Uncoated Sand Research Articles

Heat extraction through conductive proppants

The absence of direct contact between the working fluid and the geothermal formation in closed-loop geothermal system (CLGS) limits its application to direct heating. Enhancing the CLGS heat extraction performance could lead to an effective utilization of CLGS for power generation purposes. Optimizing CLGS performance is still an active area of research. In this paper, we propose a fractured closed-loop geothermal system (FCLGS) with a conductive plug installed downhole and a fracture propped with conductive proppants to improve heat exchange performance of CLGS. Using an integrated numerical approach, we first explore the potential to improve the thermal conductivity of common types of proppants by applying metallic coatings. Applying a thick layer of copper coating to Aluminum Nitride proppants yields a thermal conductivity of 44.91 W/(mK), 24 times larger than that of uncoated sand. The proppant/coating with largest thermal conductivity is used for field-scale simulations. Heat extraction performance of FCLGS is evaluated at the field-scale under different parameter combinations. Numerical results show that incorporating a conductive plug into a fractured CLGS improves net power output by 30.60 %. Conductive plug shape and size play an important role in heat extraction in FCGLS. In addition, a circulation velocity of 2 m/s is observed to yield the highest increase in net power output.

Biosurfactant-affected mobility of oxytetracycline and its variations with surface chemical heterogeneity in saturated porous media

Herein, the influences of rhamnolipid (a typical biosurfactant) on oxytetracycline (OTC) transport in the porous media and their variations with the surface heterogeneities of the media (uncoated sand, goethite (Goe)-, and humic acid (HA)-coated sands) were explored. Compared to uncoated sand, goethite and HA coatings suppressed OTC mobility by increasing deposition sites. Interestingly, rhamnolipid-affected OTC transport strongly depended on the chemical heterogeneities of aquifers and biosurfactant concentrations. Concretely, adding rhamnolipid (1–3 mg/L) inhibited OTC mobility through sand columns because of the bridging effect of biosurfactant between sand and OTC. Unexpectedly, rhamnolipid of 10 mg/L did not further improve the inhibition of OTC transport owing to the fact that the deposition capacity of rhamnolipid reached its maximum. OTC mobility in Goe-coated sand columns was inhibited by 1 mg/L rhamnolipid. However, the inhibitory effect decreased with the increasing rhamnolipid concentration (3 mg/L) and exhibited a promoted effect at 10 mg/L rhamnolipid. This surprising observation was that the increased rhamnolipid molecules gradually occupied the favorable deposition sites (i.e., the positively charged sites). In comparison, rhamnolipid facilitated OTC transport in the HA-coated sand column. The promotion effects positively correlated with rhamnolipid concentrations because of the high electrostatic repulsion and deposition site competition induced by the deposited rhamnolipid. Another interesting phenomenon was that rhamnolipid's enhanced or inhibitory effects on OTC transport declined with the increasing solution pH because of the decreased rhamnolipid deposition on porous media surfaces. These findings benefit our understanding of the environmental behaviors of antibiotics in complex soil-water systems containing biosurfactants.

Effects of biofilms on the retention and transport of PFOA in saturated porous media.

Understanding the fate and transport of perfluorooctanoic acid (PFOA) in soil and groundwater is essential to reliable assessments of its risks. This study investigated the impacts of Gram-positive Bacillus subtilis (BS), Gram-negative Pseudomonas aeruginosa (PA) and wild microbiota (WM) biofilm on the transport of PFOA in saturated sand columns at two ionic strengths (i.e., 1.0 and 20.0mM NaCl). The retention of PFOA in biofilm-coated sand columns was higher than that in uncoated sand columns, due to biofilm-induced reinforced hydrophobic interactions and surface roughness, and decreased zeta potential. However, the retention effects varied among biofilm bacterial species with PFOA retardation factors in PA, WM and BS columns of 1.29-1.38, 1.21-1.29 and 1.11-1.15, respectively. Notably, PA biofilm had the most pronounced effect on PFOA retention. While increasing ionic strength promoted the retention of PFOA in BS biofilm-coated sand, it had no significant impact on PFOA transport in PA and WM biofilm-coated sand. This could be attributed to the differences in biofilm composition, deviating the ionic strengths effects on electrostatic double layer compression. The advection dispersion equation coupled with two-site kinetic retention model well described the transport of PFOA in all saturated columns. Our findings reveal that biofilm plays important roles in PFOA transport in porous media, instructive for risk assessment and remediation of PFOA contamination.

Black coating of quartz sand towards low-cost solar-absorbing and thermal energy storage material for concentrating solar power

Solid particles based direct solar absorbing heat transfer fluid (HTF) and thermal energy storage (TES) material is gaining increasing interests for high-temperature concentrating solar power (CSP). However, there is no particulate material that simultaneously possesses the performance and cost metrics needed to achieve the target levelized cost of electricity of future CSP, including high solar absorptance, high thermal conductivity, good mechanical and thermal stability, and importantly, low cost. In this work, we demonstrate a facile and scalable method to turn low-cost quartz sand into a direct solar-absorbing HTF and TES material by coating a thin silica shell containing black spinel nanoparticles (Cu0.5Cr1.1Mn1.4O4and CuCr2O4). The black coating increases the solar absorptances of the sand from ∼0.43 to ∼0.89. The coated quartz sand also shows excellent stability and nearly unchanged optical properties after 100-hr of isothermal annealing at 800 °C and up to 24-hr ball mixing with Yttria Stabilized Zirconia (YSZ) grinding balls. The measured effective thermal conductivity of the coated quartz sand increases 0.30 W m−1 K−1 to 0.65 W m−1 K−1 from 20 °C to 700 °C and is comparable to that of uncoated sand as well as Carbo ceramic particle beds currently used in CSP plants. The coating process could represent a cost-effective and generic strategy to enhance the solar absorptance of solid particles, and when applied on low-cost sand, could turn them into low-cost and high-performance HTF and TES media for future CSP systems.

Influence of analog barrier type and impact velocity on the energy dissipation of simulant saprolitic rock particles colliding rigid and deformable barrier systems

The analysis of the collision problem of solid objects is of interest in the fundamental study and design of barrier systems protecting infrastructures against debris flows, which can be particularly useful in the sustainable design of transportation systems in urban areas with hilly terrain or extended railway/highway lines which pose additional hazards in relation to landslide and granular flow problems. However, many times debris flows encounter weathered materials in which the formed coating of microparticles on the surfaces of sand grains and fragmented rock alter significantly their contact response. In the present study, we examined with grain-scale tests the impact problem of particle – analog barrier systems encountering natural sand particles in their original form and also in the presence of a laboratory-created soft coating of microparticles, representing simulant saprolitic rock grains. Different types of analog barriers were examined including brittle and ductile rigid blocks and also analog deformable buffering and the data were analyzed in terms of the coefficient of restitution and percentage energy loss expressing the energy dissipation during collision. For impacts on analog rigid barriers, brittle to elastoplastic deformation of micro-asperities comprised a major influencing factor for uncoated sand particles on the energy dissipation during collision, however, additional (and somehow competitive) mechanisms were introduced when coated particles were encountered in the collision tests. Due to the highly hysteretic behavior of the analog deformable barrier, the results converged between uncoated and coated particles in terms of coefficient of restitution and percentage energy loss during collision. For all the combinations of particles and analog barriers, transformation of the translational (input) kinetic energy into different forms also contributed to the dissipation of energy, more prominently for the analog deformable buffering as the deviating angles from the experiments suggested. Additional comparisons were made between 2-dimensional and 3-dimensional analyses of the experiments using one and two high speed cameras, respectively, revealing small differences in the resultant energy dissipation from the two approaches. The results from the present study offer some fundamental insights into the impact problem of particle-block systems and also input parameters which can be further utilized in discrete-based numerical simulations of debris flow – barrier interaction problems.

Comparison Of Local And Import Sands Quality Laboratory Tests Results For Used In Hydraulic Fracturing Operations

Sand quality laboratory tests have been carried out on local and import uncoated sands. Based on results of sand sieve, roundness, sphericity, turbidity, acid solubility and crush resistance at 3000 psi quality tests of import uncoated sand has better quality than local sand. The import sand fulfills API – RP 56 specification requirements and will be able used in hydraulic fracturing operation. Whereas, the local uncoated sand does not achieve API - RP 56 specification requirements.

Effect of Iron Oxide Coatings on Thermal Conductivity of Silica Sands

Engineering properties of soils coated with naturally occurring or artificially manufactured particles can significantly differ from those of uncoated soils. This study aims at evaluating the effect of iron oxide coatings on the thermal conductivity of silica sands. Laboratory thermal needle probe tests were conducted on uncoated, goethite coated and hematite coated Ottawa 20/30 and Ottawa 100/200 sands. Test results demonstrated that the thermal conductivity (k) of all tested materials increases with increasing applied vertical stress, and the k of hematite coated sands was greater than that of uncoated sands under the similar relative density. However, the measured k of both uncoated and coated sands is a single function of porosity regardless of applied stress level, reflecting the limited effect of applied stress and iron oxide mineral coating on the k of tested materials. Simple resistances in series model was employed to explain the observed test results, and the results of this study was compared with k of previous studies on uncoated natural soils.

Polarity Dependence of Transport of Pharmaceuticals and Personal Care Products Through Birnessite-Coated Porous Media

Pharmaceuticals and personal care products (PPCPs) have drawn increasing concern of environmental health as they are continuously released into the environment. This study examined the effects of birnessite (δ-MnO2) on the transport and retention of five PPCPs in porous media under steady saturated flow conditions. Considering that natural birnessite occurs as discrete particles and small nodules, birnessite-coated sand was used to mimic the natural regime of birnessite in the environment. Batch isotherm experiments were conducted using uncoated and birnessite-coated sand; results showed that the difference in the affinity of the five PPCPs was correlated to their polarity characteristics. Column experiments were conducted by mixing 0, 10, and 20% birnessite-coated sands with the uncoated sands. These three percentages are equivalent to three contents of manganese (Mn) in the experimental columns (0, 55, and 109 μg Mn g−1 sand). Results suggested that polar compounds (such as bisphenol-A, tetracycline, and ciprofloxacin) had a higher affinity to birnessite-coated sands than the weak polar compounds (such as ibuprofen and carbamazepine) because the polarity was favorable to electrostatic attraction and oxidative reaction. Overall, birnessite decreased the mobility of polar PPCPs but exerted no significant effect on the mobility of weak polar PPCPs under continuous flow conditions. The polarity-based correlation extended traditional electrostatic theory while well interpreting the complicated effects of birnessite on the adsorption and transport of PPCPs, especially neutral or non-dissociated compounds like carbamazepine.

Cotransport of micro- and nano-plastics with chlortetracycline hydrochloride in saturated porous media: Effects of physicochemical heterogeneities and ionic strength

Global production and use of plastics have resulted in the wide dissemination of micro- and nano-plastics (MNPs) to the natural environment. Potentially acting as a vector, the role of MNPs on the fate and transport of environmental pollutants (e.g., antibiotics such as chlortetracycline hydrochloride; CTC) has garnered global concern recently. Herein, the cotransport of MNPs and CTC in columns packed with uncoated sand or soil colloid-coated sand (SCCS) under different degrees of physicochemical heterogeneity and ionic strength was systematically explored. Our results show that MNPs and CTC inhibit the transport of each other when they coexist. The adsorption of CTC onto sand grains, soil colloids, and MNPs, as well as the aggregation of MNPs in the presence of CTC could be the major contributors to the enhanced retention of CTC and MNPs. In SCCS with different degrees of soil colloid coating, the adsorption of CTC on soil colloids is critical to influence the transport of CTC, and the nonlinear retention of MNPs to soil colloids is mainly attributed to the alteration of collector surface roughness by soil colloids. High ionic strength slightly facilitates CTC transport due to the competition for adsorption sites and the formation of CTC macromolecules, but significantly inhibits MNPs transport by suppressing the electrostatic double layers based on colloid stability theory. Consequently, the cotransport of MNPs and CTC is governed by the coupled interplay of collector surface roughness and chemical heterogeneity, due to the soil colloid coatings and the adsorbed CTC on the surfaces associated with solution chemistries such as ionic strength. Increased cotransport of MNPs and CTC occurred under a higher concentration of MNPs due to a larger number of adsorption sites for CTC. Our findings advance the current understanding of the complex cotransport of MNPs and antibiotics in the environment. This information is valuable for understanding contaminant fate and formulating strategies for environmental remediation due to the contamination of MNPs and co-occurring contaminants.

The influence of CO2-transformed iron oxide grain coatings on the frictional stability and transport properties of simulated faults in sandstones

Carbon sequestration involves long-term containment of CO2 in ideally sealed reservoirs. However, CO2 migration can weaken rocks and faults by geochemical alteration, elevate risks of seismic hazards, and loss of inventory. Recent studies show that CO2 bleaching can alter the iron oxide grain coating of sand-sized quartz in sandstones, which may impose a significant influence on frictional stability and permeability evolution of faults in sandstones. This study investigates the influence of iron oxide grain coatings via coupled shear-flow experiments on uncoated, hematite-coated, and CO2-transformed goethite-coated synthetic sand gouge. Shear strength, frictional stability, healing/relaxation, and shear-parallel permeability are measured in velocity-stepping and slide-hold-slide loading modes. Hematite-coated sand exhibits the highest shear strength, followed by goethite-coated and uncoated sand. All samples, both coated and uncoated, show similar residual shear strength. Frictional stability measurements suggest hematite-coated sand may undergo potential seismic slip (negative (a − b) values); goethite-coated sand is aseismic (positive (a − b) values) but features higher frictional healing and relaxation. Shear-parallel permeability enhances during initial shear in all samples, followed by a sharp decline after the peak strength, except for goethite-coated sand, for which permeability reduction is moderate. SEM characterizations pre- and post-shear suggest that the competitive liberation, transport, and clogging of coating particles and shear-produced wear products can be an important mechanism in permeability evolution.

Surface heterogeneity mediated transport of hydrochar nanoparticles in heterogeneous porous media.

The effects of clay particles (montmorillonite, M) and phosphate (P) on the transport of hydrochar nanoparticles (NPs) in water-saturated porous media (uncoated and aluminum (Al) oxide-coated sands) were explored in NaCl (1-50 mM) solutions. Our results showed that the deposition behaviors of hydrochar NPs affected by M and phosphate were significantly different between pH 6.0 and pH 9.0, especially in Al oxide-coated sand. This can be attributed to their distinct surface characteristics: hydrochar agglomerates with a larger pore size distribution, more carboxylate groups, and less negative charges on the surface at pH 9.0 than those at pH 6.0. In Al oxide-coated sand, block adsorption of hydrochar was alleviated appreciably with the presence of M due to the preferential preoccupies of M on these favorable retention sites. On the contrary, M substantially increased the hydrochar retention on uncoated sand due to the formation of nanoaggregates between hydrochar and M. Differently, phosphate substantially enhanced the transport of hydrochar, even in coated sand, due to the strong phosphate adsorption onto Al oxide on the surface of sand and hydrochar. Our findings will provide useful insights into designing effective strategies for land application of hydrochar while minimizing potential environmental risks. Graphical abstract.

Removal of microcystin-LR and other water pollutants using sand coated with bio-optimized carbon submicron particles: Graphene oxide and reduced graphene oxide

Sand is the most used filter adsorbent material in a drinking water treatment plant (DWTP) or household purpose filters. However, sand poses challenge for the micropollutant removal in presence of metals, natural organic matter, total organic carbon, ammonia and other macro-pollutants. In this study, sand was coated using laboratory-synthesized graphene oxide (GO) and reduced graphene oxide (rGO) to enhance its surface properties in terms of hydrophobicity, roughness and specific surface area (6 times higher for GO/rGO as compared to 5.5 m2/g for sand), that allowed an enhanced adsorption of macro pollutants as well as one target micropollutant: Microcystin-LR (MC-LR). A more electropositive surface than sand (zeta potential: −43.2 mV) in form of iron oxide-coated sand (IOCS or Fe: −21.2 mV) and its combination with GO (FeGO: −13.4 mV) was tested to validate the hypothesis of enhanced MC-LR adsorption due to electrostatic attraction. Two known MC-LR degraders (Arthrobacter ramosus and Bacillus sp.) were screened before bioaugmentation, based on the biofilm forming potential for each coated sand composite. Additionally, the dosage of GO and rGO were bio-optimized before coating them over the sand grains (400 mg/L for GO and 520 mg/L for rGO), to obtain a non-toxic and non-disruptive effect and providing at least 60% cell viability. A highest MC-LR removal of 91% was obtained under biodegradation phase using rGO-coated sand that showed an increase of 47.2% in MC-LR removal when compared to physical adsorption phase. Sand filter (control) showed a maximum MC-LR removal of 54.7%. The highest saturation adsorbent constants of 8.5 mg/kg and 7.4 mg/kg were obtained for GO and rGO-coated sand media, respectively, which was 5–6 times higher than the uncoated sand.

Importance of Al/Fe oxyhydroxide coating and ionic strength in perfluorooctanoic acid (PFOA) transport in saturated porous media

Understanding subsurface transport of per- and polyfluoroalkyl substances (PFASs) is of critical importance for the benign use and risk management of PFASs. As one of the most commonly found PFASs, perfluorooctanoic acid (PFOA) is used as a representative PFAS and water-saturated column experiments were conducted to investigate the effect of Al/Fe oxyhydroxide coating and ionic strength on its transport at an environmentally relevant PFOA concentration (6.8 μg L−1). Our results showed a clear increase in PFOA retardation in Al/Fe oxyhydroxide coated sand (retardation factor: Al: 1.87–5.58, Fe: 1.28–4.05) than those in uncoated sand (1.00–1.05), due to the stronger electrostatic attraction between anionic PFOA and Al/Fe oxyhydroxide coated sand surface. Notably, Al oxyhydroxide have a more profound effect on PFOA retention and retardation than Fe oxyhydroxide. Besides, higher ionic strength significantly inhabited PFOA retention and retardation in positively charged sand, and the considerable retention of PFOA (∼90%) in deionized water than those in 1.5 mM and 30.0 mM NaCl (<10%) clearly proves the role of competitive adsorption of Cl− on PFOA transport in positively charged sand. In contrast, higher ionic strength (0 mM–30 mM NaCl) slightly increased PFOA retardation in negatively charged sand, illustrating the dominance of electrostatic interaction. Our findings advance current knowledge to understand PFOA transport in natural media with different surface charge property under environmental PFOA concentrations.

Coupled effect of colloids and surface chemical heterogeneity on the transport of antibiotics in porous media

Release of antibiotics into the environment has caused ecological and human health concerns in recent years. However, little is known about their transport behaviors in chemically heterogeneous porous media. In this study, we investigated the coupled effects of surface chemistry and soil colloids on the transport of ciprofloxacin and tetracycline through sand under steady state saturated flow conditions. Both antibiotics had a much higher capacity of adsorption on soil colloids (17,500 mg/kg for ciprofloxacin and 8600 mg/kg for tetracycline) than on sand (5.11 mg/kg for ciprofloxacin and 2.80 mg/kg for tetracycline). However, ciprofloxacin adsorption increased to 8.91 mg/kg after the sand was coated with iron oxide and to 8.73 mg/kg after the sand was coated with humic acid. Tetracycline, adsorption increased to 7.99 mg/kg after sand was coated with iron oxide coated sand and to 8.35 mg/kg after the sand was coated with humic acid coated The high adsorption capacity of ciprofloxacin led to a recovery rate of <4% in the effluents of the columns containing 0%, 20% and 50% of iron oxide/humic acid coated sand. The surface coating decreased the recovery rates of tetracycline from 35.4% (in uncoated sand) to 12.0% (in column containing 50% iron oxide coated sand) and 0.010% (in column containing 50% humic acid coated sand), respectively. Once adsorbed to soil colloids, the recovery rate of ciprofloxacin increased by 26.7% in uncoated sand column, 21.1% in iron oxide coated sand column, and 32.7% in humic acid coated sand column. Similarly, the presence of the colloids increased the recovery rate of tetracycline from 13.8% to 33.2% after the sand was coated with humic acid. Colloids did not significantly influence the transport and recovery of tetracycline in the uncoated sand and iron oxide coated sand due likely to its lower adsorption affinity.

Characteristic and Erosion Study of Uncoated Sand Proppant Using Impingement Test

The characterization and properties of sand from beaches in Malaysia that has potential to be used as proppant in hydraulic fracturing was investigated through impingement tests. Eight sand samples were obtained from eight different locations in Malaysia; three samples from West Coastal beaches (Selangor) and five from East Coastal beaches (Kelantan). These impingement tests were conducted by varying several parameters i.e. size ranges of sand, type of targeted metal and distance of nozzle standoff from target. In these tests, air was fed through an acrylic pipe with inner diameter of 8 mm and 4 m length at gas velocity of 100 m/s and flow rate of 200 l/min. 250 g of sand samples were fed in compressed air stream and the samples were projected toward targeted metal i.e. mild steel and aluminium with a 90° of impingement angle. These tests were conducted at different separation distance of 0.5, 2.5 and 5.0 inches of nozzle and target. The results were analysed by measuring the mass loss of the metals after impingement and the images of the impinged metals were captured using camera. The highest metal loss was obtained when the targeted metal was impacted with the largest size range of sand samples. The mass loss of mild steel ranged from 0.03 to 0.16 g and 0.10 to 0.22 g for aluminium, at a variable distance between the nozzle and metal target due to higher hardness of mild steel than aluminium. The mass loss reduced when the distance between nozzle and metal target increased due to energy loss because of the inter particle collision.

Micromechanical behaviour of a polymer-coated sand

Polymers, such as polydimethylsiloxane, can be utilised as functional coatings with water repellent properties in granular materials. These new surface-modified granular materials have potential applications in barriers to control wetting. However, limited knowledge exists on the mechanical behaviour of particles with synthetic coatings. Here, a fundamental insight is provided by investigating the micromechanical behaviour of a coated sand with extreme water repellency by means of a custom-built inter-particle loading apparatus. To synthesize the water repellent coatings, an industrial sand was subjected to 3% mass ratio of dimethyldichlorosilane. In a comparative investigation between coated and uncoated (natural) sand particles, the results revealed (1) a decrease of the normal stiffness and (2) a dependency of the inter-particle friction coefficient on the normal force for the coated sand particles. In particular, the inter-particle friction coefficient significantly increased at low normal forces (lower than 2 N) with this effect decreasing as the coatings were damaged under a high normal force (over 5 N), and with the micromechanical behaviour of the coated sand converging towards that of the uncoated (natural) sand. The results are compared to theoretical models and mechanisms are proposed to explain the findings. By changing the nature of the coatings, this paper provides a basis for future research on granular materials with functional coatings for other applications.

Effects of Escherichia coli and phosphate on the transport of titanium dioxide nanoparticles in heterogeneous porous media

Transport behaviors of titanium dioxide nanoparticles (nTiO2) were examined in the individual- and co-presence Escherichia (E.) coli and phosphate in heterogeneous sand (uncoated and iron oxyhydroxide-coated sand) columns. The results showed that for the individual presence of phosphate, the degree of nTiO2 deposition was less in uncoated than in iron oxide-coated sands. In contrast, an opposite trend that greater deposition of nTiO2 in uncoated than in coated sands occurred in the individual presence of E. coli. These observations are due to the phosphate adsorption changing the charge of NPs and iron oxyhydroxide-coated sand, or the preferential adhesion of bacterial to coated sand. In the copresence of E. coli and phosphate, interestingly, the phosphate level plays an important role in influencing nTiO2 transport. At a high phosphate concentration (>1.0 mM), the deposition of nTiO2 with the individual presence of E. coli was stronger than nTiO2 in the copresence of both E. coli and phosphate, regardless of sand type. The potential mechanism was that phosphate adsorption led to the formation of more negatively charged NPs-bacteria complexes that have higher mobility in sand columns. At a low phosphate level (≤0.1 mM), a similar observation occurred in uncoated sand. Nevertheless, the deposition of nTiO2 with copresence of E. coli and phosphate was greater than nTiO2 with E. coli in oxyhydroxide-coated sand. It was attributed to the formation of large NPs-bacteria-phosphate clusters (less mobile) and the preferential adhesion of E. coli cells to iron oxyhydroxide coating simultaneously. Taken together, our findings provide crucial knowledge for better understanding the fate, transport, and potential risks of engineered nanoparticles in complicated environmental settings where bacteria and phosphate are ubiquitous.

Removal of hexavalent chromium using mackinawite (FeS)-coated sand.

This study investigates the feasibility of mackinawite (FeS)-coated sand in permeable reactive barrier applications to treat Cr(VI)-contaminated groundwater under anoxic conditions. For this, Cr(VI) sorption experiments were conducted using both coated and uncoated sands. Solution-phase Cr speciation and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated the complete reduction of Cr(VI) to Cr(III) by coated sand. At pH 4.7, substantial amounts of Cr(III) remained in solution due to its unfavorable cationic adsorption at acidic pH. At pH 7.1 and 9.8, it was quantitatively immobilized by forming Cr(III)-bearing precipitates. In contrast, uncoated sand showed the decreasing Cr(VI) sorption with pH. In uncoated sand, magnetite impurities would mediate the partial reduction of Cr(VI). Thus, the pH-dependent sorption by uncoated sand was due to both unfavorable anionic Cr(VI) adsorption and its lesser reduction to Cr(III) with pH. Compared to uncoated sand, coated sand showed significantly increased Cr(VI) sorption at neutral to basic pH. By Fe K-edge XANES analysis, FeS was mainly responsible for Cr(VI) reduction by coated sand, with a green rust-like phase being the major Fe product. Since Fe(OH)3 is not thermodynamically stable under the redox conditions favoring formation of green rust, Fe(III)-substituted Cr(OH)3 likely represents a Cr(III)-bearing phase.

Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand.

Understanding the fate and transport including remobilization of graphene oxide nanomaterials (GONMs) in the subsurface would enable us to expedite their benign use and evaluate their environmental impacts and health risks. In this study, the retention and release of GONMs were investigated in water-saturated columns packed with uncoated sand (Un-S) or iron oxide-coated sand (FeS) at environmentally relevant solution chemistries (1-100mM KCl and 0.1-10mM CaCl2 at pH7 and 11). Our results showed that increasing ionic strength (IS) inhibited GONMs' transport, and the impact of K+ was less than Ca2+. The positively charged iron oxide coating on sand surfaces immobilized the negatively charged GONMs (pH7) in the primary minimum, yielding hyperexponential retention profiles particularly in Ca2+. A stepwise decrease in pore-water IS caused detachment of previously retained GONMs. The mass of GONMs released during each detachment step correlated positively with the difference in secondary minimum depth (ΔΦmin2) at each IS, indicating that the released GONMs were retained in the secondary minimum. While most retained GONMs were re-entrained upon lowering pore-water IS in Un-S, decreasing IS only released limited GONMs in FeS, which were captured in the primary minimum. Introducing 1mM NaOH (pH11) released most retained GONMs in FeS; and average hydrodynamic diameters of the detached GONMs upon injecting NaOH were significantly smaller than those of GONMs in the influent and retentate, suggesting that NaOH induced GONMs disaggregation. Our findings advance current knowledge to better predict NMs' fate and transport under various solution chemistries such as during rainfall events or in the mixing zones between sea water and fresh water where transient IS changes drastically.

Evaluation of nano ceramic coating on radiographic defects of thin-walled AL4-4 aluminum alloy sand casting

Internal defects are among the problems in gravity casting of aluminum parts. The main internal volumetric defects are gas and shrinkage defects which form during solidification of the melt and drastically reduce the quality of the produced parts. These defects adversely affect the mechanical properties of thin walled castings parts. In this study, ceramic nanoparticles coatings were applied on the sand mold and the effect of mold coatings on the reduction of defects were investigated. X-ray radiography was used to detect defects in sand molds with ceramic nanoparticles coatings. For comparison, this test was performed on molds with micro-ceramic and graffiti coatings and uncoated sand mold. The results showed that the maximum amount of gas and shrinkage defects was observed in casting parts from AL4-1 alloy in uncoated molds. On the other hand, the minimum defects were found in molds coated with ceramic nanoparticles. It seems that the reduced defects in casting parts in molds coated with ceramic nanoparticles may be due to high thermal and chemical stability and higher heat transfer rate of the coating. These results can facilitate the production of high quality aluminum alloys parts using nanotechnology.