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.

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AdOx is een combinatie van een adsorptieproces en een oxidatietechniek. Een veelbelovende technologie om medicijnresten uit afvalwater te halen. En winnaar van
de Waterinnovatieprijs 2021, categorie Gezond Water en Gezonde Bodem.@en

One framework type of high-silica zeolite only can effectively remove a limited range of organic micropollutants (OMPs) from water. In order to extend the OMP removal range, different types of high-silica zeolites need to be combined in the adsorption process. In this study, Mordernite (MOR) and ZSM-5 (MFI) high-silica zeolite powders were mixed in different mass ratios. The removal performances of eight OMPs by zeolite mixtures, as well as single MOR and MFI zeolites, were evaluated through batch adsorption experiments to investigate their adsorption behaviors and mechanisms. When there was only one solute in water, the adsorption isotherms of OMPs by zeolite mixtures were well predicted by combining the experimental adsorption isotherms of single zeolites based on the mass ratios of single zeolites. In multi-solute water, adsorption isotherms by zeolite mixtures were calculated with less accuracy when solely combining experimental isotherms of single zeolites, especially in the case of having a lower portion of more-effective zeolite in the mixture. This could be attributed to the competition for more-effective zeolite between different OMPs.@en

High-silica zeolites can be used for adsorption of organic compounds (OCs) from water. The adsorption efficacy could vary with the properties of OCs, as well as the porous and surface features of high-silica zeolites. In this study, the adsorption of triclosan, trichlorophenol (TCP) and phenol by ten high-silica zeolites were investigated. The plateaus of adsorption isotherms were observed in the adsorption of triclosan. The maximum adsorption capacity of triclosan could be related to the surface area and volume of micropores. The adsorption of TCP by FAU zeolites gave an S-shaped isotherm due to the possible lateral interactions of TCP molecules in the specific pore topology of FAU zeolites. The adsorption of phenol by high-silica zeolites had no adsorption plateau. Zeolites with channel structures, e.g. MFI zeolites, possess closely fitted pores for phenol, which slightly promoted its adsorption efficacy. The active adsorption sites of zeolites, i.e. Brønsted acid sites (BAS) and Lewis acid sites (LAS) failed to promote phenol adsorption. Phenol adsorption was favoured by carbon-based adsorbents with aromatic rings and functional groups, e.g. carboxyl and carbonyl, while the lack of active adsorption sites limited the phenol adsorption by high-silica zeolites, especially at the low concentration range.@en

The adsorption of organic micropollutants (OMPs) on high-silica zeolites is characterized by adsorption isotherms with various shapes. The occurrence of an S-shaped adsorption isotherm indicates the lack of adsorption affinity for OMPs at low, environmentally relevant equilibrium concentrations. In this study, S-shaped isotherms were observed during batch experiments with 2,4,6-trichlorophenol (TCP) and FAU zeolites. This is the first time that an S-shaped isotherm is reported for the adsorption of OMPs on high-silica zeolites. Monte Carlo (MC) simulations in the grand-canonical ensemble were used to obtain a better understanding of the mechanism of the S-shaped adsorption isotherms. From the MC simulation results, it was observed that multiple TCP molecules were adsorbed in the supercages of the FAU zeolites. It was found that the π-π interactions between TCP molecules give rise to the adsorption of multiple TCP molecules per supercage, and thus causing an S-shaped adsorption isotherm. Simulations also revealed that water molecules were preferentially adsorbed in the supercages and sodalite cages of the FAU zeolites. FAU zeolites with a higher Al content adsorbed a higher amount of water molecules and a lower amount of TCP, and showed less pronounced S-shaped isotherms.@en

A broad range of organic micropollutants (OMPs), including pesticides, pharmaceuticals and personal care products, are present in drinking water sources and effluent of wastewater treatment plants (Kolpin et al., 2002; Stackelberg et al., 2004). The presence of OMPs in water significantly threatens public health and thus calls for effective treatment technologies (Alan et al., 2008; Pal et al., 2010). Zeolites are highly structured minerals with uniform micropores (pore diameters < 2nm) (McCusker and Baerlocher 2001). The pores of zeolites allow for the adsorption of OMPs and potentially avoid the negative influence of natural organic matter (NOM) (de Ridder et al., 2012; Hung and Lin 2006; Knappe and Campos 2005). High-silica zeolites have hydrophobic surfaces, which could prevent water competition with OMP adsorption (Maesen 2007; Rakic et al., 2010; Tsitsishvili 1973). High-silica zeolites are thus expected to be potential alternative adsorbents for activated carbon in water treatment.@en

High-silica zeolites have been found to be effective adsorbents for the removal of organic micro-pollutants (OMPs) from impaired water, including various pharmaceuticals, personal care products, industrial chemicals, etc. In this review, the properties and fundamentals of high-silica zeolites are summarised. Recent research on mechanisms and efficiencies of OMP adsorption by high-silica zeolites are reviewed to assess the potential opportunities and challenges for the application of high-silica zeolites for OMP adsorption in water treatment. It is concluded that the adsorption capacities are well-related to surface hydrophobicity/hydrophilicity and structural features, e.g. micropore volume and pore size of high-silica zeolites, as well as the properties of OMPs. By using high-silica zeolites, the undesired competitive adsorption of background organic matter (BOM) in natural water could potentially be prevented. In addition, oxidative regeneration could be applied on-site to restore the adsorption capacity of zeolites for OMPs and prevent the toxic residues from re-entering the environment.

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