Adsorption of Ba2+ and Sr2+ on Ti3C2Tx MXene in model fracking wastewater.

Wastewater from hydraulic fracking contains both organic and inorganic pollutants; the latter include radioactive nuclides such as Ba2+ and Sr2+. We explored whether MXene (Ti3C2Tx), a novel adsorbent, could remove Ba2+ and Sr2+ from model wastewater. Zeta potential analysis showed that MXene had a high negative surface charge. MXene adsorbed Ba2+ and Sr2+ via electrostatic attraction, as confirmed by the adsorption at different solution pH values and in the presence of various concentrations of other ions (NaCl and CaCl2). MXene exhibited outstanding adsorption of Ba2+ and Sr2+, to approximately 180 and 225 mg g-1, respectively, when 1 g L-1 MXene was admixed with adsorbates at 2 g L-1. MXene exhibited very rapid adsorption kinetics, attaining equilibrium within 1 h. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy revealed that MXene adsorbed Ba2+ and Sr2+, respectively, via ion exchange and inner-sphere complex formation. Finally, we performed MXene reusability tests; reusability was excellent over at least four cycles. Thus, MXene removed Ba2+ and Sr2+ from model fracking wastewater.

Heterogeneous sonocatalytic degradation of an anionic dye in aqueous solution using a magnetic lanthanum dioxide carbonate-doped zinc ferrite-reduced graphene oxide nanostructure.

We report herein the sonochemical synthesis of a lanthanum dioxide carbonate (La2O2CO3) and zinc ferrite (ZnFe2O4)-loaded reduced graphene oxide (LZF-rGO) nanoheterostructure for ultrasound (US)-assisted degradation of methyl orange (MO) from water. The MO was chosen as a model organic dye due to its toxicological and biodegradable-resistant properties. The LZF-rGO catalyst was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results of characterizations confirmed successful synthesis of sonocatalyst. Among different removal systems, US/LZF-rGO displayed outstanding sonodegradation performance for degradation of MO. The maximum removal efficiency of 75.9% was achieved using 0.2 g/L sonocatalyst, 20 mg/L MO, and 0.71 W/cm2 US power intensity for 65 min. MO can be partially adsorbed on LZF-rGO but mostly sonodegraded by reactive radical species. The reaction conditions were optimized by investigating the effect of key operating parameters, including the sonocatalyst dosage, initial MO concentration, US power intensity, presence of inorganic salts, and use of an enhancer, on the decolorization of MO. The degradation intermediates produced from MO during the sonocatalytic process were identified by UPLC®/MS-MS, and possible mechanism and pathway for the degradation of MO in the US/LZF-rGO system were also proposed. Reusability experiments with this sonocatalyst revealed a less than 10% drop in the degradation efficiency after four adsorption-desorption cycles.

Evaluation of the colloidal stability and adsorption performance of reduced graphene oxide-elemental silver/magnetite nanohybrids for selected toxic heavy metals in aqueous solutions.

Reduced graphene oxide (rGO) hybridized with magnetite and/or elemental silver (rGO/magnetite, rGO/silver, and rGO/magnetite/silver) nanoparticles were evaluated as potential adsorbents for toxic heavy metal ions (Cd (II), Ni(II), Zn(II), Co(II), Pb(II), and Cu(II)). Although the deposition of iron oxide and silver nanoparticles on the rGO nanosheets played an inhibitory role in metal ion adsorption, the metal adsorption efficiency by the nanohybrids (NHs) was still higher than that reported for many other sorbents (e.g., activated biochar, commercial resins, and nanosized hydrated Zr(IV) oxide particles). X-ray photoelectron spectroscopy analyses revealed that complexation with deprotonated adsorbents and cation exchange was an important mechanism for Cd(II) ion removal by the rGO and NHs. Competitive adsorption tests using multi metals showed that the adsorption affinity of metal ions on the rGO and its NHs follows the order (Cu(II), Zn(II)) > Ni(II) > Co(II) > (Pb(II), Cd(II)), which is similar to the order observed for single-metal adsorption experiments. These results can be explained by the destabilization abilities of the rGO and NHs, as well as the ionic radii of the considered metal ions. Our findings demonstrate the feasibility of using rGO-based NHs as highly efficient adsorbents for heavy metal removal from water.

Modeling the Transport of the "New-Horizon" Reduced Graphene Oxide-Metal Oxide Nanohybrids in Water-Saturated Porous Media.

Little is known about the fate and transport of the "new-horizon" multifunctional nanohybrids in the environment. Saturated sand-packed column experiments ( n = 66) were therefore performed to investigate the transport and retention of reduced graphene oxide (RGO)-metal oxide (Fe3O4, TiO2, and ZnO) nanohybrids under environmentally relevant conditions (mono- and divalent electrolytes and natural organic matter). Classical colloid science principles (Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and colloid filtration theory (CFT)) and mathematical models based on the one-dimensional convection-dispersion equation were employed to describe and predict the mobility of RGO-Fe3O4, RGO-TiO2, and RGO-ZnO nanohybrids in porous media. Results indicate that the mobility of the three nanohybrids under varying experimental conditions is overall explainable by DLVO theory and CFT. Numerical simulations suggest that the one-site kinetic retention model (OSKRM) considering both time- and depth-dependent retention accurately approximated the breakthrough curves (BTCs) and retention profiles (RPs) of the nanohybrids concurrently; whereas, others (e.g., two-site retention model) failed to capture the BTCs and/or RPs. This is primarily because blocking BTCs and exponential/hyperexponential/uniform RPs occurred, which is within the framework of OSKRM featuring time- (for kinetic Langmuirian blocking) and depth-dependent (for exponential/hyperexponential/uniform) retention kinetics. Employing fitted parameters (maximum solid-phase retention capacity: Smax = 0.0406-3.06 cm3/g; and first-order attachment rate coefficient: ka = 0.133-20.6 min-1) extracted from the OSKRM and environmentally representative physical variables (flow velocity (0.00441-4.41 cm/min), porosity (0.24-0.54), and grain size (210-810 µm)) as initial input conditions, the long-distance transport scenarios (in 500 cm long sand columns) of the three nanohybrids were predicted via forward simulation. Our findings address the existing knowledge gap regarding the impact of physicochemical factors on the transport of the next-generation, multifunctional RGO-metal oxide nanohybrids in the subsurface.

Evaluation of biochar-ultrafiltration membrane processes for humic acid removal under various hydrodynamic, pH, ionic strength, and pressure conditions.

The performance of an ultrafiltration (UF)-biochar process was evaluated in comparison with a UF membrane process for the removal of humic acid (HA). Bench-scale UF experiments were conducted to study the rejection and flux trends under various hydrodynamic, pH, ionic strength, and pressure conditions. The resistance-in-series model was used to evaluate the processes and it showed that unlike stirred conditions, where low fouling resistance was observed (28.7 × 1012 m-1 to 32.5 × 1012 m-1), higher values and comparable trends were obtained for UF-biochar and UF alone for unstirred conditions (28.7 × 1012 m-1 to 32.5 × 1012 m-1). Thus, the processes were further evaluated under unstirred conditions. Additionally, total fouling resistance was decreased in the presence of biochar by 6%, indicating that HA adsorption by biochar could diminish adsorption fouling on the UF membrane and thus improve the efficiency of the UF-biochar process. The rejection trends of UF-biochar and UF alone were similar in most cases, whereas UF-biochar showed a noticeable increase in flux of around 18-25% under various experimental conditions due to reduced membrane fouling. Three-cycle filtration tests further demonstrated that UF-biochar showed better membrane recovery and antifouling capability by showing more HA rejection (3-5%) than UF membrane alone with each subsequent cycle of filtration. As a result of these findings, the UF-biochar process may potentially prove be a viable treatment option for the removal of HA from water.

Review of adsorption-membrane hybrid systems for water and wastewater treatment.

Adsorption is an effective method for the removal of inorganic and organic contaminants and has been commonly used as a pretreatment method to improve contaminant removal and control flux during membrane filtration. Over the last two decades, many researchers have reported the use of hybrid systems comprising various adsorbents and different types of membranes, such as nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) membranes, to remove contaminants from water. However, a comprehensive evaluation of the removal mechanisms and effects of the operating conditions on the transport of contaminants through hybrid systems comprising various adsorbents and NF, UF, or MF membranes has not been performed to date. Therefore, a systematic review of contaminant removal using adsorption-membrane hybrid systems is critical, because the transport of inorganic and organic contaminants via the hybrid systems is considerably affected by the contaminant properties, water quality parameters, and adsorbent/membrane physicochemical properties. Herein, we provide a comprehensive summary of the most recent studies on adsorption-NF/UF/MF membrane systems using various adsorbents and membranes for contaminant removal from water and wastewater and highlight the future research directions to address the current knowledge gap.

Removal of micropollutants and NOM in carbon nanotube-UF membrane system from seawater.

One of the main problems for seawater reverse osmosis desalination is membrane fouling associated with natural organic matter. Bisphenol-A (BPA) and 17alpha-ethinylestradiol (EE2) are well-known endocrine-disrupting compounds that have been detected in wastewater and seawater. In this study, the contribution of carbon nanotubes (CNTs, single-walled carbon nanotubes) to membrane fouling control and the potential adsorption mechanisms of BPA and EE2 were investigated using artificial seawater (ASW) in a bench scale ultrafiltration (UF) membrane coupled with CNTs. For high ionic strength ASW, UVA254 nm is a good alternative for highly aromatic dissolved organic carbon (DOC) determination, with a very strong linear relationship (R2 > or = 0.99) with increasing DOC concentrations. Approximately 80% of DOC in ASW was rejected by the CNT-UF system where 31% of DOC was removed due to adsorption by CNTs. The presence of CNTs shows a 20% increase in membrane flux in ASW. A strong linear correlation between retention and adsorption of BPA and EE2 was obtained. The percentage of adsorption/retention of BPA and EE2 in UF-CNTs follows the order: 94.0/96.6 (DI + CNTs, EE2) > 86.2/90.0 (ASW + CNTs, EE2) > 73.6/78.9 (DI + CNTS, BPA) > or = 74.1/77.3 (ASW + CNTS, BPA) > 29.8/29.8 (ASW, EE2) approximately equal to 27.3/27.3 (ASW, BPA) > or = 25.3/25.3 (DI, EE2) approximately equal to 24.8/24.8 (DI, BPA). This indicates that retention by the UF-CNT system is mainly due to adsorption. Overall, EE2 adsorption was greater than BPA during the UF-CNT experiments, presumably due to the higher hydrophobicity of EE2 than BPA.

Applications of MXene-based membranes in water purification: A review.

Since MXenes (a new family of two-dimensional materials) were first produced in 2011, they have become very attractive nanomaterials due to their unique properties and the range of potential industrial applications. Numerous recent studies have discussed the environmental applications of different MXenes in adsorption, catalysis, and membranes. Only a limited number of MXene-based membrane studies have been published to date, and most have discussed only specific MXenes (i.e., Ti3C2Tx), a small number of solutes (e.g., dyes and inorganic salts), and laboratory-scale short-term experiments under limited water-quality and operational conditions. In addition, to our knowledge, there has been no review of MXene-membrane studies. It is therefore essential to assess the current status of understanding of the performance of these membranes in liquid separation and water purification. Here, a comprehensive literature review is conducted to summarize the current preparation techniques for MXene-based membranes and their applications, particularly in terms of environmental and industrial applications (e.g., water treatment and organic solvent filtration), and to direct future research by identifying gaps in our present understanding. In particular, this review focuses on several key factors, including the effects of preparation techniques on membrane properties, operational conditions, and compound properties that influence liquid separation during MXene-based membrane filtration.

Comprehensive evaluation of the removal mechanism of carbamazepine and ibuprofen by metal organic framework.

Pharmaceutical products (PhACs) in water sources are considered to be a severe environmental issue. To mitigate this issue, we used a metal-organic framework (MOF) as an adsorbent to remove selected PhACs (i.e., carbamazepine (CBM) and ibuprofen (IBP)). This work was carried out to characterize the MOF, then confirm its feasibility for removing the selected PhACs. In particular, based on practical considerations, we investigated the effects of various water quality conditions, such as solution temperature, pH, ionic strength/background ions, and humic acid. MOF exhibited better removal rates than commercial powder activated carbon (PAC), considering pseudo-second order kinetic model. We clarified the competitive PhACs adsorption mechanisms based on the results obtained under various water quality conditions and found that hydrophobic interactions were the most important factors for both adsorbates. To confirm the practicality of MOF adsorption, we carried out regeneration tests with four adsorption and desorption cycles using acetone as a cleaning solution. Furthermore, to support the results of our regeneration tests, we characterized the MOF samples before and after adsorbate exposure using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Overall, MOF can be used in practical applications as efficient adsorbents to remove PhACs from water sources.

Enhanced adsorption of bisphenol A and sulfamethoxazole by a novel magnetic CuZnFe2O4-biochar composite.

The widespread occurrence of endocrine-disrupting compound and pharmaceutical active compounds such as bisphenol A (BPA) and sulfamethoxazole (SMX) in natural freshwater resources can cause serious environmental problems even at low exposure levels. In this work, in order to remove BPA and SMX from aqueous solutions, a novel biochar-supported magnetic CuZnFe2O4 composite (CZF-biochar) was synthesized by a facile one-pot hydrothermal process. After characterization studies, the key factors affecting BPA and SMX adsorption on CZF-biochar were comprehensively investigated. The primary mechanisms for BPA and SMX adsorption included charge-assisted H-bonding, hydrophobic, and π-π electron donor-acceptor interactions. In summary, considering the fast kinetics, high adsorption properties, easy magnetic separation, and recyclability for multiple reuses, the CZF-biochar composite has potential for the removal of BPA, SMX, and potentially other emerging organic contaminants from contaminated soil and water.

Enhanced sonocatalytic degradation of carbamazepine and salicylic acid using a metal-organic framework.

A metal-organic framework (MOF) was used as a sonocatalyst for ultrasonic (US) processes, to improve the degradation of two selected pharmaceutical active compounds (PhACs); carbamazepine (CBM) and salicylic acid (SA). The intrinsic characteristics of the MOF were characterized using a porosimeter (N2-BET) and scanning electron microscope (SEM). Various experiments were carried out under conditions with different US frequencies (28 and 1000 kHz), US power densities (45-180 W L-1), pH conditions (3.5, 7, and 10.5), and temperatures (293, 303, and 313 K) to investigate the degradation rates of the selected PhACs. Improved removal rates of PhACs were demonstrated within 60 min at 28 kHz (46% for SA; 47% for CBM) and 1000 kHz (60% for SA; 99% for CBM) with an MOF concentration of 45 mg L-1 in the US/MOF system, in comparison to 28 kHz (20% for SA; 25% for CBM) and 1000 kHz (37% for SA; 97% for CBM) under the 'US only' process. The removal of CBM was greater than that of SA under all experimental conditions due to the intrinsic properties of the PhACs. The degradation rates of PhACs are related to the quantity of H2O2; degradation is thus mostly affected by OH oxidation, which is generated by the dissociation of water molecules. The advantages of the 'US/MOF system' are as follows: (i) dispersion of MOF by US can improve sites and reactivity with respect to adsorption between the adsorbate (PhACs) and the adsorbent (MOF), and (ii) dispersed MOF acted as additional nuclei for water molecule pyrolysis, leading to the production of more OH. Therefore, based on the synergy indices, which were calculated using the removal rate constants [k1 (min-1)] of the pseudo-first order kinetic model, the 'US/MOF system' can potentially be used to treat organic pollutants (e.g., PhACs).

Comprehensive evaluation on removal of lead by graphene oxide and metal organic framework.

Graphene oxide (GO) and metal-organic framework (MOF) as adsorbents were applied to removal of Pb(II) with comprehensive characterizations and various experimental conditions. Various characterizations were conducted to clarify the physico-chemical properties of adsorbents. The analyses of adsorption experiments included (i) dosage amounts, (ii) isotherm and kinetic studies, and (iii) several factors related to water chemistry (i.e., solution pH, background ions, and humic acid). The maximum equilibrium adsorption capacity (qe) for Pb(II) using the GO and MOF was 555 and 108 mg g-1, respectively, as determined in the optimum dosage experiments. Although the surface area of the MOF (629 m2 g-1) was much larger than that of the GO (19.8 m2 g-1), the adsorption capacity of the MOF was five times lower due to electrical repulsion. Thus, the MOF was utilized as the control group for comparison with the GO to evaluate the adsorption mechanisms in the experiments related to surface charge (i.e., under various pH and humic acid conditions). The adsorption isotherms and kinetics model determined using GO followed the Langmuir model (R2 > 0.99) and pseudo-second-order model (R2 > 0.99), respectively. Additionally, three adsorption-desorption cycles were conducted with the GO adsorbent to evaluate the maintenance of the removal ratio after regeneration and the equilibrium adsorption capacity was determined. Finally, the adsorption of other heavy metals (i.e., Cu(II), Cd(II), and Zn(II)), separately and in mixtures, was also evaluated to determine the selectivity of the adsorbents.

Heterogeneous activation of persulfate by reduced graphene oxide-elemental silver/magnetite nanohybrids for the oxidative degradation of pharmaceuticals and endocrine disrupting compounds in water.

Reduced graphene oxide hybridized with zero-valent silver and magnetite nanoparticles (NPs) (rGO-Ag0/Fe3O4 nanohybrids) prepared via in situ nucleation and crystallization was used to activate peroxydisulfate (PDS) for degradation of pharmaceuticals and endocrine disrupting compounds (phenol, acetaminophen, ibuprofen, naproxen, bisphenol A, 17ß-estradiol, and 17α-ethinyl estradiol). The deposition of Ag0 and Fe3O4 in rGO nanosheet enhanced the catalytic removal of phenol in the heterogeneous activation of PDS. The adsorption capacities of rGO-Ag0/Fe3O4 for 10 µM phenol were 1.76, 1.33, and 2.04 µmol g-1-adsorbent at pH 4, 7, and 10, respectively, which are much higher than those of single NPs studied (Ag0, nanoscale zero-valent iron, and rGO). The rGO-Ag0/Fe3O4 effectively activated PDS to produce strong oxidizing SO4·and facilitate an electron transfer on the surface of the nanohybrid. The initial pseudo-first-order rate (k ini) constant for phenol degradation in PDS/rGO-Ag0/Fe3O4 system was 0.46 h-1 at pH 7, which is approximately eight times higher than that in the presence of single NPs (k ini = 0.04-0.06 h-1) due to the synergistic effects between adsorption and catalytic oxidation. Among various organic contaminants tested, the simultaneous use of rGO-Ag0/Fe3O4 (0.1 g/L) and PDS (1 mM) achieved more than 99% degradation of acetaminophen and 17ß-estradiol at pH 7. The radical scavenging studies with methanol and natural organic matter indicated that phenol was more likely to be degraded via free SO4·- and ·OH formation or a non-radical oxidative pathway. Our findings indicate that the rGO-Ag0/Fe O nanohybrids can be used as an efficient magnetically-separable nanocatalyst for removal of organic compounds in water and wastewater treatment.

Aggregation of reduced graphene oxide and its nanohybrids with magnetite and elemental silver under environmentally relevant conditions.

The aggregation and long-term (25 d) sedimentation behaviors of reduced graphene oxide (RGO) and its three successively self-assembled nanohybrids with magnetite (Fe3O4) and zerovalent silver (Ag0) nanoparticles have been investigated. The aggregation behaviors of the nanomaterials in NaCl and CaCl2 were found to be in good agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO)-type interactions and the Schulze-Hardy rule. The colloidal stability decreased with the increasing ratios of the edge-based functional groups (COO- and C=O) to the total oxygen-containing functional groups decorated on the basal planes (C-O) and edges of RGO, as quantified by X-ray photoelectron spectroscopy analysis. In the presence of natural organic matter (NOM), the aggregation of RGO and its nanohybrids was greatly inhibited as a result of the enhanced electrosteric repulsions arising from the adsorbed NOM macromolecules. The long-term sedimentation kinetics results showed that the RGO nanohybrids were less stable in synthetic groundwater containing higher electrolyte concentrations, which was likely because of the greater charge screening or neutralization effect imparted by higher monovalent and divalent electrolyte concentrations. Our findings have important implications for evaluating the environmental impact and toxicity of the emerging class of multifunctional nanohybrids whose environmental behaviors are currently largely unknown.

Fabrication of graphene-oxide/ß-Bi2O3/TiO2/Bi2Ti2O7 heterojuncted nanocomposite and its sonocatalytic degradation for selected pharmaceuticals.

A graphene-oxide (GO)/ß-Bi2O3/TiO2/Bi2Ti2O7 heterojuncted nanocomposite, designated as GBT, was synthesized via a two-step hydrothermal process. The sonocatalytic activity of the GBT was evaluated at several frequencies (28, 580, and 970 kHz) and compared with Bi-doped GO (GB) and Ti-doped GO (GT). Transmission electron microscopy images showed heterojuncted crystal structures of Bi and Ti on GO, and X-ray diffraction patterns verified that the crystal structures consisted of ß-Bi2O3, TiO2, and Bi2Ti2O7 nanocomposites. Energy-dispersive X-ray spectroscopy revealed a higher proportion of metal on GBT surfaces compared with GB and GT surfaces. The energy band gaps of GT, GB, and GBT were 3.0, 2.8, and 2.5 eV, respectively. Two pharmaceuticals (PhACs; carbamazepine [CBZ] and acetaminophen [ACE]) were selected and treated under sonolytic conditions at frequencies of 28, 580, and 970 kHz at a power level of 180 W L-1. The selected pharmaceuticals, present at initial concentrations of 20 µM, were reduced by over 99% by ultrasonic irradiation in the presence of GBT. The 580 kHz treatment achieved the most rapid organic removal among the frequencies tested. The removal kinetic of CBZ was higher than that of ACE owing to its relatively high hydrophobicity. High sonocatalytic activity of GBT was observed through measurement of H2O2 in solution. Because of its low band gaps and high surface activity, GBT exhibited higher sonolytic activity in removing selected PhACs than GT or GB.

Oxidative degradation of bisphenol A and 17α-ethinyl estradiol by Fenton-like activity of silver nanoparticles in aqueous solution.

Silver nanoparticles (AgNPs) have been reported to have antibacterial activities and cytotoxicity, resulting from the dissolved silver cation (Ag+) and its soluble complexes. However, to our knowledge, little has been reported about their potential use in degrading organic contaminants such as endocrine-disrupting compounds in aqueous solution. In this first report on the subject, we examined the effectiveness of the oxidative degradation of bisphenol A (BPA) and 17α-ethinyl estradiol (EE2) in water by reactive oxygen species formed during the decomposition of H2O2, assisted by polyvinylpyrrolidone (PVP)-stabilized AgNPs. The dissolution of AgNPs accompanied generation of OH at low pH. The fully dispersed PVP-AgNPs in the presence of H2O2 exhibited fast degradation kinetics for EE2 at a typical aquatic condition of pH (6-7). The oxidation kinetics of BPA and EE2 by PVP-AgNPs can be interpreted using three different modeling approaches: an initial pseudo-first-order, a retarded first-order rate, and Behnajady-Modirshahla-Ghanbery kinetic equation. The findings showed that AgNPs may have potential to facilitate the in situ oxidation for emerging contaminants in the aqueous environment.

Aggregation kinetics of single walled carbon nanotubes influenced by the frequency of ultrasound irradiation in the aquatic environment.

The colloidal stability of single-walled carbon nanotubes (SWNTs) sonicated at three different ultrasonication (US) frequencies (28, 580, and 1000kHz) were investigated under environmentally relevant conditions. In particular, correlations between surface chemistry, electrokinetic potential, interaction energy, and the aggregation kinetics of the aqueous SWNTs were studied. We observed that H2O2 production is negatively correlated with the yield of hydroxylation and carboxylation of SWNTs, which was dependent on the generation of ultrasonic energy by cavity collapse during US process. The SWNTs sonicated at relatively high US frequencies (580 and 1000kHz) aggregated rapidly in synthetic surface water, whereas alkalinity affected the stability of SWNTs insignificantly. This was because the SWNTs became less negatively charged under such conditions and were captured in deep primary energy wells, according to the Derjaguin-Landau-Verwey-Overbeek theory. Critical coagulation concentration values for the ultrasonicated SWNTs were determined to be 102mM NaCl for 28kHz, 22mM NaCl for 580kHz, and 43mM NaCl for 1000kHz. Suwannee River humic acid decreased the aggregation rate of SWNTs due to the steric hindrance, because of adsorbed macromolecules. Our findings show that the aggregate stability of SWNTs is controlled largely by a complex interplay between the evolution of surface functional groups on the SWNTs during US and solution chemistry.

Environmental behavior of engineered nanomaterials in porous media: a review.

A pronounced increase in the use of nanotechnology has resulted in nanomaterials being released into the environment. Environmental exposure to the most common engineered nanomaterials (ENMs), such as carbon-based and metal-based nanomaterials, can occur directly via intentional injection for remediation purposes, release during the use of nanomaterial-containing consumer goods, or indirectly via different routes. Recent reviews have outlined potential risks assessments, toxicity, and life cycle analyses regarding ENM emission. In this review, inevitable release of ENMs and their environmental behaviors in aqueous porous media are discussed with an emphasis on influencing factors, including the physicochemical properties of ENMs, solution chemistry, soil hydraulic properties, and soil matrices. Major findings of laboratory column studies and numerical approaches for the transport of ENMs are addressed, and studies on the interaction between ENMs and heavy metal ions in aqueous soil environments are examined. Future research is also presented with specific research directions and outlooks.

Modeling the effects of surfactant, hardness, and natural organic matter on deposition and mobility of silver nanoparticles in saturated porous media.

This study aims to provide insights into the mechanisms governing the deposition and retention of silver nanoparticles (AgNPs) in saturated porous media. Column experiments were conducted with quartz sand under saturated conditions to investigate the deposition kinetics of AgNPs, their mobility at different groundwater hardnesses (10-400 mg/L as CaCO3), and humic acid (HA, 0-50 mg/L as dissolved organic carbon [DOC]). An anionic surfactant, sodium dodecyl sulfate (SDS), was used as a dispersing agent to prepare a SDS-AgNPs suspension. The deposition kinetics of AgNPs were highly sensitive to the surfactant concentration, ionic strength, and cation type in solution. The breakthrough curves (BTCs) of SDS-AgNPs suggested that the transport and retention were influenced by groundwater hardness and HA. At low water hardness and high HA, high mobility of SDS-AgNPs was observed in saturated conditions. However, the retention of SDS-AgNPs increased substantially in very hard water with a low concentration of HA, because of a decreased primary energy barrier and the straining effect during the course of transport experiments. A modified clean-bed filtration theory and a two-site kinetic attachment model showed good fits with the BTCs of SDS-AgNPs. The fitted model parameters (katt and kstr) could be used successfully to describe that the retention behaviors were dominated by electrostatic and electrosteric repulsion, based on extended Derjaguin-Landau-Vaerwey-Overbeek calculations.

Ultrasonic degradation of acetaminophen and naproxen in the presence of single-walled carbon nanotubes.

Ultrasonic (US) and single-walled carbon nanotube (SWNT)-catalyzed ultrasonic (US/SWNT) degradation of a pharmaceutical (PhAC) mixture of acetaminophen (AAP) and naproxen (NPX) used as analgesics was carried out in water. In the absence of SWNTs, maximum degradations of AAP and NPX occurred at a high frequency (1000 kHz) and under acidic conditions (pH 3) and different solution temperatures (25 °C at 28 kHz and 35 °C at 1000 kHz) during US reactions. Rapid degradation of PhACs occurred within 10 min at 28 kHz (44.5% for AAP; 90.3% for NPX) and 1000 kHz (39.2% for AAP; 74.8% for NPX) at a SWNT concentration of 45 mgL(-1) under US/SWNT process, compared with 28 kHz (5.2% for AAP; 10.6% for NPX) and 1000 kHz (29.1% for AAP; 46.2% for NPX) under US process. Degradation was associated with the dispersion of SWNTs; small particles acted as nuclei during US reactions, enhancing the H2O2 production yield. NPX removal was greater than AAP removal under all US-induced reaction and SWNT adsorption conditions, which is governed by the chemical properties of PhACs. Based on the results, the optimal treatment performance was observed at 28 kHz with 45 mgL(-1) SWNTs (US/SWNT) within 10 min.