Kyoung Hoon Chu

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.

Removal of humic and tannic acids by adsorption–coagulation combined systems with activated biochar

Despite recent interest in transforming biomass into bio-oil and syngas, there is inadequate information on the compatibility of byproducts (e.g., biochar) with agriculture and water purification infrastructures. A pyrolysis at 300°C yields efficient production of biochar, and its physicochemical properties can be improved by chemical activation, resulting in a suitable adsorbent for the removal of natural organic matter (NOM), including hydrophobic and hydrophilic substances, such as humic acids (HA) and tannic acids (TA), respectively. In this study, the adsorption affinities of different HA and TA combinations in NOM solutions were evaluated, and higher adsorption affinity of TA onto activated biochar (AB) produced in the laboratory was observed due to its superior chemisorption tendencies and size-exclusion effects compared with that of HA, whereas hydrophobic interactions between adsorbent and adsorbate were deficient. Assessment of the AB role in an adsorption-coagulation hybrid system as nuclei for coagulation in the presence of aluminum sulfate (alum) showed a synergistic effect in a HA-dominated NOM solution. An AB-alum hybrid system with a high proportion of HA in the NOM solution may be applicable as an end-of-pipe solution.

Sonocatalytical degradation enhancement for ibuprofen and sulfamethoxazole in the presence of glass beads and single-walled carbon nanotubes

Sonocatalytic degradation experiments were carried out to determine the effects of glass beads (GBs) and single-walled carbon nanotubes (SWNTs) on ibuprofen (IBP) and sulfamethoxazole (SMX) removal using low and high ultrasonic frequencies (28 and 1000kHz). In the absence of catalysts, the sonochemical degradation at pH 7, optimum power of 0.18WmL(-1), and a temperature of 15°C was higher (79% and 72%) at 1000kHz than at 28kHz (45% and 33%) for IBP and SMX, respectively. At the low frequency (28kHz) H2O2 production increased significantly, from 10μM (no GBs) to 86μM in the presence of GBs (0.1mm, 10gL(-1)); however, no enhancement was achieved at 1000kHz. In contrast, the H2O2 production increased from 10μM (no SWNTs) to 31μM at 28kHz and from 82μM (no SWNTs) to 111μM at 1000kHz in the presence of SWNTs (45mgL(-1)). Thus, maximum removals of IBP and SMX were obtained in the presence of a combination of GBs and SWNTs at the low frequency (94% and 88%) for 60min contact time; however, >99% and 97% removals were achieved for 40 and 60min contact times at the high frequency for IBP and SMX, respectively. The results indicate that both IBP and SMX degradation followed pseudo-first-order kinetics. Additionally, the enhanced removal of IBP and SMX in the presence of catalysts was because GBs and SWNTs increased the number of free OH radicals due to ultrasonic irradiation and the adsorption capacity increase with SWNT dispersion.

Evaluation of Humic Acid and Tannic Acid Fouling in Graphene Oxide-Coated Ultrafiltration Membranes

Three commercially available ultrafiltration (UF) membranes (poly(ether sulfone), PES) that have nominal molecular weight cut-offs (5, 10, and 30 kDa) were coated with graphene oxide (GO) nanosheets. Field-emission scanning electron microscopy, Fourier-transform infrared spectroscopy, confocal laser scanning microscopy, water contact angle measurements, and X-ray photoelectron spectroscopy were employed to determine the changed physicochemical properties of the membranes after GO coating. The water permeability and single-solute rejection of GO-coated (GOC) membranes for humic acid (HA) molecules were significantly higher by approximately 15% and 55%, respectively, compared to those of pristine UF membranes. However, the GOc membranes for single-solute tannic acid (TA) rejection showed similar trends of higher flux decline versus pristine PES membranes, because the relatively smaller TA molecules were readily adsorbed onto the membrane pores. When the mixed-solute of HA and TA rejection tests were performed, in particular, the adsorbed small TA molecules resulted in irreversible membrane fouling due to cake formation and membrane pore blocking on the membrane surface for the HA molecules. Although both membranes showed significantly higher flux declines for small molecules rejection, the GOc membranes showed better performance than the pristine UF membranes in terms of the rejection of various mixed-solute molecules, due to higher membrane recovery and antifouling capabilities.

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.

Occurrence and Removal of Engineered Nanoparticles in Drinking Water Treatment and Wastewater Treatment Processes

The increase in use of engineered nanoparticles (ENPs) ends up in waste streams, and consequently to the environment and drinking water resources. Thus, there is a continuing need to detect and quantify ENPs in water. Understanding ENPs occurrence and behavior in aquatic environments is also required to assess the associated environmental impact. Indeed, insufficient water and wastewater treatment for ENPs may cause a serious threat to the environment and human health. The contaminant removal in conventional drinking water treatment depends largely on the influent quality and operation parameters. A question remains as to whether these processes are also efficient for removal of ENPs and thus for the protection of the public from exposure to ENPs. ENPs can be biodegraded or sorbed by bacterial communities to differing extents, affected greatly by their surface properties, speciation, and transformations in wastewater treatment. Although the available literature is insufficient, the present review attempts to summarize several topics concerning the detection and removal of ENPs that might undergo dissolution, agglomeration, bioaccumulation and transformation in drinking water and wastewater treatment processes.

Evaluation of Removal Mechanisms in a Graphene Oxide-Coated Ceramic Ultrafiltration Membrane for Retention of Natural Organic Matter, Pharmaceuticals, and Inorganic Salts

Functionalized graphene oxide (GO), derived from pure graphite via the modified Hummer method, was used to modify commercially available ceramic ultrafiltration membranes using the vacuum method. The modified ceramic membrane functionalized with GO (ceramicGO) was characterized using a variety of analysis techniques and exhibited higher hydrophilicity and increased negative charge compared with the pristine ceramic membrane. Although the pure water permeability of the ceramicGO membrane (14.4-58.6 L/m2 h/bar) was slightly lower than that of the pristine membrane (25.1-62.7 L/m2 h/bar), the removal efficiencies associated with hydrophobic attraction and charge effects were improved significantly after GO coating. Additionally, solute transport in the GO nanosheets of the ceramicGO membrane played a vital role in the retention of target compounds: natural organic matter (NOM; humic acid and tannic acid), pharmaceuticals (ibuprofen and sulfamethoxazole), and inorganic salts (NaCl, Na2SO4, CaCl2, and CaSO4). While the retention efficiencies of NOM, pharmaceuticals, and inorganic salts in the pristine membrane were 74.6%, 15.3%, and 2.9%, respectively, these increased to 93.5%, 51.0%, and 31.4% for the ceramicGO membrane. Consequently, the improved removal mechanisms of the membrane modified with functionalized GO nanosheets can provide efficient retention for water treatment under suboptimal environmental conditions of pH and ionic strength.