Low Molecular Weight Organics Research Articles

Synthesis and mechanism of Bi/SnO2 for enhanced photocatalytic activity in the degradation of humic acid under visible light

Humic acid (HA) is a typical refractory organic matter, widely existing in natural water and many types of wastewaters. In this study, Bi/SnO2 composites were synthesized by a simple one-step hydrothermal method for photocatalytic degradation of HA. Due to the surface plasmon resonance (SPR) effect of Bi, the light absorption of Bi/SnO2 composites expands from UV to visible light compared to pure SnO2. Under visible light irradiation for 120 min, the UV254 removal reached 93.58 %, color number (CN) removal reached 94.41 %. The degradation rate constant of Bi/SnO2 (0.0178 min−1) is 14.83 times higher than that of SnO2 (0.0012 min−1), indicating the superior photocatalytic degradation efficiency of Bi/SnO2 composites. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was found that photocatalysis mainly degraded most CHO-containing organic matter and reduced the amount of organic matter (from 7240 to 3886). The molecular number of organic compounds with m/z of 400–500 and 500–600 exhibited a significant decrease, specifically by 1052 and 770, respectively. This observation suggests that the photocatalytic process effectively eliminates low and moderate molecular weight organics. The molecular number, aromaticity and molecular weight of dissolved organic matter (DOM) were reduced mainly by carboxylic acid reaction, dealkylation reaction and oxygen addition reaction.

Biochar promotes microbial CO2 fixation by regulating feedback inhibition of metabolites

Chemoautotrophs, the crucial contributors to biological carbon fixation, derive energy from reducing specific inorganic substances and utilize CO2 for growth. However, the release of extracellular free organic carbon (EFOC) by chemoautotrophic microorganisms can inhibit their own growth and metabolism. To reduce the feedback inhibition effect, a low-release biochar (BC-LR) was applied to adsorb EFOC. BC-LR not only adsorbed EFOC, but also selectively adsorbed the main inhibitory component, low molecular weight organics, in EFOC. In contrast, ordinary biochar could not effectively adsorb EFOC and its addition inhibited microbial growth and CO2 fixation. In Transwell culture, BC-LR promoted microbial growth by 190% and CO2 fixation by 29%, and exhibited better economic advantage, when compared with granular activated carbon. These findings provide a novel insight into the interaction between biochar and autotrophic microbial metabolism, offering an economically feasible approach to mitigate feedback inhibition of metabolites and promoting biological CO2 fixation.

Surface functionalization of polytetrafluoroethylene membrane by polydopamine and quaternary ammonium compound for fouling mitigation in membrane distillation of secondary effluent

Considering the worldwide water scarcity issue, membrane distillation (MD) offers a promising solution for producing reclaimed water from secondary effluent generated by municipal wastewater treatment plants. However, prolonged operation of MD systems can result in membrane fouling, which impairs separation efficiency and shortens the membrane's lifespan. To overcome these challenges, novel surface-modified polytetrafluoroethylene (PTFE) membranes have been developed through a combination of polydopamine deposition and quaternary ammonium compound (QAC) grafting (named as PDQ membranes). The objectives of this preparation are to reduce the membrane hydrophobicity, impart anti-adhesive and bactericidal properties, and improve water permeability and antifouling performance in the MD system. In particular, the membrane coated by a QAC solution with the concentration of 40 mM (named as M3 membrane), achieved a flux of 47.0 LMH, surpassing the original PTFE membrane by 11.4%. During the long-term direct contact membrane distillation operation with the secondary effluent, the M3 membrane exhibited significantly superior antifouling capability than the PTFE membrane. Undergoing three runs of the MD process, the flux reduction of M3 was only 38.1%, whereas the flux of original PTFE membrane experienced a 71.4% decline. The PDQ membranes were also capable of providing stable distillate quality (over 40 h). Notably, the M3 demonstrated a high rejection capability, with an impressive removal efficiency of 96.9% for low molecular weight organics and 92.5% for humus-like substances. Additionally, the PDQ membrane exhibited good anti-bioadhesion and bactericidal properties. This study highlights the promising application potential of the PDQ membrane for fouling mitigation in water and wastewater treatment.

Catalytic ozonation of reverse osmosis concentrate from coking wastewater reuse by surface oxidation over Mn-Ce/γ-Al2O3: Effluent organic matter transformation and its catalytic mechanism

Degradation of organics in high-salinity wastewater is beneficial to meeting the requirement of zero liquid discharge for coking wastewater treatment. Creating efficient and stable performance catalysts for high-salinity wastewater treatment is vital in catalytic ozonation process. Compared with ozonation alone, Mn and Ce co-doped γ-Al2O3 could remarkably enhance activities of catalytic ozonation for chemical oxygen demand (COD) removal (38.9%) of brine derived from a two-stage reverse osmosis treatment. Experimental and theoretical calculation results indicate that introducing Mn could increase the active points of catalyst surface, and introducing Ce could optimize d-band electronic structures and promote the electron transport capacity, enhancing HO• bound to the catalyst surface ([HO•]ads) generation. [HO•]ads plays key roles for degrading the intermediates and transfer them into low molecular weight organics, and further decrease COD, molecular weights and number of organics in reverse osmosis concentrate. Under the same reaction conditions, the presence of Mn/γ-Al2O3 catalyst can reduce ΔO3/ΔCOD by at least 37.6% compared to ozonation alone. Furthermore, Mn-Ce/γ-Al2O3 catalytic ozonation can reduce the ΔO3/ΔCOD from 2.6 of Mn/γ-Al2O3 catalytic ozonation to 0.9 in the case of achieving similar COD removal. Catalytic ozonation has the potential to treat reverse osmosis concentrate derived from bio-treated coking wastewater reclamation.

Fate of low molecular weight organics in paddy vs. upland soil: A microbial biomarker approach

Low-molecular-weight organic carbon (LMWOC) from root exudate influences soil organic carbon cycling via priming of microbial activity. However, the mechanisms underlying the uptake and utilization of specific exudates by microorganisms in soils remain unclear. To address this gap in knowledge, a one-month 13C (0.1 mg C﹒g soil) tracer incubation study was conducted to investigate the fate of the most abundant root exudate groups (using 13C-labeled glucose, acetic acid, and oxalic acid) in paddy vs. upland soil. After 2 days of incubation, the microbial substrate use efficiency (SUE) was >80% in paddy soil, which was approximately 1.9, 2.9, and 1.3 times that in uplands with glucose, acetic acid, and oxalic acid addition, respectively. The SUE in paddy soil with glucose or acetic acid addition was always higher than that in uplands over time (P < 0.05). In both soils, the SUE of glucose was 1–4 times that of carboxylic acids (P < 0.05). The recovery of 13C-labeled total phospholipid fatty acids (PLFAs) in paddy soils was 1.5–2 times that in uplands (P < 0.05). In both soils, bacteria preferred to utilize glucose and acetic acid to synthesize cellular components. Throughout the incubation period, bacteria dominated over fungi in terms of LMWOC consumption. Gram-positive and -negative bacteria were dominant in upland and paddy soils, respectively. From days 11–30, the contribution of fungi and actinomycetes to LMWOC utilization began to appear. Temperature positively regulated 13C distribution in microbial groups (P < 0.05), and increased dissolved organic carbon in upland soil accelerated microbial SUE. The results of this study clarify microbial effects on the high soil carbon sequestration capacity of paddy soil as compared with upland in subtropical areas.

Isolation and identification of olive tree leaf phenols through a resin adsorption/desorption process

Agricultural by-products contain substantial amounts of phenolics with significant antioxidant properties. In this work, a method to recover high concentrations of phenolic substances from a typical agricultural by-product, such as olive tree leaves, was investigated. Phenolic compounds previously separated through extraction and fractionated on a membrane array: Ultrafiltration (UF)-Nanofiltration (NF)-Reverse Osmosis (RO), were examined, alongside standard compounds. More specifically, the NF concentrate fraction, which contained the low molecular weight organics (simple phenols, carbohydrates) was treated by sorption/desorption processes in a cylindrical column packed with XAD16 N resin. The aim of this study was to optimize the isolation and the enrichment of phenolic compounds through their selective adsorption on resin grains. Standard compounds such as gallic acid and glucose were tested to study the sorption and desorption kinetics of phenol and carbohydrate families and the sorption parameters were estimated using the Thomas model. The results showed that XAD16 N resin selectively adsorbed phenolic compounds over carbohydrates. This was evident by the adsorption capacity exhibited by the resin when treating the glucose and gallic acid solutions. Regarding the NF concentrate fraction of olive leaf phenols, after the adsorption step, the adsorbed hydrophilic carbohydrates were removed by desorption using pure water. Immediately afterwards, an alcohol/water mixture was introduced to recover and extract the phenolic compounds. To identify the distinct phenols, a thorough liquid chromatography study was performed on the sorption and desorption samples collected at the outlet of the packed column, identifying the different time frame each phenol left the system.

Removal of dissolved organic matter via a combination of UV/persulfate oxidation and biological activated carbon (BAC) process

The combination process of UV/peroxydisulfate (UV/PDS) oxidation and BAC (UV/PDS-BAC) is examined as an advanced treatment for conventional drinking water treatment through a continuous flow experimental apparatus with actual sand filter effluent as feed water. The effect of three key parameters (UV dosage, PDS dosage, and water temperature) on dissolved organic matter (DOM) is evaluated via long-term monitoring data. The removal behavior of DOM in the UV/PDS-BAC process is also compared with that in the O3-BAC process. Further, this study delves into the mechanisms behind the enhanced DOM removal achieved by UV/PDS-BAC. The achieved results reveal that impressive removal rates of 42.1 % for total organic carbon (TOC) and 62.9 % for UV254 could be achieved in the presence of specific conditions: a UV input of 200 mJ/cm2 and a PDS dosage of 0.5 mM. The efficiency of DOM removal by UV/PDS-BAC exhibits a positive correlation with the UV dosage ranging from 50 to 200 mJ/cm2, and the water temperature in the range of 4–30 ℃. While increasing the PDS dosage from 0.01 to 0.5 mM results in improved DOM removal, any further increase yields reduced removal efficiency. In comparison, subjected to similar operating conditions, UV/PDS-BAC outperforms O3-BAC in removing DOM. The superior DOM removal achieved by the UV/PDS-BAC process can be essentially attributed to several factors: firstly, it exhibits excellent mineralization capacity for low molecular weight organics (<1 kDa) and leads to increased biodegradable fraction of organics by oxidatively degrading DOM with a molecular weight >3 kDa. Secondly, the appropriate exposure of activated carbon pores is chiefly maintained by suitably controlling the biomass. Lastly, the enhanced biodegradability of BAC influent stimulated the growth of bacteria proficient in degrading DOM.

High content of low molecular weight organics does not always affect pharmaceutical adsorption on activated carbon: The case of acetate, propionate and ethanol in source-separated urine

Adsorption on activated carbon is a common process to remove pharmaceuticals in wastewater treatment. Activated carbon adsorption is usually applied to wastewater with a low content of biological degradable organics, i.e. after biological treatment. Especially low molecular weight (LMW) compounds are known to compete with pharmaceuticals for adsorption sites. The goal of this study was to test the hypothesis that biological treatment is necessary for efficient pharmaceutical removal. Source-separated urine after anaerobic storage (anaerobically stored urine) and after aerobic biological removal of organics without nitrification (organics-depleted urine) were used in this study. In anaerobically stored urine 60% of the organic compounds were LMW organics, of which about 40% were acetate and propionate. 74% of the DOC and 100% of acetate and propionate were removed during aerobic biological treatment. To investigate the effect of the organic compounds on pharmaceutical removal, sorption experiments with 19 spiked pharmaceuticals and one artificial sweetener were conducted with powdered activated carbon. Ethanol, another LMW organic, was included in the study, as it is regularly used for pharmaceutical spiking thereby strongly increasing the DOC content. The experiments showed that the adsorption of the pharmaceuticals and the sweetener were hardly affected by the easily biodegradable LMW organics or ethanol. Therefore, it was concluded that biological pre-treatment is not necessary for efficient pharmaceutical adsorption. Since acetate, propionate and ethanol contribute substantially to the DOC content but do not absorb UV light, the latter is recommended as indicator for pharmaceutical removal in solutions with high contents of biodegradable LMW organics.

Floc aging: Crystallization and improving low molecular weight organic removal in re-coagulation.

Iron coagulants have been used extensively in drinking water treatment. This typically produces substantial quantities of insoluble iron hydrolysis products which interact with natural and anthropogenic organic substances during the coagulation process. Previous studies have shown that the removal of low molecular weight (MW) organics is relatively poor by coagulation, which leads to their presence during disinfection, with the formation of halogenated byproducts, and in treated water supplies as potentially biodegradable material. Currently, there is little knowledge about the changes that occur in the nature of coagulant flocs as they age with time and how such changes affect interactions with organic matter, especially low MW organics. To improve this deficiency, this study has investigated the variation of aged flocs obtained from two commonly used iron salts and their impact on representative organic contaminants, natural organic matter (NOM) and tetracycline antibiotic (TC), in a real surface water. It was found that aging resulted in increasing crystallization of the flocs, which can play a beneficial role in activating persulfate oxidant to remove the representative organics. Furthermore, acidification was also found to further improve the removal of low MW natural organics and tetracycline. In addition, the results showed that the low MW fractions of NOM (<1K Dalton) were substantially removed by the aging flocs. These results are in marked contrast to the poor removal of low MW organic substances by conventional coagulation, with or without added oxidants, and show that aged flocs have a high potential of reuse for re-coagulation and activation of oxidants to reduce low MW organics, and enhance drinking water quality.

Changes to Struvite Growth and Morphology as Impacted by Low Molecular Weight Organics

Changes to Struvite Growth and Morphology as Impacted by Low Molecular Weight Organics

Generation and properties of organic colloids extracted by water from the organic horizon of a boreal forest soil

Organic colloids are an important part of dissolved organic matter (DOM) yet many of their properties remain elusive. The main aims of this study were to assess how the colloidal properties of DOM extracted with water from an organic boreal soil horizon varied with the extraction protocol, and thereby provide insight into the nature of the DOM colloids and develop a mechanistic understanding of how the colloids were generated from the parent soil aggregates. This was accomplished by systematic variations of extraction temperature (4 °C–100 °C), time, mechanical agitation, and pH, together with a combination of chemical analyses, and light and X-ray scattering. Our results agreed with the previous identification of two main colloidal DOM species, one fractal cluster and a second, smaller colloidal DOM species described as chains or coils. Fractal clusters completely dominated the colloidal DOM in extracts from our soil at room temperature and below. Colloidal coils only existed in DOM extracted above room temperature, and their amount increased significantly between 50 °C–100 °C. Moreover, the temperature variation indicated that the fractal clusters partly dissolved into colloidal coils at elevated temperatures. Mechanical agitation at 4 °C significantly increased the amount of DOM extracted, increasing the concentrations of both fractal clusters and low-molecular weight organic compounds. While the clusters were extracted from agitated and non-agitated soil suspensions, the low molecular weight organics were mainly released by agitation. Based on the experimental observations, we propose a conceptual model where parent soil aggregates contain the fractal clusters in mobile and occluded forms, and that the occluded clusters co-exist with occluded low molecular weight organics. These occluded forms may be released by mechanical forces, increasing pH and temperature. At higher temperatures, the soil aggregates and the fractal clusters start to break up, and subsequently individual colloidal coils, presumably carbohydrates, disperse in the water phase. The model explains the origin and properties of the fractal clusters that completely dominate the colloidal DOM extracted from our soil at room temperature and below.

Treatment of sewage sludge hydrothermal carbonization aqueous phase by Fe(II)/CaO2 system: Oxidation behaviors and mechanism of organic compounds

The Fe(II)/CaO2 system with a stable oxidant and a low-cost homogeneous activating agent has been considered as a prospective process for the disposal of wastewater. The system was constructed to treat sewage sludge hydrothermal carbonization aqueous phase (HTC-AP) in this study. As the hydrothermal temperature increased, the organics in the HTC-AP were first decomposed and then cyclized, while the Maillard reaction occurs throughout the stage. The oxidation efficiency of the Fe(II)/CaO2 system was related to the composition of organics in HTC-AP, and the removal of dissolved organic carbon (DOC) by the system was 38.56 % in the HTC-AP obtained by hydrothermal treatment at 220 °C. Redundancy analysis showed that the low molecular weight organics, hydrophobic acids, and hydrophobic neutral components were beneficial to DOC removal, while Maillard products and cyclization products were hard to be oxidized to CO2 and H2O. The CN functional group of the protein facilitated DOC removal, and some organics in HTC-AP were oxidized to acids and phenols. The energy input to remove DOC in Fe(II)/CaO2 system was 27.74 MJ per kg carbon. This study provides a low-energy consumption Fe(II)/CaO2 system for the post-treatment of HTC-APs and explores the applicability of the system.

Interaction of operating HRT and temperature on fouling of tertiary membranes treating municipal wastewater

This paper presents the first study to quantify and demonstrate the interactions between SBR operating conditions (hydraulic retention time (HRT) and temperature) and soluble microbial product (SMP) generation, as well as the impact of SBR operating conditions and filtration temperature on fouling of membranes used in tertiary treatment. Reducing SBR operating HRT from 20 to 10 h resulted in an increase in SMP concentrations, however, the extent of the increase in high and low molecular weight (MW) organics was different for the effluents from SBRs operated at 8 and 20 °C. Results of SMP modelling demonstrated that a reduction in SBR operating HRT induced decreased utilization associated product (UAP) yields and the influence was greater at the SBR operating temperature of 8 °C. In contrast, biomass associated product (BAP) yields were relatively stable with SBR operating HRT but greater at lower SBR operating temperature. The effects of SBR operating HRT and temperature on fouling indices were also interactive. Reducing SBR operating HRT led to a lower increase in hydraulically reversible resistances and a greater increase in hydraulically irreversible resistances for the effluent from the SBR operated at 8 °C. Reducing the filtration temperature resulted in additional increase in membrane resistances, and the increase was greater at lower SBR operating HRT. The contribution of filtration temperature was observed to have the greatest impact on membrane resistances, followed in importance by SBR operating HRT and temperature. The comprehensive analysis undertaken in the present study provides insights into the interaction between secondary and tertiary operations on fouling development. The results can be employed to understand the limits of fouling control for tertiary treatment under challenging conditions.

Renewable energy powered membrane technology: Implications of adhesive interaction between membrane and organic matter on spontaneous osmotic backwash cleaning

Organic matter (OM) in surface and ground waters may cause membrane fouling that is laborious to clean once established. Spontaneous osmotic backwash (OB) induced by solar irradiance fluctuation has been demonstrated for early mineral scaling/organic fouling control in decentralised small-scale photovoltaic powered-nanofiltration/reverse osmosis (PV-NF/RO) membrane systems. However, various OM types will interact differently with membranes which in turn affects the effectiveness of OB. This work evaluates the suitability of spontaneous OB cleaning for eleven OM types (covering low-molecular-weight organics (LMWO), humic substances, polyphenolic compounds and biopolymers) regarding adhesive interactions with NF/RO membranes. The adhesive interactions were quantified by an asymmetric flow field-flow fractionation coupled with an organic carbon detector (FFFF-OCD). The underlying mechanism of OM-membrane adhesive interactions affecting OB cleaning was elucidated. The results indicate that humic acid (a typical humic substance) and tannic acid (a typical polyphenolic compound) induced stronger adhesive interaction with NF/RO membranes than biopolymers and LMWO. When the mass loss of an OM due to adhesion was below a critical range, the spontaneous OB is most effective (>85% flux recovery); and above this range, the OB becomes ineffective (<50% flux recovery). Polyphenolic compounds and humic substances resulted in lower OB cleaning efficiency, due to their higher aromatic content, enhancing hydrophobic interactions and hydrogen bonding. Calcium-facilitated adhesion of some OM types (such as humic substances, polyphenolics and biopolymers) increased irreversible organic fouling potential and weakened OB cleaning, which was verified by both FFFF-OCD and membrane filtration results. This work provides a guidance to formulate strategies to enhance spontaneous OB cleaning, such as first identifying the adhesion of OM in feedwater (surface and ground waters) using FFFF-OCD, and then removing “sticky” OM using suitable pre-treatment processes.

Degradation performance of high-concentration coking wastewater by manganese oxide ore acidic oxidation.

The degradation of coking wastewater using a manganese oxide ore acidic oxidation was investigated. This work was performed in three stages. Firstly, the advantageous degradation conditions were measured by the degradation tests, and under the optimal conditions percentage degradation was obtained of 91.6% chemical oxygen demand measured by potassium dichromate oxidation (CODcr), 94.7% total nitrogen (TN), 98.3% phenols, 98.2% fatty acid, 89.5% tar, and 98.9% sulphide for the oxidized effluent, simultaneously cogenerating a Mn2+concentration of 46.2 g/L for Mn-electrolytic stock solution. Secondly, the transformation analysis of the special chemical group of coking wastewater contaminants illustrated that the employment of manganese oxide ore generated the degradation of low and high molecular weight organics, especially causing polymers to break down into oligomers. Thirdly, the electrochemical characteristics of the interface between wastewater and ore revealed that the contaminant degradation of coking wastewater greatly depended on the oxidation capacity of the surface oxide species, involving a simple answer to the MnO2 oxidation for small-molecule organic materials and a strengthening response to the MnO·OH oxidation for high-weight molecule organic substances. The treatment of coking wastewater using the Mn-oxide ore acidic oxidation process is an effective and value-added method, which is particularly applicable to high-concentration coking wastewater.

Evaluation of the impact of SBR operating temperature and filtration temperature on fouling of membranes used for tertiary treatment

Ultrafiltration (UF) is a cost-effective technology for water reclamation, however, its use in tertiary filtration in cold climates is impacted by changes in secondary effluent characteristics and membrane properties under low temperatures. In the present study, a series of filtration tests with the effluents from sequencing batch reactors (SBR) were conducted to evaluate the impact of SBR operating temperature and UF filtration temperature on fouling of membranes used for tertiary treatment. The results demonstrated increased total membrane resistances of 27%, 45% and 82% over multiple filtration cycles when the SBR operating temperature was 20, 14 and 8 ℃, respectively. The greater total membrane resistance at low SBR operating temperatures was attributed to increased hydraulically reversible and irreversible resistances. Moreover, a reduction in filtration temperature induced 55% and 122% increases in total membrane resistance when compared to a filtration temperature of 20 ℃, with the effluents from the SBRs operated at 14 and 8 ℃, respectively. The greater total membrane resistance at low filtration temperature was attributed to increased intrinsic resistance and the greater retention of hydraulically reversible and irreversible foulants. Pearson correlation revealed that the fouling of membranes used for tertiary treatment was governed by both high and low molecular weight organics, which were generated to a greater extent at low SBR operating temperatures. In addition, the generation of biomass associated products (BAP) was greatly affected by SBR operating temperature resulting in higher fouling potential at lower temperatures. The outcomes of the present study provide insights into the effects of operating temperature on biological and filtration processes that influence the fouling behavior of membranes used for tertiary treatment and can be employed to assist with the design of fouling mitigation measures in cold climates.

Degradation of pesticide wastewater with simultaneous resource recovery via ozonation coupled with anaerobic biochemical technology

The effective treatment of pesticide wastewater with high organic content, complex composition and high-toxicity has attracted enormous attention of researchers. This work proposes a new idea for removing the pesticide wastewater with simultaneous resource recovery, which is different from the traditional view of mineralization of pesticide wastewater via composite technology. This novel strategy involved a sequential three-step treatment: (a) acidic Ozonation process, to remove the venomous aromatic heterocyclic compounds; (b) hydrolysis and ozonation in alkaline conditions, enhancing the biodegradability of pesticide wastewater, mainly due to the dehalogenation, elimination of C=C bonds and production of low molecular-weight carboxylate anions; (c) the final step is anaerobic biological reactions. Based on the characterizations, this two-stage acidic-alkaline ozonation can efficiently degraded the virulence of pesticide wastewater and enhance its biodegradability from 0.08 to 0.32. The final anaerobic biochemical treatment can stably remove the residuals and convert the low molecular-weight organics into CH4, achieving the resource recovery. This work explored the pH-dependent of ozonized degradation of pesticide wastewater and gives a new perspective of wastewater treatment.

Unraveling competition versus adsorbability of dissolved organic matter against organic micropollutants onto activated carbon

It was widely acknowledged that dissolved organic matter (DOM) in natural water has ubiquitous competitiveness against organic micropollutants (OMPs) during adsorption onto activated carbon. However, some (model) low molecular weight organics have been reported to adsorb onto activated carbon, but were not competitive against co-adsorbates. The objective of this study is to identify which adsorbable DOM fractions in natural water contribute to the DOM competitiveness, and what is the impact of the OMP adsorbability and initial OMP concentration on this competitiveness. We, therefore, disassociated the adsorption of DOM fractions and OMPs (carbamazepine, caffeine and sulfamethoxazole) using a two-stage adsorption procedure, removing various adsorbable DOM fractions with powdered activated carbon pretreatment and then unraveling the competitiveness against OMPs of the remaining DOM. Our results demonstrated that DOM competition was not ubiquitous for all adsorbable fractions in natural water, and ∼ 25% of the adsorbable DOM was not competitive. The poorly adsorbable DOM was shown to be a non-competitive co-adsorbate, and its complexation even elevated the adsorption capacity of one of the OMPs (carbamazepine). The amount of DOM competitors increased for weaker adsorbable OMPs, and at higher initial OMP concentrations. The variability in DOM competition, differentiated by DOM adsorbability, has advanced the understanding of DOM competition, from ubiquitous competition to variable roles (varying competitiveness/complementary adsorption) of differently adsorbable DOM fractions during OMP adsorption.

Removal of organics from Bayer liquors via foam flotation

Rich foam is often formed in Bayer liquors, providing a promising short green process for the removal of organics by foam flotation. In this study, foam flotation of sodium oxalate (SO), sodium benzoate (SB), sodium citrate (SC), and sodium stearate (SS) was investigated using sodium dodecyl sulfate (SDS) as a frother and collector in a synthetic sodium aluminate solution. The feasibility of foam flotation in industrial production was then verified using Bayer liquor from alumina refineries. Low molecular weight organics achieved high maximum surface excesses, and elevated temperatures reduced them in the synthetic solution. Foams containing bubbles of different sizes were generated in the synthetic solution, and the stability of the foam increased with decreasing temperature. Medium molecular weight organics readily adhered to the bubble surface owing to their good surface activities, and the removal rate followed the order SO < SB < SS < SC. Increasing the temperature increased the removal rate of organics in both the synthetic solution and the Bayer liquor. Additionally, two types of Bayer liquor with a high total organic carbon (TOC) concentration produced a rich foam consisting of small, uniform bubbles. Over 31% of the removed organics were enriched in the foam for the Bayer liquor with TOC above 9 g L−1. A concentration coefficient of 3.46 could be achieved, resulting in a TOC greater than 51.9 g L−1 in the foam. These foam enrichment results were mainly attributed to small bubbles, high TOC concentrations, and multilayer adsorption.

Assessing the performance of coral reef-like floc towards the removal of low molecular weight organic contaminant

The removal of low molecular weight (MW) organics by coagulation is always a challenge in water treatment. In this study, we proposed a novel coagulation strategy: continuous dosing coagulation (CDC). The metallic coagulant and alkali were continuously dosed into water that was pre-acidized, rather than adding all the coagulant and alkali at once as in conventional coagulation (CC). The CDC process promoted the removal of different low MW organics, performing 15% better than the CC process. The best performance occurred at initial pH 6 and the coagulant dosing rate was 2 mg/(L·min). Under optimal conditions, the continuously dosed coagulant formed medium polymer Al in the early stages, which bound low MW organics to form complexes. Then, the subsequently dosed coagulant could adhere to the primary complexes and form coral reef-like surfaces with higher zeta potential and specific surface area. Each freshly formed surface bound contaminants and covered the previous surface. As a result, more dissolved low MW organic contaminants were included in the interior of flocs. However, in the CC process, all the coagulant was dosed at once, resulting in the rapid formation of aluminum hydroxide clusters, which had cotton-like surfaces with fewer binding sites. To achieve similar organic removal in treating secondary effluent, the CDC dosage was half of the CC dosage, indicating the potential economic benefits. The CDC process is a promising technology and the application in various water treatments should be further investigated.