Effects of solution chemistry on adsorption of selected pharmaceuticals and personal care products (PPCPs) by graphenes and carbon nanotubes

Theoretical and experimental adsorption studies of sulfamethoxazole and ketoprofen on synthesized ionic liquids modified CNTs

The adsorption of sulfamethoxazole (SMZ) and ketoprofen (KET) using carbon nanotubes (CNTs) and CNTs modified with ionic liquids (ILs) was investigated. Two ionic liquids (1-benzyl, 3-hexyl imidazolium, IL1 and 1-benzyl, 3-decahexyl imidazolium, IL2) were synthesized, and characterized by nuclear magnetic resonance (¹H and ¹³C NMR) and high resolution-mass spectrometry (HR-MS). CNTs and modified CNTs were characterized using FT-IR, X-ray diffraction (XRD), surface area and porosity analysis, thermal gravimetric analysis (TGA), Zeta potential, Raman and scanning electron microscopy (SEM). Kinetics, isotherm and computational studies were carried out to determine the efficiency and adsorption mechanism of SMZ and KET on modified CNTs. A density functional theory (DFT) method was applied to shed more light on the interactions between the pharmaceutical compounds and the adsorbents at the molecular level. The effects of adsorbent dosage, concentration, solution pH, energetics and contact time of SMZ and KET on the adsorption process were investigated. The adsorption of SMZ and KET on CNTs and modified CNTs were pH dependent, and adsorption was best described by pseudo-second-order kinetics and the Freundlich adsorption isotherm. Ionic liquid modified CNTs showed improved adsorption capacities compared to the unmodified ones for both SMZ and KET, which is in line with the computational results showing performance order; CNT+KET/SMZ < CNT-ILs+SMZ < CNT-ILs+KET.

Adsorption mechanism of emerging and conventional phenolic compounds on graphene oxide nanoflakes in water

Emerging contaminants (ECs) such as bisphenol A (BPA), 4-nonylphenol (4-NP) and tetrabromobisphenol A (TBBPA) have gained immense attention worldwide due to their potential threat to humans and environment. Graphene oxide (GO) nanomaterial is considered as an important sorbent due to its exceptional range of environmental application owing to its unique properties. GO was also considered as one of ECs because of its potential hazard. The adsorption of organic contaminants such as phenolic ECs on GO affects the stability of GO nanoflakes in water and the fate of organic contaminants, which would cause further environmental risk. Therefore, the adsorption behaviors of emerging and common phenolic compounds (PCs) including phenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 4-NP, BPA and TBBPA on GO nanoflakes and their stability in water were studied. The adsorption equilibrium for all the compounds was reached <10h and was fitted with Langmuir and Freundlich isotherms. In addition to hydrophobic effect, adsorption mechanisms included π-π bonding and hydrogen bonding interactions between the adsorbate and GO, especially the electrostatic interactions were observed. Phenol has the highest adsorption affinity due to the formation of hydrogen bond. GO has a good stability in water even after the adsorption of PCs in the presence of a common electrolyte, which could affect its transport with organic contaminants in the environment. These better understandings illustrate the mechanism of emerging and common PC interaction with GO nanoflakes and facilitate the prediction of the contaminant fate in the aquatic environment.

Influence of sodium dodecyl sulfate coating on adsorption of methylene blue by biochar from aqueous solution

Biochar is regarded as a promising new class of materials due to its multifunctional character and the possibility of effectively coupling different properties. With increasing introduction into the environment, environmental chemicals such as surfactants will load onto the released biochar and change its physicochemical characteristics and adsorption behavior toward pollutants. In this study, sodium dodecyl sulfate (SDS), as one type of anionic surfactant, was coated onto biochar with different loading amounts. The influence of SDS loading onto biochar's physicochemical properties were investigated by Fourier transform infrared (FT-IR) spectroscopy, elemental analysis, zeta potential and Brunauer-Emmett-Teller (BET) surface area and pore size distribution analysis. Results showed that the pore size of the biochar decreased gradually with the increase of SDS loading because of the surface-adsorption and pore-blocking processes; the pH of the point of zero charge (pH_PZC) decreased with increasing SDS loading. Although surface-coating with SDS decreased the pore size of the biochar, its adsorption capacity toward Methylene Blue (MB) significantly increased. The biochar-bound SDS introduced functional groups and negative charges to the biochar surface, which could thus enhance the adsorption of MB via hydrogen bonding and electrostatic interaction. The results can shed light on the underlying mechanism of the influence of anionic surfactants on the adsorption of MB by biochar.

Wrinkle- and Edge-Adsorption of Aromatic Compounds on Graphene Oxide as Revealed by Atomic Force Microscopy, Molecular Dynamics Simulation, and Density Functional Theory

In this work, the favorable adsorption sites of aromatic compounds (ACs) on graphene oxide (GO) are characterized with both experimental and theoretical approaches. The results show that ACs exhibit a strong preference in adsorbing near the wrinkles and edges. Further analyses reveal that the edge-adsorption is mainly guided by the stronger π-π interaction near edges, accompanied by a stronger hydrogen bond interaction between carboxyl groups and ACs. Additionally, the water-mediated steric hindrance and flexibility of carboxyl groups also contribute to the edge-adsorption. A higher density of atoms and electrons is the main mechanism for the wrinkle-adsorption, and structural investigations indicate that the roughness serving as a steric hindrance for the ACs migration also contributes to the wrinkle-adsorption. This wrinkle- and edge-adsorption pattern will shed light on the design of GO-related environmental materials.

Nanomaterials for treating emerging contaminants in water by adsorption and photocatalysis: Systematic review and bibliometric analysis

Emerging contaminants in the aquatic environment have become a worldwide problem. Conventional wastewater treatment processes are ineffective for eliminating the emerging contaminants at trace concentrations. Nanomaterials possessing novel size-dependent properties, however, have shown great potential for removing these contaminants. Herein we reviewed nanomaterials reported for removing emerging contaminants by adsorption and/or photocatalysis, and their removal capacity, mechanism, and influencing factors are discussed. Meanwhile, a large-scale bibliometric analysis is conducted on the trends of the emerging contaminants, nanoadsorbents, nanophotocatalysts, and related research topics from the literature during 1998-2017.

Metal nanoparticles by doping carbon nanotubes improved the sorption of perfluorooctanoic acid

Due to considerable application of perfluorooctanoic acid (PFOA) and its refractory degradation, the widespread distribution of PFOA has already resulted in its' risks to environment and organisms. However, the intrinsic characteristic of pristine multi-walled carbon nanotubes (MWCNTs) limited their application for removing PFOA from aqueous medium. Therefore, three nano-metals (nano-crystalline iron, copper and zinc) grafted MWCNTs were synthesized and characterized by BET-N₂, TEM, FTIR, XPS and XRD as well as MWCNTs (as the control treatment) in this study. The results showed that nano metals were well grafted on the surface of MWCNTs. Adsorption were investigated by using radioactive labeled PFOA (¹⁴C-PFOA) to quantify the trace PFOA. Adsorption kinetics showed the adsorption of PFOA on the metal doped MWCNTs (MDCNTs) was controlled by intra-particle diffusion. Adsorption isotherms showed the sorption amounts on the MDCNTs were higher than the control. This attributed much to the hydrophobic interaction, electrostatic interaction and the formation of the inner sphere complexes. Ionic strength (0-100 mM) and ionic species (Ca²⁺) had little effects on the sorption of MDCNTs. PFOA adsorption on MDCNTs strongly depended on pH value in the medium. These results provide an innovative approach for removing trace PFOA from liquid medium.

Improved sorption of perfluorooctanoic acid on carbon nanotubes hybridized by metal oxide nanoparticles

Multi-walled carbon nanotubes (MWCNTs) are often used as adsorbent because of their strong adsorption capacity. However, due to the nature of MWCNTs, their ability to adsorb perfluorooctanoic acid (PFOA), a highly hydrophobic pollutant, is low. In this study, MWCNTs were modified by three nano metal oxides (nano iron oxide, copper oxide, and zinc oxide). The pristine (as the control) and modified MWCNTs were characterized by BET-N₂, TEM, FTIR, XPS, and XRD, which showed that nano metal oxides were well hybridized on the surface of MWCNTs. Radioactive-labeled PFOA (¹⁴C-PFOA) was used to quantify it at trace level. Adsorption kinetics showed that intra-particle diffusion was the control step of PFOA adsorbing on metal oxides hybridized MWCNTs (MOHCNTs). Adsorption capacity of PFOA on the MOHCNTs was higher than that on the control due to electrostatic and hydrophobic interactions. In addition, PFOA formed inner-sphere complexes with metal oxide nanoparticles via ligand exchange. The alteration of PFOA adsorption capacity by increasing ionic strength was attributed to the aggregation degree of MWCNTs, electrostatic shielding, and/or salting out effect. The presence of Ca²⁺ increased the adsorption, owing to not only its higher electrostatic shielding ability than Na⁺ but also its formation of bridge between PFOA and MOHCNTs. PFOA adsorption on MOHCNTs strongly depended on medium pH value. These results provide an innovative approach for removing trace PFOA from liquid medium.

Removal of emerging contaminants from the environment by adsorption

Emerging contaminants (EC's) are pollutants of growing concern. They are mainly organic compounds such as: pesticides, pharmaceuticals and personal care products, hormones, plasticizers, food additives, wood preservatives, laundry detergents, surfactants, disinfectants, flame retardants, and other organic compounds that were found recently in natural wastewater stream generated by human and industrial activities. A majority of ECs does not have standard regulations and could lead to lethal effects on human and aquatic life even at small concentrations. The conventional primary and secondary water treatment plants do not remove or degrade these toxic pollutants efficiently and hence need cost effective tertiary treatment method. Adsorption is a promising method worldwide for EC removal since it is low initial cost for implementation, highly-efficient and has simple operating design. Research has shown that the application of different adsorbents such as, activated carbons(ACs), modified biochars (BCs), nanoadsorbents (carbon nanotubes and graphene), composite adsorbents, and other are being used for EC's removal from water and wastewater. The current review intends to investigate adsorption process as an efficient method for the treatment of ECs. The mechanism of adsorption has also been discussed.

Ultrahigh selective adsorption of zwitterionic PPCPs both in the absence and presence of humic acid: Performance and mechanism

Since zwitterionic PPCPs could be combined with humic acid (HA) leading to certain contaminants residue in aqueous solution, adsorbents with much stronger binding with zwitterionic PPCPs were needed to avoid this phenomenon. Through comparison of serial magnetic carboxyl modified hypercrosslinked resins including MA10, MA30, MA40 and MA70, MA50 was found to exhibit ultrahigh selective adsorption of zwitterionic PPCPs including tetracycline and quinolone antibiotics due to the remarkable synergistic effects generated from cation exchange interaction and non-ion exchange interaction. To figure out the effect of HA, other five adsorbents including hypercrosslinked resin Q100, aminated hypercrosslinked resin GMA50, anion exchange resin MIEX, wood-based activated carbon F400D and coal-based activated carbon Norit were chosen as comparison to MA50 in adsorption performance of tetracycline (TC) and ciprofloxacin (CPX). It turned out that the percentage of CPX or TC combined with HA in aqueous solution varied greatly for studied adsorbents. There existed serious false-positive adsorption during the adsorption process by some commercial adsorbents such as MIEX and F400D, while MA50 exhibited relatively lower false-positive adsorption, ensuring the maximum safety of effluents. Breakthrough tests showed that MA50 had significant advantages in PPCPs removal at environment concentration, indicating its potential in application for real water.

Competitive adsorption of PPCP and humic substances by carbon nanotube membranes: Effects of coagulation and PPCP properties

Natural organic matter (NOM) and pharmaceuticals and personal care products (PPCP) are known to compete for adsorption sites on carbon nanotubes (CNT), resulting in decreasing PPCP adsorption onto CNT. In this study, four types of PPCP, as such acetaminophen (AAP), caffeine (CAF), triclosan (TCS), and carbendazim (CBD) were used to investigate the effects of PPCP properties and NOM coagulation on the competitive adsorption of PPCP and NOM. Coagulation preferentially removed HS from a natural surface water, thereby increasing adsorption of AAP, CAF, TCS and CBD by 19%, 13%, 17% and 11%, respectively. Similar trends were obtained with synthetic natural waters, for which the adsorption of AAP, CAF, TCS, and CBD increased by 29%, 7%, 44% and 69%, respectively, as humic acid (HA) concentration decreased from 10mgL^-1 to 0mgL^-1. Furthermore, PPCP properties also affected their competition with NOM for adsorption by CNT membranes Because CAF existed in cationic form at pH ranging from 7 to 8.3, its adsorption was less affected by the presence/coagulation of NOM than AAP, CBD, and TCS. Based upon these findings, coagulation has the potential to be integrated with CNT adsorption for the removal of PPCP compounds during advanced drinking water treatment.

Electronic-property dependent interactions between tetracycline and graphene nanomaterials in aqueous solution

Understanding the interactions between graphene nanomaterials (GNMs) and antibiotics in aqueous solution is critical to both the engineering applications of GNMs and the assessment of their potential impact on the fate and transport of antibiotics in the aquatic environment. In this study, adsorption of one common antibiotic, tetracycline, by graphene oxide (GO) and reduced graphene oxide (RGO) was examined with multi-walled carbon nanotubes (MWCNTs) and graphite as comparison. The results showed that the tetracycline adsorption capacity by the four selected carbonaceous materials on the unit mass basis followed an order of GO>RGO>MWCNTs>graphite. Upon normalization by surface area, graphite, RGO and MWCNTs had almost the same high tetracycline adsorption affinity while GO exhibited the lowest. We proposed π-electron-property dependent interaction mechanisms to explain the observed different adsorption behaviors. Density functional theory (DFT) calculations suggested that the oxygen-containing functional groups on GO surface reduced its π-electron-donating ability, and thus decreased the π-based interactions between tetracycline and GO surface. Comparison of adsorption efficiency at different pH indicated that electrostatic interaction also played an important role in tetracycline-GO interactions. Site energy analysis confirmed a highly heterogeneous distribution of the binding sites and strong tetracycline binding affinity of GO surface.

Effective adsorption of sulfamethoxazole, bisphenol A and methyl orange on nanoporous carbon derived from metal-organic frameworks

Nanoporous carbons (NPCs) derived from metal-organic frameworks (MOFs) are attracting increasing attention in many areas by virtue of their high specific surface area, large pore volume and unique porosity. The present work reports the preparation of an NPC with high surface area (1731m²/g) and pore volume (1.68cm³/g) by direct carbonization of MOF-5. We examined the adsorption of three typical contaminants from aqueous solutions, i.e., sulfamethoxazole (SMX), bisphenol A (BPA) and methyl orange (MO), by using the as-prepared NPC. The results demonstrated that NPC could adsorb the contaminants effectively, with adsorption capacity (q_m) of 625mg/g (SMX), 757mg/g (BPA) and 872mg/g (MO), respectively. These values were approximately 1.0-3.2 times higher than those obtained for single-walled carbon nanotubes (SWCNTs) and commercial powder active carbon (PAC) under the same conditions. With its high surface area and unique meso/macropore structure, the enhanced adsorption of NPC most likely originates from the cooperative interaction of a pore-filling mechanism, electrostatic interaction, and hydrogen bonding. In particular, the pH value has a crucial impact on adsorption, suggesting the significant contribution of electrostatic interaction between NPC and the contaminants. This study provides a proof-of-concept demonstration of MOF-derived nanoporous carbons as effective adsorbents of contaminants for water treatment.

Adsorption and desorption of phthalic acid esters on graphene oxide and reduced graphene oxide as affected by humic acid

The implications of humic acid (HA) regarding surface properties of graphene materials and their interactions with phthalic acid esters (PAEs) are not vivid. We report the role of HA on graphene oxide (GO) and reduced graphene oxide (RGO) for sorption-desorption behavior of PAEs. Besides higher surface area and pore volume, the hydrophobic π-conjugated carbon atoms on RGO ensured prominent adsorption capacity towards PAEs in comparison to hydrophilic GO, highlighting the hydrophobic effect. After adjusting for the hydrophobic effect by calculating the hexadecane-water partition coefficient (K_HW) normalized adsorption coefficient (K_d/K_HW), the dimethyl phthalate (DMP) molecule portrayed a higher adsorption affinity towards RGO by π-π electron donor-acceptor (EDA) interaction for active sites on graphene interface via sieving effect. In contrast to RGO, the weak π-π EDA interactions and H-bonding was observed between the carbonyl groups of PAEs and oxygen containing functional groups on GO. There was no obvious change in morphologies of GO and RGO before and desorption as revealed by SEM and TEM images, as desorption hysteresis did not occur in all conditions. The presence of HA also resulted in shielding effect thereby decreasing the adsorption rate and capacity of diethyl phthalate (DEP) on GO and RGO, while it had little effect on DMP, probably due to the adsorbed HA as new active sites. The desorption of DMP and DEP on RGO in presence of HA was quick and enhanced. These results should be important for evaluating the fate and health risk of graphene materials and PAEs in the environment.

Cotton fiber-supported layered double hydroxides for the highly efficient adsorption of anionic organic pollutants in water

Cotton fiber is used as a substrate for the immobilization of LDHs and applied for the adsorption anionic pharmaceuticals and dyes.

Adsorption of organic contaminants by graphene nanosheets: A review

Graphene nanosheets (GNS) such as graphenes and graphene oxides (GOs) have been widely investigated as next-generation adsorbents in both water and wastewater treatment processes due to their unique physicochemical properties and their affinity towards different classes of organic contaminants (OCs). In the last five years, more than 40 articles investigating adsorption of different classes of OCs by graphene and GO were published in peer-reviewed journals. Adsorption mechanisms were controlled by molecular properties of OCs (e.g., aromatic vs aliphatic, molecular size and hydrophobicity), characteristics of adsorbents (e.g., surface area, pore size distribution, and surface functional groups), and background solution properties (e.g., pH, ionic strength, surfactants, NOM, and temperature). This literature survey includes: (i) a summary of adsorption of OCs by GNS, (ii) a comprehensive discussion of the mechanisms and factors controlling the adsorption of OCs by GNS and a comparison of their adsorption behaviors with those of CNT. This literature survey also identifies future research needs and challenges on the adsorption of OCs by GNS.

Graphene: A new activator of sodium persulfate for the advanced oxidation of parabens in water

Graphene was successfully employed as a catalyst for the activation of sodium persulfate, towards the effective degradation of propylparaben, an emerging micro-pollutant, representative of the parabens family. A novel process is proposed which utilizes a commercial graphene nano-powder as the catalyst and sodium persulfate as the oxidizing agent. It was found that over 95% of micro-pollutant degradation occurs within 15 min of reaction time. The effects of catalyst loading (75 mg/L to 1 g/L), sodium persulfate (SPS) concentration (10 mg/L to 1 g/L), initial solution pH (3-9) and initial paraben concentration (0.5 mg/L to 5 mg/L) were examined. Experiments were carried out in different aqueous conditions, including ultrapure water, bottled water and wastewater in order to investigate their effect on the degradation rate. The efficiency of the process was lower at complex water matrices signifying the role of organic matter as scavenger of the oxidant species. The role of radical scavengers was also investigated through the addition of methanol and tert-butanol in several concentrations, which was found to be important only in relatively high values. An experiment in which propylparaben was substituted by methylparaben was conducted and similar results were obtained. The consumption of SPS was found to be high in all pH conditions tested, surpassing 80% in near neutral environment. However, the results indicate that the sulfate radicals formed react with water in alkaline conditions, which are the optimal for the reaction, producing hydroxyl radicals which appear to be the dominant species leading to the rapid degradation of propylparaben. To the best of our knowledge, this is the first time pristine graphene has been implemented as an activator of sodium persulfate for the effective oxidation of micro-pollutants.

Unveiling Adsorption Mechanisms of Organic Pollutants onto Carbon Nanomaterials by Density Functional Theory Computations and Linear Free Energy Relationship Modeling

Predicting adsorption of organic pollutants onto carbon nanomaterials (CNMs) and understanding the adsorption mechanisms are of great importance to assess the environmental behavior and ecological risks of organic pollutants and CNMs. By means of density functional theory (DFT) computations, we investigated the adsorption of 38 organic molecules (aliphatic hydrocarbons, benzene and its derivatives, and polycyclic aromatic hydrocarbons) onto pristine graphene in both gaseous and aqueous phases. Polyparameter linear free energy relationships (pp-LFERs) were developed, which can be employed to predict adsorption energies of aliphatic and aromatic hydrocarbons on graphene. Based on the pp-LFERs, contributions of different interactions to the overall adsorption were estimated. As suggested by the pp-LFERs, the gaseous adsorption energies are mainly governed by dispersion and electrostatic interactions, while the aqueous adsorption energies are mainly determined by dispersion and hydrophobic interactions. It was also revealed that curvature of single-walled carbon nanotubes (SWNTs) exhibits more significant effects than the electronic properties (metallic or semiconducting) on gaseous adsorption energies, and graphene has stronger adsorption abilities than SWNTs. The developed models may pave a promising way for predicting adsorption of environmental chemicals onto CNMs with in silico techniques.

Removal of micropollutants from municipal wastewater by graphene adsorption and simultaneous electrocoagulation/electrofiltration process

In this work the optimal operating conditions for removing selected micropollutants (also known as emerging contaminants, ECs) from actual municipal wastewater by graphene adsorption (GA) and simultaneous electrocoagulation/electrofiltration (EC/EF) process, respectively, were first determined and evaluated. Then, performance and mechanisms for the removal of selected phthalates and pharmaceuticals from municipal wastewater simultaneously by the GA and EC/EF process were further assessed. ECs of concern included di-n-butyl phthalate (DnBP), di-(2-ethylhexyl) phthalate (DEHP), acetaminophen (ACE), caffeine (CAF), cefalexin (CLX) and sulfamethoxazole (SMX). It was found that GA plus EC/EF process yielded the following removal efficiencies: DnBP, 89 ± 2%; DEHP, 85 ± 3%; ACE, 99 ± 2%; CAF, 94 ± 3%; CLX, 100 ± 0%; and SMX, 98 ± 2%. Carbon adsorption, size exclusion, electrostatic repulsion, electrocoagulation, and electrofiltration were considered as the main mechanisms for the removal of target ECs by the integrated process indicated above.

The role of graphene oxide and graphene oxide-based nanomaterials in the removal of pharmaceuticals from aqueous media: a review

In this review paper, the ill effects of pharmaceuticals (PhAs) on the environment and their adsorption on graphene oxide (GO) and graphene oxide-based (GO-based) nanomaterials have been summarised and discussed. The adsorption of prominent PhAs discussed herein includes beta-blockers (atenolol and propranolol), antibiotics (tetracycline, ciprofloxacin and sulfamethoxazole), pharmaceutically active compounds (carbamazepine) and analgesics such as diclofenac. The adsorption of PhAs strictly depends upon the experimental conditions such as pH, adsorbent and adsorbate concentrations, temperature, ionic strength, etc. To understand the adsorption mechanism and feasibility of the adsorption process, the adsorption isotherms, thermodynamics and kinetic studies were also considered. Except for some cases, GO and its derivatives show excellent adsorption capacities for PhAs, which is crucial for their applications in the environmental pollution cleanup.

High efficiency removal of triclosan by structure-directing agent modified mesoporous MIL-53(Al)

In order to expand the potential applications of metal-organic frameworks (MOFs), structure directing agents modified mesoporous MIL-53(Al) (MIL-53(Al)-1) was investigated to adsorb triclosan (TCS) with two different initial concentrations. MIL-53(Al)-1 with high mesoporosity and total pore volume exhibited higher adsorption capacity and 4.4 times faster adsorption of TCS at low concentration (1 mg L^-1) than that of microporous MIL-53(Al). Also, mesoporous as well as microporous MIL-53(Al) showed significant higher adsorption capacity and two orders of magnitude greater fast uptake of TCS than two kinds of mesoporous-activated carbon. The adsorption of TCS onto MIL-53(Al)-1 released more energy and had higher disorderliness than TCS on MIL-53(Al). The superior adsorption characteristics of MIL-53(Al)-1 were preserved over a wide pH range (4-9), at high concentration of ionic strengths, and in the presence of coexisting compounds (anions, cations, phenol, aniline, and humic acid). The selectivity adsorption and Fourier transform infrared (FT-IR) spectra revealed that TCS adsorption on MIL-53(Al)s was mainly driven by hydrophobicity interaction assisted with hydrogen bonding on MIL-53(Al)s. MIL-53(Al)s can be effectively regenerated several times by washing with 90% methanol-water (pH 11). All of the above results demonstrated MIL-53(Al)s are promising adsorbents for water purification. Graphical abstract.

Sorption of triclosan to carbon nanotubes: The combined effects of sonication, functionalization and solution chemistry

The sonication effect on the sorption behavior of carbon nanotubes (CNTs) with different functional groups has been poorly understood in previous studies, especially when combined with solution chemistry that may affect both the sorption and dispersion of CNTs. Our results show that sonication accelerated sorption of triclosan and increased the sorption capacities of CNTs with and without functionalizations at a neutral pH. Regardless of how sonication on CNTs was applied, the sorption decreased in the following order: pristine CNTs>CNTs-OH>CNTs-COOH. Sorption decreased with the increase of pH for all types of CNTs due to the electrostatic repulsion between triclosan and CNTs. The pH effect is greater for pristine CNTs than functionalized CNTs, while the sonication effect is greater for functionalized CNTs than pristine CNTs. Sonication increased sorption at a low pH (i.e., <9.5) but decreased sorption at a high pH (i.e., >9.5) for all types of CNTs, which may be due to the difference in the structure of CNTs at different pHs. Within 0.001-0.1M NaCl, sorption increased and then decreased for all types of CNTs with and without sonication, which can be explained by the balance between the salting-out effect and electrostatic screening effect.

Comparison of chemical, ultrasonic and thermal regeneration of carbon nanotubes for acetaminophen, ibuprofen, and triclosan adsorption

Recovering the adsorption capacity of multi-walled carbon nanotubes (MWCNT) is of importance to the sustainable use of MWCNT for the adsorption of pharmaceuticals and personal care products (PPCP).

Activated biochar derived from pomelo peel as a high-capacity sorbent for removal of carbamazepine from aqueous solution

In recent years, the application of biochar to remove contaminants from aqueous solutions has become interesting due to favorable physical/chemical properties and abundant feedstocks.

Adsorption of anti-inflammatory nimesulide by graphene materials: a combined theoretical and experimental study

Interactions of anti-inflammatory nimesulide with different graphene material species were explored employing both ab initio calculations and a batch adsorption process.

Removal of triazine-based pollutants from water by carbon nanotubes: Impact of dissolved organic matter (DOM) and solution chemistry

Adsorption of organic pollutants by carbon nanotubes (CNTs) in the environment or removal of pollutants during water purification require deep understanding of the impacts of the presence of dissolved organic matter (DOM). DOM is an integral part of environmental systems and plays a key role affecting the behavior of organic pollutants. In this study, the effects of solution chemistry (pH and ionic strength) and the presence of DOM on the removal of atrazine and lamotrigine by single-walled CNTs (SWCNTs) was investigated. The solubility of atrazine slightly decreased (∼5%) in the presence of DOM, whereas that of lamotrigine was significantly enhanced (by up to ∼70%). Simultaneous introduction of DOM and pollutant resulted in suppression of removal of both atrazine and lamotrigine, which was attributed to DOM-pollutant competition or blockage of adsorption sites by DOM. However the decrease in removal of lamotrigine was also a result of its complexation with DOM. Pre-introduction of DOM significantly reduced pollutant adsorption by the SWCNTs, whereas introduction of DOM after the pollutant resulted in the release of adsorbed atrazine and lamotrigine from the SWCNTs. These data imply that DOM exhibits higher affinity for the adsorption sites than the triazine-based pollutants. In the absence of DOM atrazine was a more effective competitor than lamotrigine for adsorption sites in SWCNTs. However, competition between pollutants in the presence of DOM revealed lamotrigine as the better competitor. Our findings help unravel the complex DOM-organic pollutant-CNT system and will aid in CNT-implementation in water-purification technologies.

Removal of pharmaceuticals and personal care products (PPCPs) from wastewater: A review

The pharmaceutical and personal care products (PPCPs) are emerging pollutants which might pose potential hazards to environment and health. These pollutants are becoming ubiquitous in the environments because they cannot be effectively removed by the conventional wastewater treatment plants due to their toxic and recalcitrant performance. The presence of PPCPs has received increasing attention in recent years, resulting in great concern on their occurrence, transformation, fate and risk in the environments. A variety of technologies, including physical, biological and chemical processes have been extensively investigated for the removal of PPCPs from wastewater. In this paper, the classes, functions and the representatives of the frequently detected PPCPs in aquatic environments were summarized. The analytic methods for PPCPs were briefly introduced. The removal efficiency of PPCPs by wastewater treatment plants was analyzed and discussed. The removal of PPCPs from wastewater by physical, chemical and biological processes was analyzed, compared and summarized. Finally, suggestions are made for future study of PPCPs. This review can provide an overview for the removal of PPCPs from wastewater.

Lessons learned: Are engineered nanomaterials toxic to terrestrial plants?

The expansion of nanotechnology and its ubiquitous applications has fostered unavoidable interaction between engineered nanomaterials (ENMs) and plants. Recent research has shown ambiguous results with regard to the impact of ENMs in plants. On one hand, there are reports that show hazardous effects, while on the other hand, some reports highlight positive effects. This uncertainty whether the ENMs are primarily hazardous or whether they have a potential for propitious impact on plants, has raised questions in the scientific community. In this review, we tried to demystify this ambiguity by citing various exposure studies of different ENMs (nano-Ag, nano-Au, nano-Si, nano-CeO2, nano-TiO2, nano-CuO, nano-ZnO, and CNTs, among others) and their effects on various groups of plant families. After scrutinizing the most recent literature, it seems that the divergence in the research results may be possibly attributed to multiple factors such as ENM properties, plant species, soil dynamics, and soil microbial community. The analysis of the literature also suggests that there is a knowledge gap on the effects of ENMs towards changes in color, texture, shape, and nutritional aspects on ENM exposed plants.

Effect of resin charged functional group, porosity, and chemical matrix on the long-term pharmaceutical removal mechanism by conventional ion exchange resins

This study attempted to clarify the long-term pharmaceutical removal mechanism from sewage treatment plant effluent during the cyclical adsorption-regeneration operation of 5 commercial resin-based fixed-bed reactors with the simultaneous occurrence of electrostatic interactions and complex non-electrostatic interactions. It examined 12 pharmaceuticals belonging to 10 therapeutic classes with different predominant existing forms and hydrophobicities. Furthermore, the effect of the resin charged functional group (strong-base vs. strong-acid vs. non-ionic), porosity (macroporous vs. gel), and chemical matrix (polystyrenic vs. polyacrylic) on the mechanism was investigated to optimize resin properties and achieve higher pharmaceutical removal. The results reported herein indicate the importance of non-electrostatic interactions between pharmaceuticals and the resin backbone during short-term cyclical operation (i.e., the 1st adsorption-regeneration cycle). With the development of cyclical operation, however, non-electrostatic interaction-induced pharmaceutical removal generally decreased and even disappeared when equilibrium was achieved between the influent and the resin. Despite pharmaceutical therapeutic class or hydrophilicity, anion (or cation) exchange resin preferentially removed those pharmaceuticals that were predominantly present as organic anions (or cations) by ion exchange process during long-term cyclical operation (i.e., ≥6 adsorption-regeneration cycles). Besides pharmaceuticals predominantly present as undissociated molecules, some amphoteric pharmaceuticals containing large amounts of zwitterions were also difficult to remove by ion exchange resin. Additionally, neither resin porosity nor chemical matrix had any significant effect on the long-term pharmaceutical removal mechanism.

Effect of humic acid on ciprofloxacin removal by magnetic multifunctional resins

Background organic matter significantly influences the removal of emerging contaminants in natural water. In this work, the adsorption of ciprofloxacin (CPX) onto a series of magnetic multifunctional resins (GMA10-GMA90) in the presence and absence of humic acid (HA) was conducted to demonstrate the effect of HA. Both hydrophobic and ion exchange interactions contributed to CPX adsorption. Negative charge-assisted hydrogen bonds also participated in the adsorption process, resulting in the high adsorption amount of anionic CPX onto the negatively charged GMA30 under basic solutions. HA could impact CPX adsorption not only as a competitive adsorbate but also as an additional adsorbent. At pH 5.6, the additional adsorption sites provided by adsorbed HA molecules on the resins dominated and thus facilitated the adsorption process. While at pH 10, HA inhibited the adsorption of CPX by directly competing for ion exchange sites and coexisting with CPX in the solution. The ratio of the amount of CPX adsorbed by dissolved HA to that by the resin reached as high as 1.61 for GMA90. The adsorbed HA molecules onto the resins could provide additional adsorption sites for CPX as proven by the enhanced CPX adsorption in HA-preloading systems at pH 5.6.

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.

Insight into the adsorption of PPCPs by porous adsorbents: Effect of the properties of adsorbents and adsorbates

Adsorption is an efficient method for removal of pharmaceuticals and personal care products (PPCPs). Magnetic resins are efficient adsorbents for water treatment and exhibit potential for PPCP removal. In this study, the magnetic hypercrosslinked resin Q100 was used for adsorption of PPCPs. The adsorption behavior of this resin was compared with those of two activated carbons, namely, Norit and F400D. Norit exhibited the fastest adsorption kinetics, followed by Q100. Norit featured a honeycomb shape and long-range ordered pore channels, which facilitated the diffusion of PPCPs. Moreover, the large average pore size of Q100 reduced diffusion resistance. The adsorbed amounts of 11 PPCPs on the three adsorbents increased with increasing adsorbate hydrophobicity. For Q100, a significant linear correlation was observed between the adsorption performance for PPCPs and hydrophobicity (logD value) of adsorbates (R(2) = 0.8951); as such, PPCPs with high logD values (>1.69) could be efficiently removed. Compared with those of Norit and F400D, the adsorption performance of Q100 was less affected by humic acid because of the dominant hydrophobic interaction. Furthermore, Q100 showed improved regeneration performance, which renders it promising for PPCP removal in practical applications.

Considerations of Environmentally Relevant Test Conditions for Improved Evaluation of Ecological Hazards of Engineered Nanomaterials

Engineered nanomaterials (ENMs) are increasingly entering the environment with uncertain consequences including potential ecological effects. Various research communities view differently whether ecotoxicological testing of ENMs should be conducted using environmentally relevant concentrations-where observing outcomes is difficult-versus higher ENM doses, where responses are observable. What exposure conditions are typically used in assessing ENM hazards to populations? What conditions are used to test ecosystem-scale hazards? What is known regarding actual ENMs in the environment, via measurements or modeling simulations? How should exposure conditions, ENM transformation, dose, and body burden be used in interpreting biological and computational findings for assessing risks? These questions were addressed in the context of this critical review. As a result, three main recommendations emerged. First, researchers should improve ecotoxicology of ENMs by choosing test end points, duration, and study conditions-including ENM test concentrations-that align with realistic exposure scenarios. Second, testing should proceed via tiers with iterative feedback that informs experiments at other levels of biological organization. Finally, environmental realism in ENM hazard assessments should involve greater coordination among ENM quantitative analysts, exposure modelers, and ecotoxicologists, across government, industry, and academia.

Adsorption of Emerging Ionizable Contaminants on Carbon Nanotubes: Advancements and Challenges

The superior adsorption capacity of carbon nanotubes has been well recognized and there is a wealth of information in the literature concerning the adsorption of unionized organic pollutants on carbon nanotubes. Recently, the adsorption of emerging environmental pollutants, most of which are ionizable, has attracted increasing attention due to the heightened concerns about the accumulation of these emerging contaminants in the environment. These recent studies suggest that the adsorption of emerging ionizable contaminants on carbon nanotubes exhibit different characteristics than unionized ones. For example, a new charge-assisted intermolecular force has been proposed for ionizable compounds because some adsorption phenomenon cannot be easily explained by the conventional force theory. The adsorption of ionizable compounds also displayed much stronger dependence on solution pH and ionic strength than unionized compounds. This article aims to present a brief review on the current understanding of the adsorption of emerging ionizable contaminants to carbon nanotubes and discuss further research needs required to advance the mechanistic understanding of the interactions between ionizable contaminants and carbon nanotubes.

Adsorption of sulfonamides on reduced graphene oxides as affected by pH and dissolved organic matter

With the significant increase in use and application of graphene and the frequent presence of sulfonamides (SAs) in water environments, their interactions have attracted extensive attention. In this study, adsorption of two selected SAs (sulfapyridine and sulfathiazole) by two reduced graphene oxides (rGO1 and rGO2) was examined as affected by pH and dissolved organic matter (DOM). Adsorption of SAs by rGOs was highly pH-dependent, and adsorption affinity of different SAs species followed the order of SA(0) > SA(+) > SA(-). The contribution of SA(0) to the overall adsorption was greater than its species fraction, implying the importance of the neutral species to adsorption. SAs adsorption isotherms at three selected pHs were in the order of pH 5.0 > pH 1.0 > pH 11.0, which was in accordance with the variation of site energy distribution analysis. Hydrophobic interaction, π-π EDA interaction and electrostatic interaction were the main mechanisms responsible for SAs adsorption by rGOs. Three representative natural DOMs including humic acid (HA), bovine serum albumin (BSA), and sodium alginate together with sodium dodecylbenzenesulfonate (SDBS) as a synthetic DOM were used to investigate their effect on SAs adsorption. The inhibition impact of DOM on SAs adsorption was lower for rGOs compared with carbon nanotubes and graphite, which might be attributed to the higher oxygen contents of rGOs. Also, the suppression effect of DOM generally followed an order of SDBS > HA ≥ BSA > alginate, indicating the importance role of DOM compositions. These results should be important for assessing the fate and transport of graphene and antibiotics in the environment.

Effect of humic acid on the sulfamethazine adsorption by functionalized multi-walled carbon nanotubes in aqueous solution: mechanistic study

The presence of humic acid (HA) inhibited sulfamethazine (SMZ) adsorption by three types of multi-walled carbon nanotubesviacompetitive interactions, molecular sieving, and pore blockage in solution (pH < 9).

Synthesis and application of magnetic reduced graphene oxide composites for the removal of bisphenol A in aqueous solution—a mechanistic study

Magnetic reduced graphene oxide composites (MRGO) with different reduction degree for removal of bisphenol A from aqueous solution

Removal of beta-blockers from aqueous media by adsorption onto graphene oxide

The aim of the present study is the evaluation of graphene oxide (GhO) as adsorbent material for the removal of beta-blockers (pharmaceutical compounds) in aqueous solutions. The composition and morphology of prepared materials were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Atenolol (ATL) and propranolol (PRO) were used as model drug molecules and their behavior were investigated in terms of GhO dosage, contact time, temperature and pH. Adsorption mechanisms were proposed and the pH-effect curves after adsorption were discussed. The kinetic behavior of GhO-drugs system was analyzed after fitting to pseudo-first and -second order equations. The adsorption equilibrium data were fitted to Langmuir, Freundlich and Langmuir-Freundlich model calculating the maximum adsorption capacity (67 and 116 mg/g for PRO and ATL (25 °C), respectively). The temperature effect on adsorption was tested carrying out the equilibrium adsorption experiments at three different temperatures (25, 45, 65 °C). Then, the thermodynamic parameters of enthalpy, free energy and entropy were calculated. Finally, the desorption of drugs from GhO was evaluated by using both aqueous eluants (pH2-10) and organic solvents.

Adsorption of 4-n-Nonylphenol and Bisphenol-A on Magnetic Reduced Graphene Oxides: A Combined Experimental and Theoretical Studies

Adsorption of 4-n-nonylphenol (4-n-NP) and bisphenol A (BPA) on magnetic reduced graphene oxides (rGOs) as a function of contact time, pH, ionic strength and humic acid were investigated by batch techniques. Adsorption of 4-n-NP and BPA were independent of pH at 3.0- 8.0, whereas the slightly decreased adsorption was observed at pH 8.0-11.0. Adsorption kinetics and isotherms of 4-n-NP and BPA on magnetic rGOs can be satisfactorily fitted by pseudo-second-order kinetic and Freundlich model, respectively. The maximum adsorption capacities of magnetic rGOs at pH 6.5 and 293 K were 63.96 and 48.74 mg/g for 4-n-NP and BPA, respectively, which were significantly higher than that of activated carbon. Based on theoretical calculations, the higher adsorption energy of rGOs + 4-n-NP was mainly due to π-π stacking and flexible long alkyl chain of 4-n-NP, whereas adsorption of BPA on rGOs was energetically favored by a lying-down configuration due to π-π stacking and dispersion forces, which was further demonstrated by FTIR analysis. These findings indicate that magnetic rGOs is a promising adsorbent for the efficient elimination of 4-n-NP/BPA from aqueous solutions due to its excellent adsorption performance and simple magnetic separation, which are of great significance for the remediation of endocrine-disrupting chemicals in environmental cleanup.

pH/temperature dependent selective removal of trace Cr(vi) from aqueous solution by imidazolium ionic liquid functionalized magnetic carbon nanotubes

Ionic liquid functionalized magnetic carbon nanotube (Fe₃O₄/CNT-IL) was prepared and used for selective removal of Cr(vi).