in the flocculated effluent. During the O₃/H₂O₂ process, the completely and partially removed DOM were mainly low-oxygen unsaturated and phenolic compounds that were mainly in a reduced state. Flocculation can remove many (condensed) aromatic compounds, and methylation and hydrogenation reactions occurred during flocculation. In the O₃/H₂O₂ process, dearomatization, demethylation, carboxylation, and carbonylation reactions further achieved the degradation of DOM that was resistant to flocculation. Overall, the combined flocculation-O₃/H₂O₂ process collectively eliminated a broader range of DOM than the single processes could achieve. The results of this study provide an in-depth understanding of DOM transformation in an NFC treatment.

Coagulation behavior of polyaluminum-titanium chloride composite coagulant with humic acid: A mechanism analysis

The hydrolysate species of metal-based coagulants and the binding sites of humic acid (HA) are highly dependent on the pH conditions. Exploring the binding sites and modes between coagulant hydrolysates and HA molecules is critical to understanding the coagulation mechanism. In this paper, the binding behavior between HA molecules and the hydrolysates of a polyaluminum-titanium chloride composite coagulant (PATC) was investigated under different pH conditions by semi-quantitative FTIR and XPS. It was found that oligomeric and mesopolymeric hydrolysates were the dominant species under acid conditions, which could complex with the hydroxyl and carboxyl groups of HA by forming COAl/Ti coordinate bonds. However, large amounts of H⁺ could compete with Al³⁺ and weaken the removal efficiency of HA. With the increase of pH, the hydrolysis process of the PATC and the deprotonation of HA were simultaneously underway. Under weakly acid conditions, the complexation of the aluminum hydrolysates with carboxyl groups was improved due to the gradually diminishing competition of H⁺ and the enhanced charge neutralization of the further polymerized hydrolysates. Consequently, the maximum UV₂₅₄ removal by adding PATC was observed at pH 6. Under alkaline conditions, the sweeping effect of amorphous hydroxide dominated the HA removals, which was accompanied by the surface complexation of Al/Ti nuclear with carboxyl groups as well as the hydrogen bonds between hydroxyl and hydroxide. This study provides a new clue for the interaction mechanisms between the hydrolysates of composite coagulants and the dominant functional groups of HA at various pH conditions.