Critical Review of Advances in Engineering Nanomaterial Adsorbents for Metal Removal and Recovery from Water: Mechanism Identification and Engineering Design.

Nanomaterial adsorbents (NAs) have shown promise to efficiently remove toxic metals from water, yet their practical use remains challenging. Limited understanding of adsorption mechanisms and scaling up evaluation are the two main obstacles. To fully realize the practical use of NAs for metal removal, we review the advanced tools and chemical principles to identify mechanisms, highlight the importance of adsorption capacity and kinetics on engineering design, and propose a systematic engineering scenario for full-scale NA implementation. Specifically, we provide in-depth insight for using density functional theory (DFT) and/or X-ray absorption fine structure (XAFS) to elucidate adsorption mechanisms in terms of active site verification and molecular interaction configuration. Furthermore, we discuss engineering issues for designing, scaling, and operating NA systems, including adsorption modeling, reactor selection, and NA regeneration, recovery, and disposal. This review also prioritizes research needs for (i) determining NA microstructure properties using DFT, XAFS, and machine learning and (ii) recovering NAs from treated water. Our critical review is expected to guide and advance the development of highly efficient NAs for engineering applications.

Coupling Light Emitting Diodes with Photocatalyst-Coated Optical Fibers Improves Quantum Yield of Pollutant Oxidation.

A photocatalyst-coated optical fiber was coupled with a 318 nm ultraviolet-A light emitting diode, which activated the photocatalysts by interfacial photon-electron excitation while minimizing photonic energy losses due to conventional photocatalytic barriers. The light delivery mechanism was explored via modeling of evanescent wave energy produced upon total internal reflection and photon refraction into the TiO2 surface coating. This work explores aqueous phase LED-irradiated optical fibers for treating organic pollutants and for the first time proposes a dual-mechanistic approach to light delivery and photocatalytic performance. Degradation of a probe organic pollutant was evaluated as a function of optical fiber coating thickness, fiber length, and photocatalyst attachment method and compared against the performance of an equivalent catalyst mass in a completely mixed slurry reactor. Measured and simulated photon fluence through the optical fibers decreased as a function of fiber length, coating thickness, or TiO2 mass externally coated on the fiber. Thinner TiO2 coatings achieved faster pollutant removal rates from solution, and dip coating performed better than sol-gel attachment methods. TiO2 attached to optical fibers achieved a 5-fold higher quantum yield compared against an equivalent mass of TiO2 suspended in a slurry solution.

Potential Environmental Impacts and Antimicrobial Efficacy of Silver- and Nanosilver-Containing Textiles.

For textiles containing nanosilver, we assessed benefit (antimicrobial efficacy) in parallel with potential to release nanosilver (impact) during multiple life cycle stages. The silver loading and method of silver attachment to the textile highly influenced the silver release during washing. Multiple sequential simulated household washing experiments for fabric swatches in deionized water with or without detergent showed a range of silver release. The toxicity of washing experiment supernatants to zebrafish (Danio rerio) embryos was negligible, with the exception of the very highest Ag releases (∼1 mg/L Ag). In fact, toxicity tests indicated that residual detergent exhibited greater adverse response than the released silver. Although washing the fabrics did release silver, it did not affect their antimicrobial efficacy, as demonstrated by >99.9% inhibition of E. coli growth on the textiles, even for textiles that retained as little as 2 µg/g Ag after washing. This suggests that very little nanosilver is required to control bacterial growth in textiles. Visible light irradiation of the fabrics reduced the extent of Ag release for textiles during subsequent washings. End-of-life experiments using simulated landfill conditions showed that silver remaining on the textile is likely to continue leaching from textiles after disposal in a landfill.

Potential health implications of water resources depletion and sewage discharges in the Republic of Macedonia.

Potential health implications of deficient sanitation infrastructure and reduced surface water flows due to climate change are examined in the case study of the Republic of Macedonia. Changes in surface water flows and wastewater discharges over the period 1955-2013 were analyzed to assess potential future surface water contamination trends. Simple model predictions indicated a decline in surface water hydrology over the last half century, which caused the surface waters in Macedonia to be frequently dominated by >50% of untreated sewage discharges. The surface water quality deterioration is further supported by an increasing trend in modeled biochemical oxygen demand trends, which correspond well with the scarce and intermittent water quality data that are available. Facilitated by the climate change trends, the increasing number of severe weather events is already triggering flooding of the sewage-dominated rivers into urban and non-urban areas. If efforts to develop a comprehensive sewage collection and treatment infrastructure are not implemented, such events have the potential to increase public health risks and cause epidemics, as in the 2015 case of a tularemia outbreak.

Characterization, Recovery Opportunities, and Valuation of Metals in Municipal Sludges from U.S. Wastewater Treatment Plants Nationwide.

U.S. sewage sludges were analyzed for 58 regulated and nonregulated elements by ICP-MS and electron microscopy to explore opportunities for removal and recovery. Sludge/water distribution coefficients (KD, L/kg dry weight) spanned 5 orders of magnitude, indicating significant metal accumulation in biosolids. Rare-earth elements and minor metals (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) detected in sludges showed enrichment factors (EFs) near unity, suggesting dust or soils as likely dominant sources. In contrast, most platinum group elements (i.e., Ru, Rh, Pd, Pt) showed high EF and KD values, indicating anthropogenic sources. Numerous metallic and metal oxide colloids (<100-500 nm diameter) were detected; the morphology of abundant aggregates of primary particles measuring <100 nm provided clues to their origin. For a community of 1 million people, metals in biosolids were valued at up to US$13 million annually. A model incorporating a parameter (KD × EF × $Value) to capture the relative potential for economic value from biosolids revealed the identity of the 13 most lucrative elements (Ag, Cu, Au, P, Fe, Pd, Mn, Zn, Ir, Al, Cd, Ti, Ga, and Cr) with a combined value of US $280/ton of sludge.

Membrane fouling by vesicles and prevention through ozonation.

Membrane fouling is a major challenge in water and wastewater treatment. Recent observations that ozone mitigates membrane fouling during filtration of secondary effluent prompted this study into the impact of preozonation on membrane fouling caused by biogenic colloids. The focus of this study was on liposomes, synthetic vesicles composed of (phospho)lipid bilayers, which are representative of the diverse cellular vesicles present in all biologically impacted waters. The overarching hypothesis was that these biologically produced, nonrigid or "soft" colloids (e.g., vesicles) present in wastewater give rise to unique fouling behavior that can be mitigated by preozonation. Using dead-end ultrafiltration (UF) and batch ozonation tests, the key findings of this study were (1) liposomes fouled UF membranes faster (4-13 times membrane cake resistance (RC) per mgC filtered) than polysaccharides, fatty acids, and NOM on a DOC-normalized basis; (2) based on the estimated carbon distribution of secondary effluent, liposome-like biogenic nanomaterials could be responsible for 20-60% of fouling during UF; and (3) preozonation reduces liposomal fouling during UF, likely due to the disruption of the liposome structure through cleavage of the fatty acid tails at carbon-carbon double bonds.

Characterization of food-grade titanium dioxide: the presence of nanosized particles.

Titanium dioxide (TiO2) is widely used in food products, which will eventually enter wastewater treatment plants and terrestrial or aquatic environments, yet little is known about the fraction of this TiO2 that is nanoscale, or the physical and chemical properties of TiO2 that influence its human and environmental fate or toxicity. Instead of analyzing TiO2 properties in complex food or environmental samples, we procured samples of food-grade TiO2 obtained from global food suppliers and then, using spectroscopic and other analytical techniques, quantified several parameters (elemental composition, crystal structure, size, and surface composition) that are reported to influence environmental fate and toxicity. Another sample of nano-TiO2 that is generally sold for catalytic applications (P25) and widely used in toxicity studies, was analyzed for comparison. Food-grade and P25 TiO2 are engineered products, frequently synthesized from purified titanium precursors, and not milled from bulk scale minerals. Nanosized materials were present in all of the food-grade TiO2 samples, and transmission electron microscopy showed that samples 1-5 contained 35, 23, 21, 17, and 19% of nanosized primary particles (<100 nm in diameter) by number, respectively (all primary P25 particles were <100 nm in diameter). Both types of TiO2 aggregated in water with an average hydrodynamic diameter of >100 nm. Food-grade samples contained phosphorus (P), with concentrations ranging from 0.5 to 1.8 mg of P/g of TiO2. The phosphorus content of P25 was below inductively coupled plasma mass spectrometry detection limits. Presumably because of a P-based coating detected by X-ray photoelectron spectroscopy, the ζ potential of the food-grade TiO2 suspension in deionized water ranged from -10 to -45 mV around pH 7, and the iso-electric point for food-grade TiO2 (

Mechanistic Study of Arsenate Adsorption onto Different Amorphous Grades of Titanium (Hydr)Oxides Impregnated into a Point-of-Use Activated Carbon Block.

Millions of households still rely on drinking water from private wells or municipal systems with arsenic levels approaching or exceeding regulatory limits. Arsenic is a potent carcinogen, and there is no safe level of it in drinking water. Point-of-use (POU) treatment systems are a promising option to mitigate arsenic exposure. However, the most commonly used POU technology, an activated carbon block filter, is ineffective at removing arsenic. Our study aimed to explore the potential of impregnating carbon blocks with amorphous titanium (hydr)oxide (THO) to improve arsenic removal without introducing titanium (Ti) into the treated water. Four synthesis methods achieved 8-16 wt.% Ti loading within the carbon block with 58-97% amorphous THO content. The THO-modified carbon block could adsorb both oxidation states of arsenic (arsenate and arsenite) in batch or column tests. Modified carbon block with higher Ti and amorphous content always led to better arsenate removal, achieving arsenic loadings up to 31 mg As/mg Ti after 70,000 bed volumes in continuous flow tests. Impregnating carbon block with amorphous THO consistently outperformed impregnation using crystalline TiO2. The best-performing system (TTIP-EtOH carbon block) was an amorphous THO derived using titanium isopropoxide, ethanol, and acetic acid via sol-gel technique, aged at 80° for 18 hours and dried overnight at 60°. Comparable pore size distribution and surface area of the impregnated carbon blocks suggested that chemical properties play a more crucial role than physical and textural properties in removing arsenate via amorphous Ti-impregnated carbon block. Freundlich isotherms indicated energetically favorable adsorption for amorphous chemically synthesized adsorbents. The mass transport coefficients for the amorphous TTIP-EtOH carbon block were fitted using a pore surface diffusion model, resulting in Dsurface = 3.1×10-12 cm2/s and Dpore = 3.2×10-6 cm2/s. Impregnating the carbon block with THO enabled effective arsenic removal from water without adversely affecting the pressure drop across the unit or the carbon block's ability to remove polar organic chemical pollutants efficiently.

Titanium dioxide nanoparticles in food and personal care products.

Titanium dioxide is a common additive in many food, personal care, and other consumer products used by people, which after use can enter the sewage system and, subsequently, enter the environment as treated effluent discharged to surface waters or biosolids applied to agricultural land, incinerated wastes, or landfill solids. This study quantifies the amount of titanium in common food products, derives estimates of human exposure to dietary (nano-) TiO(2), and discusses the impact of the nanoscale fraction of TiO(2) entering the environment. The foods with the highest content of TiO(2) included candies, sweets, and chewing gums. Among personal care products, toothpastes and select sunscreens contained 1% to >10% titanium by weight. While some other crèmes contained titanium, despite being colored white, most shampoos, deodorants, and shaving creams contained the lowest levels of titanium (<0.01 µg/mg). For several high-consumption pharmaceuticals, the titanium content ranged from below the instrument detection limit (0.0001 µg Ti/mg) to a high of 0.014 µg Ti/mg. Electron microscopy and stability testing of food-grade TiO(2) (E171) suggests that approximately 36% of the particles are less than 100 nm in at least one dimension and that it readily disperses in water as fairly stable colloids. However, filtration of water solubilized consumer products and personal care products indicated that less than 5% of the titanium was able to pass through 0.45 or 0.7 µm pores. Two white paints contained 110 µg Ti/mg while three sealants (i.e., prime coat paint) contained less titanium (25 to 40 µg Ti/mg). This research showed that, while many white-colored products contained titanium, it was not a prerequisite. Although several of these product classes contained low amounts of titanium, their widespread use and disposal down the drain and eventually to wastewater treatment plants (WWTPs) deserves attention. A Monte Carlo human exposure analysis to TiO(2) through foods identified children as having the highest exposures because TiO(2) content of sweets is higher than other food products and that a typical exposure for a US adult may be on the order of 1 mg Ti per kilogram body weight per day. Thus, because of the millions of tons of titanium-based white pigment used annually, testing should focus on food-grade TiO(2) (E171) rather than that adopted in many environmental health and safety tests (i.e., P25), which is used in much lower amounts in products less likely to enter the environment (e.g., catalyst supports, photocatalytic coatings).

Nanomaterial transformation and association with fresh and freeze-dried wastewater activated sludge: implications for testing protocol and environmental fate.

Engineered nanomaterials (ENMs) are an emerging class of contaminants entering wastewater treatment plants (WWTPs), and standardized testing protocols are needed by industry and regulators to assess the potential removal of ENMs during wastewater treatment. A United States Environmental Protection Agency (USEPA) standard method (OPPTS 835.1110) for estimating soluble pollutant removal during wastewater treatment using freeze-dried, heat-treated (FDH) activated sludge (AS) has been recently proposed for predicting ENM fate in WWTPs. This study is the first to evaluate the use of FDH AS in batch experiments for quantifying ENM removal from wastewater. While soluble pollutants sorbed equally to fresh and FDH AS, fullerene, silver, gold, and polystyrene nanoparticles' removals with FDH AS were approximately 60-100% less than their removals with fresh AS. Unlike fresh AS, FDH AS had a high concentration of proteins and other soluble organics in the liquid phase, an indication of bacterial membrane disintegration due to freeze-drying and heat exposure. This cellular matter stabilized ENMs such that they were poorly removed by FDH AS. Therefore, FDH AS is not a suitable sorbent for estimating nanoparticle removal in WWTPs, whereas fresh AS has been shown to reasonably predict full-scale performance for titanium removal. This study indicates that natural or engineered processes (e.g., anaerobic digestion, biosolids decomposition in soils) that result in cellular degradation and matrices rich in surfactant-like materials (natural organic matter, proteins, phospholipids, etc.) may transform nanoparticle surfaces and significantly alter their fate in the environment.

Occurrence and removal of titanium at full scale wastewater treatment plants: implications for TiO2 nanomaterials.

Titanium dioxide nanoparticles increasingly will be used in commercial products and have a high likelihood of entering municipal sewage that flows to centralized wastewater treatment plants (WWTPs). Treated water (effluent) from WWTPs flows into rivers and lakes where nanoparticles may pose an ecological risk. To provide exposure data for risk assessment, titanium concentrations in raw sewage and treated effluent were determined for 10 representative WWTPs that use a range of unit processes. Raw sewage titanium concentrations ranged from 181 to 1233 µg L(-1) (median of 26 samples was 321 µg L(-1)). The WWTPs removed more than 96% of the influent titanium, and all WWTPs had effluent titanium concentrations of less than 25 µg L(-1). To characterize the morphology and presence of titanium oxide nanoparticles in the effluent, colloidal materials were isolated via rota-evaporation, dialysis and lyophilization. High resolution transmission electron microscopy and energy dispersive X-ray analysis indicated the presence of spherical titanium oxide nanoparticles (crystalline and amorphous) on the order of 4 to 30 nm in diameter in WWTP effluents. This research provides clear evidence that some nanoscale particles will pass through WWTPs and enter aquatic systems and offers a methodological framework for collecting and analyzing titanium-based nanomaterials in complex wastewater matrices.

Octanol-water distribution of engineered nanomaterials.

The goal of this study was to examine the effects of pH and ionic strength on octanol-water distribution of five model engineered nanomaterials. Distribution experiments resulted in a spectrum of three broadly classified scenarios: distribution in the aqueous phase, distribution in the octanol, and distribution into the octanol-water interface. Two distribution coefficients were derived to describe the distribution of nanoparticles among octanol, water and their interface. The results show that particle surface charge, surface functionalization, and composition, as well as the solvent ionic strength and presence of natural organic matter, dramatically impact this distribution. Distributions of nanoparticles into the interface were significant for nanomaterials that exhibit low surface charge in natural pH ranges. Increased ionic strengths also contributed to increased distributions of nanoparticle into the interface. Similarly to the octanol-water distribution coefficients, which represent a starting point in predicting the environmental fate, bioavailability and transport of organic pollutants, distribution coefficients such as the ones described in this study could help to easily predict the fate, bioavailability, and transport of engineered nanomaterials in the environment.

Measurement of free chlorine levels in water using potentiometric responses of biofilms and applications for monitoring and managing the quality of potable water.

Residual free chlorine is not monitored continuously at scale in drinking water distribution systems because existing real-time sensor technologies require frequent maintenance, cleaning, and calibration, which makes these products too costly to be used throughout a distribution system. As a result, current measurement approaches require manual sampling, which is not feasible for the consistent monitoring of free chlorine because chlorine concentrations vary significantly throughout pipeline distribution and over time and space. This research presents an alternative and cost-effective method of predicting free chlorine levels in drinking water using graphite electrodes coated with naturally grown microbial biofilms. This Microbial Potentiometric Sensor (MPS) array was installed in a Continuously Mixed Batch Reactor (CMBR), and drinking water containing variable free chlorine concentrations. The chlorine concentrations were introduced in a controlled manner, and the MPS signals were monitored over time. MPS signals were measured from the change in Open Circuit Potential (OCP) across the MPS array in real-time. An empirically derived relationship between the normalized change in OCP and free chlorine was established by fitting individual and average MPS data to a decaying exponential growth function in order to predict free chlorine levels. The results show that free chlorine can be predicted with reasonable accuracy, with model validation showing an average absolute error of ±0.09 ppm below 1.1 ppm and ±0.30 ppm between 1.1 and 2.7 ppm. However, the accuracy of predictions was reduced at higher free chlorine levels. The researchers conclude that MPS systems may benefit drinking water distribution systems by measuring free chlorine. These advantages of the MPS are especially pronounced in the developing world because this system is inexpensive and does not require routine maintenance or cleaning. The system relies on a naturally forming and regenerating biofilm and an inexpensive potentiometric meter to produce stable measurements.

Microbial potentiometric sensor array measurements in unsaturated soils.

The overarching goal of this study is to demonstrate a novel technology for monitoring changes in electrical potential of unsaturated soils using biofilm-populated electrodes. The novelty of the study stems from the fact that it demonstrates a method for measuring open-circuit potentials (OCP) in environments without the presence of an electrolyte solution. This study also reveals that using a biofilm-populated electrode as a reference in stable environments could successfully be employed to assess and monitor the electrochemical potential generated by plants and microorganisms. The findings imply that long-term (months to years) and real-time measurements of the open-circuit potential in unsaturated soils are possible. Because MPS arrays can directly measure open-circuit potential from the biofilm, the challenges related to locally induced electrochemical changes caused by microorganisms in the soil to achieve optimum physiological levels are eliminated. The simplicity of the technology, which allows for multiple indicator electrodes to be referenced against an "internal" reference electrode, enables spatial-temporal monitoring of the changes in the soil and the generation of 2D- or 3D-signal patterns. Once a signal pattern, generated by an array of sensors, develops (usually after 30 to 90 days), it does not significantly change unless the soil is exposed to external stimuli. The observed OCP phenomena, however, suggests that the change in OCP signal is independent of changes in soil conductivity resulting from the addition of water. In brief, findings suggest that the proposed technology can enable multidimensional profiling and long-term monitoring of changes occurring in unsaturated soils without direct implications of presence of water. The changes in the 2D or 3-D signal patterns, however, can be correlated to other important parameters that characterize soil health.

Real-time monitoring and prediction of water quality parameters and algae concentrations using microbial potentiometric sensor signals and machine learning tools.

The overarching hypothesis of this study was that temporal microbial potentiometric sensor (MPS) signal patterns could be used to predict changes in commonly monitored water quality parameters by using artificial intelligence/machine learning tools. To test this hypothesis, the study first examines a proof of concept by correlating between MPS's signals and high algae concentrations in an algal cultivation pond. Then, the study expanded upon these findings and examined if multiple water quality parameters could be predicted in real surface waters, like irrigation canals. Signals generated between the MPS sensors and other water quality sensors maintained by an Arizona utility company, including algae and chlorophyll, were collected in real time at time intervals of 30 min over a period of 9 months. Data from the MPS system and data collected by the utility company were used to train the ML/AI algorithms and compare the predicted with actual water quality parameters and algae concentrations. Based on the composite signal obtained from the MPS, the ML/AI was used to predict the canal surface water's turbidity, conductivity, chlorophyll, and blue-green algae (BGA), dissolved oxygen (DO), and pH, and predicted values were compared to the measured values. Initial testing in the algal cultivation pond revealed a strong linear correlation (R2 = 0.87) between mixed liquor suspended solids (MLSS) and the MPSs' composite signals. The Normalized Root Mean Square Error (NRMSE) between the predicted values and measured values were <6.5%, except for the DO, which was 10.45%. The results demonstrate the usefulness of MPSs to predict key surface water quality parameters through a single composite signal, when the ML/AI tools are used conjunctively to disaggregate these signal components. The maintenance-free MPS offers a novel and cost-effective approach to monitor numerous water quality parameters at once with relatively high accuracy.

The release of nanosilver from consumer products used in the home.

Nanosilver has become one of the most widely used nanomaterials in consumer products because of its antimicrobial properties. Public concern over the potential adverse effects of nanosilver's environmental release has prompted discussion of federal regulation. In this paper, we assess several classes of consumer products for their silver content and potential to release nanosilver into water, air, or soil. Silver was quantified in a shirt, a medical mask and cloth, toothpaste, shampoo, detergent, a towel, a toy teddy bear, and two humidifiers. Silver concentrations ranged from 1.4 to 270,000 microg Ag g product(-1). Products were washed in 500 mL of tap water to assess the potential release of silver into aqueous environmental matrices (wastewater, surface water, saliva, etc.). Silver was released in quantities up to 45 microg Ag g product(-1), and size fractions were both larger and smaller than 100 nm. Scanning electron microscopy confirmed the presence of nanoparticle silver in most products as well as in the wash water samples. Four products were subjected to a toxicity characterization leaching procedure to assess the release of silver in a landfill. The medical cloth released an amount of silver comparable to the toxicity characterization limit. This paper presents methodologies that can be used to quantify and characterize silver and other nanomaterials in consumer products. The quantities of silver in consumer products can in turn be used to estimate real-world human and environmental exposure levels.

Microbial potentiometric sensor: A new approach to longstanding challenges.

The underlying hypothesis of this study is that simple potentiometric measurements between sensing electrodes and a shared reference electrode - Microbial Potentiometric Sesnor (MPS) system - can be employed in a long-term, continuous mode of operation to resolve the spatial and temporal changes in environmental systems. To address the hypothesis, (1) a conceptual description of the MPS system and its postulated principle of operation are provided; (2) the MPS system performance is documented under controlled laboratory conditions; and (3) the capabilities of the MPS system are documented under quiescent and dynamic field condition. In a laboratory setting, the variability among different MPS signals was insignificant confirming the postulated high accuracy and reproducibility of the sensor performance. It also demonstrated statistically significant correlations with dissolved oxygen (DO) and oxidation-reduction potential (ORP) sensors, while showing capabilities of operating under anoxic and anaerobic conditions. Regardless of their locations in the model wetland system, three MPS sensors functioned without interruption and cleaning for a period >2 years, and thus demonstrating long-term durability of the MPS technology. In real batch-wastewater treatment facility, the deployed MPS system signals were able to describe the organic carbon trends and correlate with each treatment phase in a cycle. Data reproducibility and reliability exceeded the expectations better describing the carbon treatment levels than the DO and ORP sensors (p < 4.4 × 10-162 vs phase adjusted p < 3.0 × 10-58). While MPS signals correlate with specific parameters that describe the local process or environments, it is more prudent to employ both the magnitude and pattern of a composite signal like the MPS signal describe the change to reflect any shift in the local environment. When compared to a baseline pattern, this change in signal magnitude and pattern reveals important information that can be employed to tailor and optimize any condition or process which involves microorganisms.

User-oriented batch reactor solutions to the homogeneous surface diffusion model for different activated carbon dosages.

This paper presents a simplified approach and user-oriented solutions to the homogeneous surface diffusion model (HSDM) equations for determining the surface diffusivity using a batch reactor system. Once the surface diffusivity is known, this model could also be used to estimate the performance of activated carbon (AC) applications as a function of contact time. In addition, fixed-bed performance can be predicted using the user-oriented solutions to the HSDM for fixed beds. The step-by-step procedure for determining surface diffusion coefficients of an activated carbon adsorber, which was initially developed by Hand, Crittenden and Thacker in 1983 for a carbon dose where C(equilibrium)/C(0)=0.5, is modified to allow calculations for different carbon dosages. This modification provides solutions to the HSDM equations for different activated carbon dosages. The solutions to the HSDM framework are provided as simplified algebraic equations suitable for quick and easy estimations of D(S). The excel spread sheet is provided in the supplemental information and a detailed example is discussed.

An experimental approach for determining thermodynamic surface complexation descriptors of weak-acid oxyanions onto metal (hydr)oxides: Case study of arsenic and titanium dioxide.

The extensive literature review suggests that there are two main reasons for contradictory thermodynamic parameter values obtained via sorption experiments: (1) many of the studies are conducted under unrealistic conditions where the sorbate/sorbent ratios are so high that physisorption is artificially induced; or (2) many of the studies incorrectly calculate the equilibrium constants. The goal of this study is to demonstrate a methodology that describes how to properly determine and verify theoretically predicted thermodynamic descriptors. The study employs arsenate and titanium dioxide as a model sorbate-sorbent pair, which is equilibrated under realistic conditions for a period of 3 days at two different pH conditions (~6.5 and ~8.5) and three different temperatures (7 °C, 25 °C and 35 °C) in 10 mM NaHCO3. At pH ≈ 8.55, ΔGo values were -83.38 ±â€¯1.62 kJ/mol, -88.13 ±â€¯0.66 kJ/mol, and -90.78 ±â€¯0.61 kJ/mol for sorption performed at 7 °C, 25 °C and 35 °C, respectively. Decreasing the pH to about 6.65 resulted in slightly less negative values of ΔGo to -73.38 ±â€¯1.58 kJ/mol, -77.14 ±â€¯1.52 kJ/mol, and -78.75 ±â€¯1.53 kJ/mol for sorption conducted at the same respective temperature conditions. These values overlap with the ΔGo ranges reported for sorption of arsenate on metal oxides. Change in enthalpy values of ΔHo = -19.04 kJ/mol at pH ≈ 6.65 and ΔHo =-9.35 kJ/mol at pH ≈ 8.55 were observed. Based on reports, which suggest that at lower pH more bidentate ligands are being formed, these values are expected. The change in entropy values ranged from ΔSo = 0.19 kJ/mol K at pH ≈ 6.55 to ΔSo = 0.26 kJ/mol K at pH ≈ 8.55, which suggests lower level of disorder among the created complexes at lower pH and it is in line with the rationale that bidentate complexes are better organized on the surface of the sorbent and less susceptible with desorption. These findings clearly demonstrate that experimentally obtained ΔG0 and other thermodynamic values and trends could be obtained to reflect and confirm model predictions when the existing sorption theory is properly translated into experimental practice. The sorbate-sorbent bond in chemisorption has covalent character, characterized with short bond length and higher bond energy, which makes it less reversible when compared to physisorption, and therefore highly significant from a sorbent remediation-performance practical point of view and long-term waste sorbents disposal. While thermodynamic parameter modeling represents a good first step in determining the suitability of an initial design, experimental techniques potentially have the ability to provide far more superior description of the thermodynamic sorbent/sorbate interactions under realistic conditions.

Copper release and transformation following natural weathering of nano-enabled pressure-treated lumber.

Commercially available lumber, pressure-treated with micronized copper azole (MCA), has largely replaced other inorganic biocides for residential wood treatment in the USA, yet little is known about how different outdoor environmental conditions impact the release of ionic, nano-scale, or larger (micron-scale) copper from this product. Therefore, we weathered pressure treated lumber for 18 months in five different climates across the continental United States. Copper release was quantified every month and local weather conditions were recorded continuously to determine the extent to which local climate regulated the release of copper from this nano-enabled product during its use phase. Two distinct release trends were observed: In cooler, wetter climates release occurred primarily during the first few months of weathering, as the result of copper leaching from surface/near-surface areas. In warmer, drier climates, less copper was initially released due to limited precipitation. However, as the wood dried and cracked, the exposed copper-bearing surface area increased, leading to increased copper release later in the product lifetime. Single-particle-ICP-MS results from laboratory prepared MCA-wood leachate solutions indicated that a) the predominant form of released copper passed through a filter smaller than 0.45 micrometers and b) released particles were largely resistant to dissolution over the course of 6 wks. Toxicity Characteristic Leaching Procedure (TCLP) testing was conducted on nonweathered and weathered MCA-wood samples to simulate landfill conditions during their end-of-life (EoL) phase and revealed that MCA wood released <10% of initially embedded copper. Findings from this study provide data necessary to complete a more comprehensive evaluation of the environmental and human health impacts introduced through release of copper from pressure treated lumber utilizing life cycle assessment (LCA).