Quantitative structure property relationships for the adsorption of pharmaceuticals onto activated carbon

Universal machine-learning algorithm for predicting adsorption performance of organic molecules based on limited data set: Importance of feature description

Adsorption of organic molecules from aqueous solution offers a simple and effective method for their removal. Recently, there have been several attempts to apply machine learning (ML) for this problem. To this end, polyparameter linear free energy relationships (pp-LFERs) were employed, and poor prediction results were observed outside model applicability domain of pp-LFERs. In this study, we improved the applicability of ML methods by adopting a chemical-structure (CS) based approach. We used the prediction of adsorption of organic molecules on carbon-based adsorbents as an example. Our results show that this approach can fully differentiate the structural differences between any organic molecules, while providing significant information that is relevant to their interaction with the adsorbents. We compared two CS feature descriptors: 3D-coordination and simplified molecular-input line-entry system (SMILES). We then built CS-ML models based on neural networks (NN) and extreme gradient boosting (XGB). They all outperformed pp-LFERs based models and are capable to accurately predict adsorption isotherm of isomers with similar physiochemical properties such as chiral molecules, even though they are trained with achiral molecules and racemates. We found for predicting adsorption isotherm, XGB shows better performance than NN, and 3D-coordinations allow effective differentiation between organic molecules.

Insight into selected emerging micropollutant interactions with wastewater colloidal organic carbon: implications for water treatment and analysis

This study reports how adding a membrane filter (0.45-μm cellulose nitrate filter) between a glass fibre filter and the solid phase extraction (SPE) cartridge affected the GC/MS analysis of 48 emerging organic micropollutants in wastewater. Most of them are widely used as active pharmaceuticals, cosmetic and packaging material ingredients including classes of parabens, benzophenones and bisphenols among other chemicals tested. A high artificial organic carbon (OC) content in wastewater (DOC = 280 ± 14 mg/L) was investigated to gain insight into micropollutants/colloidal OC filter cake interactions. The results show that even with the use of matrix-matched calibration, the introduction of a second (membrane) filtration step can affect the analysis. Both positive, negative and no effects on the theoretical concentrations calculated from the calibration curves with and without additional filtration were observed. Positive effects on the concentration for the same analyte peak area relative to its surrogate standard were the consequence of a reduced signal for the same concentration, while the negative effects are the consequence of increasing signal for the same concentration. Effect types were dependent on the concentration and the nature of the analytes. Results show that bisphenols and parabens significantly interact with colloidal OC. Statistical analysis of molecular descriptor distribution with effect type showed that micropollutants that have a stronger interaction with colloidal OC have significantly higher ability to act as hydrogen bond donors (HBD) and have larger molar volume (MV). All compounds that experienced either positive or negative effects have a significantly higher median logD. However, further exploration within a single class of compounds (parabens, benzophenones and bisphenols) revealed that selected descriptors are unrelated to an effect type. Pearson's correlations showed that a correlation exists for certain concentration levels and groups of compounds between a negative effect and MV and logD and a positive effect with MV, MW and rotatable bond (RB) count.

A Review on the Removal of Carbamazepine from Aqueous Solution by Using Activated Carbon and Biochar

Carbamazepine (CBZ), one of the most used pharmaceuticals worldwide and a Contaminant of Emerging Concern, represents a potential risk for the environment and human health. Wastewater treatment plants (WWTPs) are a significant source of CBZ to the environment, polluting the whole water cycle. In this review, the CBZ presence and fate in the urban water cycle are addressed, with a focus on adsorption as a possible solution for its removal. Specifically, the scientific literature on CBZ removal by activated carbon and its possible substitute Biochar, is comprehensively scanned and summed up, in view of increasing the circularity in water treatments. CBZ adsorption onto activated carbon and biochar is analyzed considering several aspects, such as physicochemical characteristics of the adsorbents, operational conditions of the adsorption processes and adsorption kinetics and isotherms models. WWTPs usually show almost no removal of CBZ (even negative), whereas removal is witnessed in drinking water treatment plants through advanced treatments (even >90%). Among these, adsorption is considered one of the preferable methods, being economical and easier to operate. Adsorption capacity of CBZ is influenced by the characteristics of the adsorbent precursors, pyrolysis temperature and modification or activation processes. Among operational conditions, pH shows low influence on the process, as CBZ has no charge in most pH ranges. Differently, increasing temperature and rotational speed favor the adsorption of CBZ. The presence of other micro-contaminants and organic matter decreases the CBZ adsorption due to competition effects. These results, however, concern mainly laboratory-scale studies, hence, full-scale investigations are recommended to take into account the complexity of the real conditions.

Activated carbon coupled with advanced biological wastewater treatment: A review of the enhancement in micropollutant removal

This study consists of a review on the removal efficiencies of a wide spectrum of micropollutants (MPs) in biological treatment (mainly membrane bioreactor) coupled with activated carbon (AC) (AC added in the bioreactor or followed by an AC unit, acting as a post treatment). It focuses on how the presence of AC may promote the removal of MPs and the effects of dissolved organic matter (DOM) in wastewater. Removal data collected of MPs are analysed versus AC dose if powdered AC is added in the bioreactor, and as a function of the empty bed contact time in the case of a granular activated carbon (GAC) column acting as a post treatment. Moreover, the enhancement in macropollutant (organic matter, nitrogen and phosphorus compounds) removal is analysed as well as the AC mitigation effect towards membrane fouling and, finally, how sludge properties may change in the presence of AC. To sum up, it was found that AC improves the removal of most MPs, favouring their sorption on the AC surface, promoted by the presence of different functional groups and then enhancing their degradation processes. DOM is a strong competitor in sorption on the AC surface, but it may promote the transformation of GAC in a biologically activated carbon thus enhancing all the degradation processes. Finally, AC in the bioreactor increases sludge floc strength and improves its settling characteristics and sorption potential.

Contemporary Techniques for Remediating Endocrine-Disrupting Compounds in Various Water Sources: Advances in Treatment Methods and Their Limitations

Over the years, the persistent occurrence of superfluous endocrine-disrupting compounds (EDCs) (sub µg L⁻¹) in water has led to serious health disorders in human and aquatic lives, as well as undermined the water quality. At present, there are no generally accepted regulatory discharge limits for the EDCs to avert their possible negative impacts. Moreover, the conventional treatment processes have reportedly failed to remove the persistent EDC pollutants, and this has led researchers to develop alternative treatment methods. Comprehensive information on the recent advances in the existing novel treatment processes and their peculiar limitations is still lacking. In this regard, the various treatment methods for the removal of EDCs are critically studied and reported in this paper. Initially, the occurrences of the EDCs and their attributed effects on humans, aquatic life, and wildlife are systematically reviewed, as well as the applied treatments. The most noticeable advances in the treatment methods include adsorption, catalytic degradation, ozonation, membrane separation, and advanced oxidation processes (AOP), as well as hybrid processes. The recent advances in the treatment technologies available for the elimination of EDCs from various water resources alongside with their associated drawbacks are discussed critically. Besides, the application of hybrid adsorption–membrane treatment using several novel nano-precursors is carefully reviewed. The operating factors influencing the EDCs’ remediations via adsorption is also briefly examined. Interestingly, research findings have indicated that some of the contemporary techniques could achieve more than 99% EDCs removal.

Adsorption of ciprofloxacin from aqueous solution using surface improved tamarind shell as an economical and effective adsorbent

Antibiotics in water bodies are emerging as an alarming new pollutant because of its persistent and recombinant nature. In recent period of human lifestyle, pharmaceutical products play a vital role in many perspectives. Due to this unpredictable usage of products, the unreacted components release into waterbodies in trace quantities. Eventhough these trace quantities initiate a crisis of developing resistant antibacterial strains which pose health risks to humans and animals. This work reports the batch adsorption of a fluoroquinolone, a fourth-generation antibiotic compound by a biosorbent made by acid-treated tamarind shells. The shells were treated with zinc chloride and hydrochloric acid. The characterization of biosorbent was performed by Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The optimized adsorption parameters of time, pH and temperature were 30 minutes, 6 and 60 °C. The adsorbent can be reused up to seven times with negligible loss in its adsorption capacity. Adsorption followed by Langmuir, Freundlich and Tempkin model where used to determine the correlation coefficient. Pseudo first-order, second-order and intra-particle kinetic model were used to fit the experimental data. The results are best described by pseudo second-order denoting chemisorption and Freundlich isotherm model describing multilayer adsorption.Novelty StatementThe proposed work is to investigate about improved tamarind shell as biomass used in the removal unreacted PPCP components that have been released into aquatic environment.The novelty of this paper lies in that it puts forward a better resource utilization method for treating PPCP component wastewater, and studies the method theoretically from the perspective of mechanism and proves its feasibility.Identifying the maximum adsorption of antibiotic component from wastewater under different conditions and finding the optimum range.In addition to the existing literatures, this study has compared the adsorption efficiency of raw and treated adsorbent material prepared using Tamarind shell.

Correlations and prediction of adsorption capacity and affinity of aromatic compounds on activated carbons

Correlations capable of predicting organic compound adsorption by activated carbons (ACs) are essential to the applications of ACs as environmental adsorbents in water treatment. Adsorption isotherms of 21 aromatic compounds on 11 ACs both with various physicochemical properties were conducted and fitted by Dubinin-Ashtakhov model to develop the predictive correlations in this study. In addition to the correlations of adsorption capacity with total surface area of ACs, micropore surface area ratios (R_micro) of ACs and chemical molar volume reported in previous studies, the negative correlation of adsorption capacity with chemical melting point was newly observed in this study. This negative correlation could be attributed to expansion of chemicals adsorbed on the mesopore or external surface of ACs. Meanwhile, in addition to the positive correlations of adsorption affinity with R_micro of ACs, chemical polarity/polarizability and hydrogen bonding donor ability reported also in previous studies, the negative correlation of adsorption affinity with H/C of ACs was newly observed in this study, which should be attributed to that ACs with higher aromaticity could have stronger π-π interaction potential, hydrogen bonding interaction potential and hydrophobic effects for aromatic compounds. These observed correlations can be used to predict aromatic compound adsorption by ACs with readily available properties of both ACs (i.e., surface area, R_micro and H/C) and aromatic compounds (i.e., molar volume, melting point and solvatochromic parameters). Moreover, these predictive correlations, incorporating various adsorptive forces, steric hindrance effect and packing efficiency in adsorption and having clearly physicochemical significance, are important for exploring the adsorption mechanisms, and guiding the synthesis of ACs with desired physicochemical properties, and selecting ACs as adsorbents in water treatment applications.

QSARs to predict adsorption affinity of organic micropollutants for activated carbon and β-cyclodextrin polymer adsorbents

The removal of organic micropollutants (MPs) from water by means of adsorption is determined by the physicochemical properties of the adsorbent and the MPs. It is challenging to predict the removal of MPs by specific adsorbents due to the extreme diversity in physicochemical properties among MPs of interest. In this research, we established Quantitative Structure-Activity Relationships (QSARs) between the physicochemical properties of a diverse set of MPs and their distribution coefficients (K_D) measured on coconut shell activated carbon (CCAC) and porous β-cyclodextrin polymer (P-CDP) adsorbents. We conducted batch experiments with a mixture of 200 MPs and used the data to calculate K_D values for each MP on each adsorbent under conditions of infinite dilution (i.e., low adsorbate concentrations). We used computational software to calculate 3656 molecular descriptors for each MP. We then developed and applied a model-selection workflow to identify the most significant molecular descriptors for each adsorbent. The functional stability and predictive power of the resulting QSARs were confirmed with internal cross validation and external validation. The applicability domain of the QSARs was defined based on the most significant molecular descriptors selected into each QSAR. The QSARs are predictive tools for evaluating adsorption-based water treatment processes and provide new insights into CCAC and P-CDP adsorption mechanisms.

Predictive models for adsorption of organic compounds by Graphene nanosheets: comparison with carbon nanotubes

The Linear Solvation Energy Relationships (LSER) technique was applied in the present study for predicting models of organic compounds (OCs) adsorption by Graphene and Graphene oxide (GO), and the results were compared with those of multi-walled carbon nanotube (MWCNT) and single-walled carbon nanotube (SWCNT). Adsorption database of 38 OCs (28 aromatic and 10 aliphatic) for Graphene and 69 OCs (59 aromatic and 10 aliphatic) for GO were collected from the literature and our laboratory. The r² of the LSER models on the adsorption of aromatic OCs by Graphene and GO at three different equilibrium concentrations gradually increased up to OC molecular weight of 400 g/mol, after which a declining trend was observed for GO, while there was no visible change for Graphene. Among descriptors for all LSER models, V (molecular volume) and B (hydrogen bond accepting) for Graphene nanosheets (GNS) and carbon nanotubes (CNT) were the most significant descriptors (p values ≤ 0.05). B term had high value and was negatively correlated with adsorption of all OCs by Graphene (-1.24 to -9.45), GO (-0.55 to -9.31), SWCNT (-0.10 to -5.38) and MWCNT (-1.24 to -1.85). LSER successfully trained models for adsorption of OCs by GNS, and model coefficients were dependent on adsorbent type and OC properties.

Fate of pharmaceuticals during membrane bioreactor treatment: Status and perspectives

Pharmaceuticals in surface waters and wastewater treatment plants (WWTPs) as emerging pollutants have become a major concern. In comparison with other wastewater treatments, removal of pharmaceuticals in MBR has received much attention. This review presents the source and occurrence of pharmaceuticals in WWTPs influents. Experimental studies related to the removal of pharmaceuticals during MBR treatment, key affecting factors (including the different stages of MBR process configuration and the process parameters), and the underlying mechanisms proposed to explain the biodegradation and adsorption behaviors, have been comprehensively discussed. Several transformation products of pharmaceuticals are also reviewed in this paper. Furthermore, further research is needed to gain more information about the multiple influence factors of the pharmaceuticals elimination, appropriate methods for promoting pharmaceuticals elimination, more essential removal pathways, effect of pharmaceuticals on membrane fouling, and the detection and analysis of transformation products.

Linear solvation energy relationships (LSER) for adsorption of organic compounds by carbon nanotubes

The objective of this paper was to create a comprehensive database for the adsorption of organic compounds by carbon nanotubes (CNTs) and to use the Linear Solvation Energy Relationship (LSER) technique for developing predictive adsorption models of organic compounds (OCs) by multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). Adsorption data for 123 OCs by MWCNTs and 48 OCs by SWCNTs were compiled from the literature, including some experimental results obtained in our laboratory. The roles of selected OCs properties and CNT types were examined with LSER models. The results showed that the r(2) values of the LSER models displayed small variability for aromatic compounds smaller than 220 g/mol, after which a decreasing trend was observed. The data available for aliphatics was mainly for molecular weights smaller than 250 g/mol, which showed a similar trend to that of aromatics. The r(2) values for the LSER model on the adsorption of aromatic and aliphatic OCs by SWCNTs and MWCNTs were relatively similar indicating the linearity of LSER models did not depend on the CNT types. Among all LSER model descriptors, V term (molecular volume) for aromatic OCs and B term (basicity) for aliphatic OCs were the most predominant descriptors on both type of CNTs. The presence of R term (excess molar refractivity) in LSER model equations resulted in decreases for both V and P (polarizability) parameters without affecting the r(2) values. Overall, the results demonstrate that successful predictive models can be developed for the adsorption of OCs by MWCNTs and SWCNTs with LSER techniques.

Fate and behaviour of diclofenac during hydrothermal carbonization

Hydrothermal carbonization (HTC) has become an esteemed method to convert sewage sludge into biochar. Besides dewatering and disinfection the process is suggested to reduce the micropollutant load, which would be beneficial for the use of biochar as fertilizer. This study was designed to examine reduction of micropollutants and formation of transformation products during HTC using the example of diclofenac. We investigated compounds' removal at HTC conditions in inert experiments and in real samples. Results showed that HTC temperature (>190 °C) and pressure (∼15 bar) have the potential to fully degrade diclofenac in inert experiments and spiked sewage sludge (>99%) within 1 h. However, interfering effects hinder full removal in native samples resulting in 44% remaining diclofenac. Additionally, a combination of suspected-target and non-target analysis using LC-MS/MS and LC-HRMS resulted in the determination of six transformation products. These products have been reported in biochar from HTC for the first time, although other studies described them for other processes like advanced oxidation. Based on the detected transformation products, we proposed a degradation mechanism reflecting HTC reactions such as dehydroxylation and decarboxylation.

Spoilt for choice: A critical review on the chemical and biological assessment of current wastewater treatment technologies

The knowledge we have gained in recent years on the presence and effects of compounds discharged by wastewater treatment plants (WWTPs) brings us to a point where we must question the appropriateness of current water quality evaluation methodologies. An increasing number of anthropogenic chemicals is detected in treated wastewater and there is increasing evidence of adverse environmental effects related to WWTP discharges. It has thus become clear that new strategies are needed to assess overall quality of conventional and advanced treated wastewaters. There is an urgent need for multidisciplinary approaches combining expertise from engineering, analytical and environmental chemistry, (eco)toxicology, and microbiology. This review summarizes the current approaches used to assess treated wastewater quality from the chemical and ecotoxicological perspective. Discussed chemical approaches include target, non-target and suspect analysis, sum parameters, identification and monitoring of transformation products, computational modeling as well as effect directed analysis and toxicity identification evaluation. The discussed ecotoxicological methodologies encompass in vitro testing (cytotoxicity, genotoxicity, mutagenicity, endocrine disruption, adaptive stress response activation, toxicogenomics) and in vivo tests (single and multi species, biomonitoring). We critically discuss the benefits and limitations of the different methodologies reviewed. Additionally, we provide an overview of the current state of research regarding the chemical and ecotoxicological evaluation of conventional as well as the most widely used advanced wastewater treatment technologies, i.e., ozonation, advanced oxidation processes, chlorination, activated carbon, and membrane filtration. In particular, possible directions for future research activities in this area are provided.

LSER model for organic compounds adsorption by single-walled carbon nanotubes: Comparison with multi-walled carbon nanotubes and activated carbon

LSER models for organic compounds adsorption by single and multi-walled carbon nanotubes and activated carbon were successfully developed. The cavity formation and dispersion interactions (vV), hydrogen bond acidity interactions (bB) and π-/n-electron interactions (eE) are the most influential adsorption mechanisms. SWCNTs is more polarizable, less polar, more hydrophobic, and has weaker hydrogen bond accepting and donating abilities than MWCNTs and AC. Compared with SWCNTs and MWCNTs, AC has much less hydrophobic and less hydrophilic adsorption sites. The regression coefficients (e, s, a, b, v) vary in different ways with increasing chemical saturation. Nonspecific interactions (represented by eE and vV) have great positive contribution to organic compounds adsorption, and follow the order of SWCNTs > MWCNTs > AC, while hydrogen bond interactions (represented by aA and bB) demonstrate negative contribution. These models will be valuable for understanding adsorption mechanisms, comparing adsorbent characteristics, and selecting the proper adsorbents for certain organic compounds.

Predicting trace organic compound breakthrough in granular activated carbon using fluorescence and UV absorbance as surrogates

This study investigated the applicability of bulk organic parameters like dissolved organic carbon (DOC), UV absorbance at 254 nm (UV254), and total fluorescence (TF) to act as surrogates in predicting trace organic compound (TOrC) removal by granular activated carbon in water reuse applications. Using rapid small-scale column testing, empirical linear correlations for thirteen TOrCs were determined with DOC, UV254, and TF in four wastewater effluents. Linear correlations (R(2) > 0.7) were obtained for eight TOrCs in each water quality in the UV254 model, while ten TOrCs had R(2) > 0.7 in the TF model. Conversely, DOC was shown to be a poor surrogate for TOrC breakthrough prediction. When the data from all four water qualities was combined, good linear correlations were still obtained with TF having higher R(2) than UV254 especially for TOrCs with log Dow>1. Excellent linear relationship (R(2) > 0.9) between log Dow and the removal of TOrC at 0% surrogate removal (y-intercept) were obtained for the five neutral TOrCs tested in this study. Positively charged TOrCs had enhanced removals due to electrostatic interactions with negatively charged GAC that caused them to deviate from removals that would be expected with their log Dow. Application of the empirical linear correlation models to full-scale samples provided good results for six of seven TOrCs (except meprobamate) tested when comparing predicted TOrC removal by UV254 and TF with actual removals for GAC in all the five samples tested. Surrogate predictions using UV254 and TF provide valuable tools for rapid or on-line monitoring of GAC performance and can result in cost savings by extended GAC run times as compared to using DOC breakthrough to trigger regeneration or replacement.

A comparative study of coagulation, granular- and powdered-activated carbon for the removal of perfluorooctane sulfonate and perfluorooctanoate in drinking water treatment

Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are persistent organic pollutants in the environment and their occurrence causes toxicological effects on humans. We examined different conventional coagulant treatments such as alum, ferric chloride and polyaluminium chloride in removing these compounds. These were then compared with a natural coagulant (Moringa oleifera). We also investigated the powdered-activated carbon (PAC) and granular-activated carbon (GAC) for removing these compounds. At an initial dose of 5 mg/L, polyaluminium chloride led to a higher reduction of PFOS/PFOA compared with alum which in turn was higher than ferric. The removal efficiency increased with the increase in coagulant dose and decrease in pH. M. oleifera was very effective in reducing PFOS and PFOA than conventional coagulants, with a reduction efficiencies of 65% and 72%, respectively, at a dose of 30 mg/L. Both PAC and GAC were very effective in reducing these compounds than coagulations. PAC led to a higher reduction in PFOS and PFOA than GAC due to its greater surface area and shorter internal diffusion distances. The addition of PAC (10 min contact time) with coagulation (at 5 mg/L dosage) significantly increased the removal efficiency, and the maximum removal efficiency was for M. oleifera with 98% and 94% for PFOS and PFOA, respectively. The reduction efficiency of PFOS/PFOA was reduced with the increase in dissolved organic concentration due to the adsorption competition between organic molecules and PFOS/PFOA.

Adsorption of synthetic organic contaminants by carbon nanotubes: a critical review

In last ten years, a large number (80⁺) of articles regarding aqueous phase adsorption of a variety of synthetic organic compound (SOC) by CNTs were published in peer-reviewed journals. Adsorption depends upon the physicochemical properties of the adsorbates and CNTs as well as the background water chemistry. Among all properties reported in the literature, no parameter was reported as solely controlling SOC adsorption by CNTs. In this article, these contributing parameters were reviewed and the associated explanations were discussed. This comprehensive literature survey provides (i) a thorough CNT characterization summary, (ii) a discussion of adsorption mechanisms of SOCs by CNTs and (iii) a summary of the statistical adsorption model development efforts. It also includes discussions of agreements and differences in the literature, and identifies some research needs.

Reducing the discharge of micropollutants in the aquatic environment: the benefits of upgrading wastewater treatment plants

Micropollutants (MPs) as individual compounds or in complex mixtures are relevant for water quality and may trigger unwanted ecological effects. MPs originate from different point and diffuse sources and enter water bodies via different flow paths. Effluents from conventional wastewater treatment plants (WWTPs), in which various MPs are not or not completely removed, is one major source. To improve the water quality and avoid potential negative ecological effects by micropollutants, various measures to reduce the discharge should be taken. In this feature we discuss one of these measures; the benefits of upgrading WWTPs toward reduced MP loads and toxicities from wastewater effluents, using the recently decided Swiss strategy as an example. Based on (i) full-scale case studies using ozonation or powder activated carbon treatment, showing substantial reduction of MP discharges and concomitant reduced toxicities, (ii) social and political acceptance, (iii) technical feasibility and sufficient cost-effectiveness, the Swiss authorities recently decided to implement additional wastewater treatment steps as mitigation strategy to improve water quality. Since MPs are of growing global concern, the concepts and considerations behind the Swiss strategy are explained in this feature, which could be of use for other countries as well. It should be realized that upgrading WWTPs is not the only solution to reduce the discharge of MPs entering the environment, but is part of a broader, multipronged mitigation strategy.

Comparison of two PAC/UF processes for the removal of micropollutants from wastewater treatment plant effluent: process performance and removal efficiency

Two hybrid membrane processes combining powdered activated carbon (PAC) adsorption with ultrafiltration (UF) were investigated regarding operational performance and efficiency to remove organic micropollutants from municipal wastewater treatment plant effluent. A pressurized PAC/UF (pPAC/UF) and a submerged PAC/UF (sPAK/UF) system were operated continuously over a period of six months. Both UF membrane systems showed good compatibility with the application of PAC showing no abrasion or other negative impacts. The pPAC/UF system reached permeability values up to 290 L/(m² h bar) at high fluxes of 80 L/(m² h) compared to the sPAC/UF with a permeability of up to 200 L/(m² h bar) at fluxes of up to 23 L/(m² h). Surface analysis of both membranes with scanning electron microscopy revealed no membrane deterioration after the six-month period of operation. On the surface of the pressurized membrane the formation of a PAC layer was observed, which may have contributed to the high permeability by forming a protective coating. Five micropollutants, i.e. sulfamethoxazole, carbamazepine, mecoprop, diclofenac and benzotriazole in ambient effluent concentrations were investigated. Both PAC/UF systems removed 60-95% of the selected micropollutants at a dosage of 20 mg PAC/L and 4 mg Fe(3+)/L. However, extreme peak loads of sulfamethoxazole with concentrations of up to 30 μg/L caused a considerable performance decrease for more than a week.

Nanofiltration and granular activated carbon treatment of perfluoroalkyl acids

Perfluoroalkyl acids (PFAAs) are of concern because of their persistence in the environment and the potential toxicological effects on humans exposed to PFAAs through a variety of possible exposure routes, including contaminated drinking water. This study evaluated the efficacy of nanofiltration (NF) and granular activated carbon (GAC) adsorption in removing a suite of PFAAs from water. Virgin flat-sheet NF membranes (NF270, Dow/Filmtec) were tested at permeate fluxes of 17-75 Lm(-2)h(-1) using deionized (DI) water and artificial groundwater. The effects of membrane fouling by humic acid on PFAA rejection were also tested under constant permeate flux conditions. Both virgin and fouled NF270 membranes demonstrated >93% removal for all PFAAs under all conditions tested. GAC efficacy was tested using rapid small-scale columns packed with Calgon Filtrasorb300 (F300) carbon and DI water with and without dissolved organic matter (DOM). DOM effects were also evaluated with F600 and Siemens AquaCarb1240C. The F300 GAC had <20% breakthrough of all PFAAs in DI water for up to 125,000 bed volumes (BVs). When DOM was present, >20% breakthrough of all PFAAs by 10,000 BVs was observed for all carbons.

Coupling granular activated carbon adsorption with membrane bioreactor treatment for trace organic contaminant removal: breakthrough behaviour of persistent and hydrophilic compounds

This study investigated the removal of trace organic contaminants by a combined membrane bioreactor - granular activated carbon (MBR-GAC) system over a period of 196 days. Of the 22 compounds investigated here, all six hydrophilic compounds with electron-withdrawing functional groups (i.e., metronidazole, carbamazepine, ketoprofen, naproxen, fenoprop and diclofenac) exhibited very low removal efficiency by MBR-only treatment. GAC post-treatment initially complemented MBR treatment very well; however, a compound-specific gradual deterioration of the removal of the above-mentioned problematic compounds was noted. While a 20% breakthrough of all four negatively charged compounds namely ketoprofen, naproxen, fenoprop and diclofenac occurred within 1000-3000 bed volumes (BV), the same level of breakthrough of the two neutral compounds metronidazole and carbamazepine did not occur until 11,000 BV. Single-solute isotherm parameters did not demonstrate any discernible correlation individually with any of the parameters that may govern adsorption onto GAC, such as log D, number of hydrogen-bond donor/acceptor groups, dipole moment or aromaticity ratio of the compounds. The isotherm data, however, could differentiate the breakthrough behaviour between negatively charged and neutral trace organic contaminants.

Predictive model development for adsorption of aromatic contaminants by multi-walled carbon nanotubes

In the present study, Quantitative Structure-Activity Relationship (QSAR) and Linear Solvation Energy Relationship (LSER) techniques were used to develop predictive models for adsorption of organic contaminants by multi-walled carbon nanotubes (MWCNTs). Adsorption data for 29 aromatic compounds from literature (i.e., the training data) including some of the experimental results obtained in our laboratory were used to develop predictive models with multiple linear regression analysis. The generated QSAR (r(2) = 0.88), and LSER (r(2) = 0.83) equations were validated externally using an independent validation data set of 30 aromatic compounds. External validation accuracies indicated the success of parameter selection, data fitting ability, and the prediction strength of the developed models. Finally, the combination of training and validation data were used to obtain a combined LSER equation (r(2) = 0.83) that would be used for predicting adsorption of a wide range of low molecular weight aromatics by MWCNTs. In addition, LSER models at different concentrations were generated, and LSER parameter coefficients were examined to gain insights to the predominant adsorption interactions of low molecular weight aromatics on MWCNTs. The molecular volume term (V) of the LSER model was the most influential descriptor controlling adsorption at all concentrations. At higher equilibrium concentrations, hydrogen bond donating (A) and hydrogen bond accepting (B) terms became significant in the models. The results demonstrate that successful predictive models can be developed for the adsorption of organic compounds by CNTs using QSAR and LSER techniques.

Removal of trace organic contaminants by a membrane bioreactor-granular activated carbon (MBR-GAC) system

The removal of trace organics by a membrane bioreactor-granular activated carbon (MBR-GAC) integrated system were investigated. The results confirmed that MBR treatment can be effective for the removal of hydrophobic (log D>3.2) and readily biodegradable trace organics. The data also highlighted the limitation of MBR in removing hydrophilic and persistent compounds (e.g. carbamazepine, diclofenac, and fenoprop) and that GAC could complement MBR very well as a post-treatment process. The MBR-GAC system showed high removal of all selected trace organics including those that are hydrophilic and persistent to biological degradation at up to 406 bed volumes (BV). However, over an extended period, breakthrough of diclofenac was observed after 7320 BV. This suggests that strict monitoring should be applied over the lifetime of the GAC column to detect the breakthrough of hydrophilic and persistent compounds which have low removal by MBR treatment.