Cr(VI) Reduction and Sequestration by FeS Nanoparticles Formed in situ as Aquifer Material Coating to Create a Regenerable Reactive Zone

Remediation of large and dilute plumes of groundwater contaminated by oxidized pollutants such as chromate is a common and difficult challenge. Herein, we show that in situ formation of FeS nanoparticles (using dissolved Fe(II), S(-II), and natural organic matter as a nucleating template) results in uniform coating of aquifer material to create a regenerable reactive zone that mitigates Cr(VI) migration. Flow-through columns packed with quartz sand are amended first with an Fe2+ solution and then with a HS- solution to form a nano-FeS coating on the sand, which does not hinder permeability. This nano-FeS coating effectively reduces and immobilizes Cr(VI), forming Fe(III)-Cr(III) coprecipitates with negligible detachment from the sand grains. Preconditioning the sand with humic or fulvic acid (used as model natural organic matter (NOM)) further enhances Cr(VI) sequestration, as NOM provides additional binding sites of Fe2+ and mediates both nucleation and growth of FeS nanoparticles, as verified with spectroscopic and microscopic evidence. Reactivity can be easily replenished by repeating the procedures used to form the reactive coating. These findings demonstrate that such enhancement of attenuation capacity can be an effective option to mitigate Cr(VI) plume migration and exposure, particularly when tackling contaminant rebound post source remediation.

Battery metal recycling by flash Joule heating

The staggering accumulation of end-of-life lithium-ion batteries (LIBs) and the growing scarcity of battery metal sources have triggered an urgent call for an effective recycling strategy. However, it is challenging to reclaim these metals with both high efficiency and low environmental footprint. We use here a pulsed dc flash Joule heating (FJH) strategy that heats the black mass, the combined anode and cathode, to >2100 kelvin within seconds, leading to ~1000-fold increase in subsequent leaching kinetics. There are high recovery yields of all the battery metals, regardless of their chemistries, using even diluted acids like 0.01 M HCl, thereby lessening the secondary waste stream. The ultrafast high temperature achieves thermal decomposition of the passivated solid electrolyte interphase and valence state reduction of the hard-to-dissolve metal compounds while mitigating diffusional loss of volatile metals. Life cycle analysis versus present recycling methods shows that FJH significantly reduces the environmental footprint of spent LIB processing while turning it into an economically attractive process.

Gypsum scale formation and inhibition kinetics with implications in membrane system

Water desalination using membrane technology is one of the main technologies to resolve water pollution and scarcity issues. In the membrane treatment process, mineral scale deposition and fouling is a severe challenge that can lead to filtration efficiency decrease, permeate quality compromise, and even membrane damage. Multiple methods have been developed to resolve this problem, such as scale inhibitor addition, product recovery ratio adjustment, periodic membrane surface flushing. The performance of these methods largely depends on the ability to accurately predict the kinetics of mineral scale deposition and fouling with or without inhibitors. Gypsum is one of the most common and troublesome inorganic mineral scales in membrane systems, however, no mechanistic model is available to accurately predict the induction time of gypsum crystallization and inhibition. In this study, a new gypsum crystallization and inhibition model based on the classical nucleation theory and a Langmuir type adsorption isotherm has been developed. Through this model, it is believed that gypsum nucleation may gradually transit from homogeneous to heterogeneous nucleation when the gypsum saturation index (SI) decreases. Such transition is represented by a gradual decrease of surface tension at smaller SI values. This model assumes that the adsorption of inhibitors onto the gypsum nucleus can increase the nucleus superficial surface tension and prolong the induction time. Using the new model, this study accurately predicted the gypsum crystallization induction times with or without nine commonly used scale inhibitors over wide ranges of temperature (25-90 °C), SI (0.04-0.96), and background NaCl concentration (0-6 mol/L). The fitted affinity constants between scale inhibitors and gypsum show a good correlation with those between the same inhibitors and barite, indicating a similar inhibition mechanism via adsorption. Furthermore, by incorporating this model with the two-phase mineral deposition model our group developed previously, this study accurately predicts the gypsum deposition time on the membrane material surfaces reported in the literature. We believe that the model developed in this study can not only accurately predict the gypsum crystallization induction time with or without scale inhibitors, elucidate the gypsum crystallization and inhibition mechanisms, but also optimize the mineral scale control in the membrane filtration system.

Rare earth elements from waste

Rare earth elements (REEs) are critical materials in electronics and clean technologies. With the diminishing of easily accessible minerals for mining, the REE recovery from waste is an alternative toward a circular economy. Present methods for REE recovery suffer from lengthy purifications, low extractability, and high wastewater streams. Here, we report an ultrafast electrothermal process (~3000°C, ~1 s) based on flash Joule heating (FJH) for activating wastes to improve REE extractability. FJH thermally degrades or reduces the hard-to-dissolve REE species to components with high thermodynamic solubility, leading to ~2× increase in leachability and high recovery yields using diluted acid (e.g., 0.1 M HCl). The activation strategy is feasible for various wastes including coal fly ash, bauxite residue, and electronic waste. The rapid FJH process is energy-efficient with a low electrical energy consumption of 600 kWh ton^-1. The potential for this route to be rapidly scaled is outlined.

A rapid experimental protocol to determine the desorption resistant fraction of sediment-sorbed hydrophobic organic contaminants

Desorption of hydrophobic organic contaminants (HOCs) from sedimentary materials plays a vital role in dictating the fate and transport of HOCs in the environment. Desorption irreversibility is a commonly observed phenomenon in laboratory sorption/desorption studies of HOCs. A desorption-resistant fraction (DRF) typically exists during the desorption process. To correctly evaluate the DRF of HOCs can considerably contribute to the understanding of availability and bioavailability of HOCs. This can substantially benefit contaminant remediation and cleanup operations. Conventional batch method to measure the DRF replies on repetitive washing of the sediments, which is time-consuming and can be impractical. This study presents an experimental protocol to quantify the DRF of the sediment-sorbed organic contaminants in a rapid manner. This protocol utilizes cosolvent to expedite desorption kinetics and adopts an ultrafiltration/centrifugation combined method to achieve a complete separation of sediment and solution phases. This proposed experimental protocol can facilitate the quantification of the DRF of sorbed contaminants to understand and minimize the uncertainties associated with risk-based pollution remediation approach. This protocol has the potential to be widely used in environmental studies to characterize sorption and desorption properties of HOCs with soil and sedimentary materials.

Identification of a new high-molecular-weight Fe-citrate species at low citrate-to-Fe molar ratios: Impact on arsenic removal with ferric hydroxide

Ferric hydroxide precipitation and flocculation is the most commonly used method for the removal of arsenic in water treatment. However, citrate often interrupts the precipitation of ferric hydroxides and thus affects arsenic removal. To date, the mechanisms controlling the effects of citrate on arsenic removal with ferric hydroxide flocculation and precipitation at very low citrate-to-Fe molar ratios are not well understood. Herein, we report a new mechanism by which citrate inhibits arsenic removal using ferric hydroxide. At a substoichiometric citrate-to-Fe molar ratio of 0.28, citrate forms a high-molecular-weight Fe-citrate (Fe₄Cit) species. The optimized structure of the Fe₄Cit species was obtained by the density functional theory calculation. To the best of our knowledge, this study is the first to report the formation and to identify the structure of dominant Fe-citrate species at a very low citrate-to-Fe molar ratio.

Sorption and desorption characteristics of anionic surfactants to soil sediments

Surfactants are important environmental chemicals due to their extensive domestic and industrial applications, such as subsurface organic pollution remediation and enhanced oil recovery. However, the interaction of surfactants with subsurface material particularly the desorption behavior of surfactants is less understood. Surfactant desorption is essential to control the fate and transport of surfactants as well as organic pollutants. In this study, the sorption and desorption of linear sodium dodecylbenzene sulfonate (SDBS) and sodium hexadecyl diphenyl oxide disulfonate (DPDS) with two types of soil sediment samples are compared. Sorption of surfactants can be modeled by hydrophobic sorption. Less DPDS sorption is observed at a higher aqueous concentration, which is attributed to the competition between surfactant micelles and sediment organic matter for DPDS sorption. A significant fraction of the sorbed surfactants resists desorption, and this is not a result of surfactant precipitation or desorption kinetics. Surfactant desorption behavior is similar to the irreversible desorption of hydrocarbons from soil with only half of the resistant phase surfactant being readily extracted by heated solvent extraction. The sorption/desorption data are interpreted with a molecular topology and irreversible sorption model. The knowledge of this study can be useful in understanding the environmental fate and transport of these common anionic surfactants. The methodology developed in this study can be expanded to study the sorptive nature of a wider range of surfactants in the environment.

Critical Uncertainties and Gaps in the Environmental- and Social-Impact Assessment of the Proposed Interoceanic Canal through Nicaragua

The proposed interoceanic canal will connect the Caribbean Sea with the Pacific Ocean, traversing Lake Nicaragua, the major freshwater reservoir in Central America. If completed, the canal would be the largest infrastructure-related excavation project on Earth. In November 2015, the Nicaraguan government approved an environmental and social impact assessment (ESIA) for the canal. A group of international experts participated in a workshop organized by the Academy of Sciences of Nicaragua to review this ESIA. The group concluded that the ESIA does not meet international standards; essential information is lacking regarding the potential impacts on the lake, freshwater and marine environments, and biodiversity. The ESIA presents an inadequate assessment of natural hazards and socioeconomic disruptions. The panel recommends that work on the canal project be suspended until an appropriate ESIA is completed. The project should be resumed only if it is demonstrated to be economically feasible, environmentally acceptable, and socially beneficial.

Enhanced transport of novel crystalline calcium-phosphonate scale inhibitor nanomaterials and their long term flow back performance in laboratory squeeze simulation tests

Prepared crystalline Si–Ca–DTPMP scale inhibitor nanomaterials with enhanced transportability and extended squeeze lifetime potentially for oilfield mineral scale control.

Transport and return of an oilfield scale inhibitor reverse micelle nanofluid: impact of preflush and overflush

Systematic evaluation of the transport and return behavior of a Ca–DTPMP reverse micelle nanomaterial and nanofluid for oilfield mineral scale control.

Synthesis and laboratory testing of a novel calcium-phosphonate reverse micelle nanofluid for oilfield mineral scale control

Developed calcium-phosphonate scale inhibitor reverse micelle nanomaterial for oilfield mineral scale control in low water cut or water sensitive wells.

Phosphino-polycarboxylic acid modified inhibitor nanomaterial for oilfield scale control: transport and inhibitor return in formation media

Evaluation of the transport and return behavior of phosphino-polycarboxylic acid modified scale inhibitor nanomaterial for oilfield mineral scale control.

Mechanistic understanding of calcium–phosphonate solid dissolution and scale inhibitor return behavior in oilfield reservoir: formation of middle phase

Mechanistic understanding of the precipitation chemistry of calcium–phosphonate solid and its phase transition process to extend oilfield scale squeeze lifetime.

Functional scale inhibitor nanoparticle capsule delivery vehicles for oilfield mineral scale control

This study synthesized phosphonate–polymer nanoparticle capsules using SiO₂ nanoparticles as the building blocks and polymer aggregates as the template for the purpose of oilfield mineral scale control.

Carbon-based nanoreporters designed for subsurface hydrogen sulfide detection

Polyvinyl alcohol functionalized carbon black with H2S-sensor moieties can be pumped through oil and water in porous rock and the H2S content can be determined based on the fluorescent enhancement of the H2S-sensor addends.

Contaminant-mobilizing capability of fullerene nanoparticles (nC60): Effect of solvent-exchange process in nC60 formation

Fullerene nanoparticles (nC(60)) in aqueous environments can significantly enhance the transport of hydrophobic organic contaminants by serving as a contaminant carrier. In the present study, the authors examine the effect of the solvent-exchange process on nC(60) aggregate formation and, subsequently, on nC(60) 's contaminant-mobilizing capability. A series of nC(60) samples were prepared using a modified toluene-water solvent-exchange method through the inclusion of a secondary organic solvent in the phase transfer of molecular C(60) in toluene to nC(60) in water. Two groups of solvents--a water-miscible group and a non-water-miscible group-of varied polarity were selected as secondary solvents. The involvement of a secondary solvent in the phase transfer process had only small effects on the particle size and distribution, ζ potential, and mobility of the nC(60) products but significantly influenced the capability of nC(60) to enhance the transport of 2,2',5,5'-polychlorinated biphenyl (PCB) in a saturated sandy soil column, regardless of whether the secondary solvent was water-miscible or non-water-miscible. The two groups of secondary solvents appear to affect the aggregation properties of nC(60) in water via different mechanisms. In general, nC(60) products made with a secondary water-miscible solvent have stronger capabilities to enhance PCB transport. Taken together, the results indicate that according to formation conditions and solvent constituents, nC(60) will vary significantly in its interactions with organic contaminants, specifically as related to adsorption or desorption as well as transport in porous media.

Transport of fullerene nanoparticles (nC60) in saturated sand and sandy soil: controlling factors and modeling

Understanding subsurface transport of fullerene nanoparticles (nC(60)) is of critical importance for the benign use and risk management of C(60). We examined the effects of several important environmental factors on nC(60) transport in saturated porous media. Decreasing flow velocity from approximately 10 to 1 m/d had little effect on nC(60) transport in Ottawa sand (mainly pure quartz), but significantly inhibited the transport in Lula soil (a sandy, low-organic-matter soil). The difference was attributable to the smaller grain size, more irregular and rougher shape, and greater heterogeneity of Lula soil. Increasing ionic strength and switching background solution from NaCl to CaCl(2) enhanced the deposition of nC(60) in both sand and soil columns, but the effects were more significant for soil. This was likely because the clay minerals (and possibly soil organic matter) in soil responded to changes of ionic strength and species differently than quartz. Anions in the mobile phase had little effect on nC(60) transport, and fulvic acid in the mobile phase (5.0 mg/L) had a small effect in the presence of 0.5 mM Ca(2+). A two-site transport model that takes into account both the blocking-affected attachment process and straining effects can effectively model the breakthrough of nC(60).

Enhanced transport of 2,2',5,5'-polychlorinated biphenyl by natural organic matter (NOM) and surfactant-modified fullerene nanoparticles (nC60)

Stable colloidal suspensions of buckminsterfullerene (nC(60)) in aqueous environments can significantly affect the fate and transport of hydrophobic organic contaminants by serving as a contaminant carrier. In this study, we examined enhanced transport of 2,2',5,5'-polychlorinated biphenyl (PCB) in saturated sandy soil columns by a variety of nC(60) samples, including an nC(60) sample prepared by the typical solvent exchange method, as well as eight natural organic matter (NOM) or surfactant-modified nC(60) samples, prepared by phase-transferring C(60) from toluene to an NOM or a surfactant solution. Whereas the NOM- and surfactant-modified nC(60) samples have mobility similar to the unmodified nC(60), their contaminant-mobilizing capabilities are significantly greater: breakthrough of PCB increases by 47.2 to 227% with the surfactant-modified nC(60) samples and by 233 to 370% with the NOM-modified nC(60) samples. The significantly enhanced contaminant-mobilizing capability of the modified nC(60) is likely due to a combined effect of increased adsorption affinities and increased tendency of desorption nonequilibrium, likely caused by the changes of nC(60) aggregation properties induced by the presence of NOM or surfactant. Findings in this study indicate that nC(60) formed in different processes might have vastly different effects on contaminant fate and transport.

Mobilization of trace metals and inorganic compounds during resuspension of anoxic sediments from Trepangier Bayou, Louisiana

The release of trace metals (Mn, Ni, Co, Cu, Zn, Pb, and Cd) and inorganic compounds (As) from initially anoxic Trepangier Bayou sediments, Louisiana and the sources of the released metals were investigated. After 1 to 2 d aeration, significant amounts of trace metals (Mn, Zn, Cd, Ni, and Co) were released to the aqueous phase with increased acidity, primarily due to the oxidation of acid-volatile sulfide and ferrous iron and iron sulfide minerals. The addition of a bacterial inhibitor, NaN,, to the Trepangier sediment during resuspension inhibited metal release, suggesting that microbial catalysis can regulate metal mobilization during sediment resuspension. In a well buffered system, oxidation of iron sulfides alone did not appear to induce trace metal release. Moreover, when Trepangier sediment was resuspended in anoxic conditions at neutral pH, <1% of the trace metal content was released, whereas a significant release of metal was observed under acidic anoxic conditions. Although oxidation of iron sulfide minerals is an essential prerequisite for the release of Zn, Co, Cd, and Ni, carbonates and oxides also play a role. The trace metals and inorganic compounds investigated could be classified into three groups according to their release characteristics: (i) Mn, Zn, Cd, Ni, and Co; (ii) Fe, Pb, and As; and (iii) Cu. The groupings appeared to depend on the sources of compounds and their relative affinity, after oxidation, to iron oxyhydroxides or organic matter.

Facilitated transport of 2,2',5,5'-polychlorinated biphenyl and phenanthrene by fullerene nanoparticles through sandy soil columns

The potential environmental implications of buckminsterfullerene (C60) and its derivatives have received much attention. In this study, we investigated facilitated transport of 2,2',5,5'-polychlorinated biphenyl (PCB) and phenanthrene by nC60 (a stable aqueous-phase aggregate of C60) through two sandy soil columns. We found that low-level (from 1.55 to 12.8 mg/L) nC60 could significantly enhance the mobility of PCB and phenanthrene. However, none of the three model dissolved organic matters (DOMs)-a humic acid, a fulvic acid, and a bovine serum albumin-had a noticeable effect on the transport of PCB when these DOMs were present at concentrations equivalent to approximately 10-11 mg/L organic carbon. We propose that the contaminant-mobilizing ability of nC60 is a result of irreversible adsorption of a fraction of nC60-associated PCB/phenanthrene (whereas DOM-associated PCB is readily desorbable). Additionally, slow desorption kinetics of nC60-adsorbed PCB/phenanthrene is another possible mechanism. The findings in this study indicate that nC60 in the subsurface environment can greatly enhance the mobility of nonionic, highly hydrophobic organic contaminants, which typically exhibit very low mobility. Such effects should be taken into account when assessing the potential environmental risks of engineered carbonaceous nanomaterials.

pH-dependent effect of zinc on arsenic adsorption to magnetite nanoparticles

The effect of Zn(2+) on both the kinetic and equilibrium aspects of arsenic adsorption to magnetite nanoparticles was investigated at pH 4.5-8.0. At pH 8.0, adsorption of both arsenate and arsenite to magnetite nanoparticles was significantly enhanced by the presence of small amount of Zn(2+) in the solution. With less than 3 mg/L of Zn(2+) added to the arsenic solution prior to the addition of magnetite nanoparticles, the percentage of arsenic removal by magnetite nanoparticles increased from 66% to over 99% for arsenate, and from 80% to 95% for arsenite from an initial concentration of ∼100 μg/L As at pH 8.0. Adsorption rate also increased significantly in the presence of Zn(2+). The adsorption-enhancement effect of Zn(2+) was not observed at pH 4.5-6.0, nor with ZnO nanoparticles, nor with surface-coated Zn-magnetite nanoparticles. The enhanced arsenic adsorption in the presence of Zn(2+) cannot be due to reduced negative charge of the magnetite nanoparticles surface by zinc adsorption. Other cations, such as Ca(2+) and Ag(+), failed to enhance arsenic adsorption. Several potential mechanisms that could have caused the enhanced adsorption of arsenic have been tested and ruled out. Formation of a ternary surface complex by zinc, arsenic and magnetite nanoparticles is a possible mechanism controlling the observed zinc effect. Zinc-facilitated adsorption provides further advantage for magnetite nanoparticle-enhanced arsenic removal over conventional treatment approaches.

SYNOPSIS

Arsenic adsorption to magnetite nanoparticles at neutral or slightly basic pH can be significantly enhanced with trace amount of Zn(2+) due to the formation of a ternary complex.

Mineralization kinetics: a constant composition approach

A new method is described for studying, reproducibly, the kinetics of crystallization of minerals under conditions of constant solution composition even at very low supersaturations. For calcium phosphates the method provides direct evidence for octacalcium phosphate as the precursor to hydroxyapatite precipitation at physiological pH.

A sorption kinetics model for arsenic adsorption to magnetite nanoparticles

INTRODUCTION

Arsenic is a well known water contaminant that causes toxicological and carcinogenic effects. In this work magnetite nanoparticles were examined as possible arsenic sorbents. The objective of this work was to develop a sorption kinetics model, which could be used to predict the amount of arsenic adsorbed by magnetite nanoparticles in the presence of naturally occurring species using a first-order rate equation, modified to include adsorption, described by a Langmuir isotherm.

DISCUSSION

Arsenate and arsenite adsorption to magnetite nanoparticles was studied, including the effect of naturally occurring species (sulfate, silica, calcium magnesium, dissolved organic matter, bicarbonate, iron, and phosphate) on adsorption.

CONCLUSION

The model accurately predicts adsorption to magnetite nanoparticles used in a batch process to remove arsenic from spiked Houston, TX tap water, and contaminated Brownsville, TX groundwater.

Adsorption of arsenic to magnetite nanoparticles: effect of particle concentration, pH, ionic strength, and temperature

Little work has been conducted on the adsorption of arsenic to the mixed iron [Fe(II)/(III)] oxide magnetite and the effect that environmental parameters, such as pH, ionic strength, and temperature, have on adsorption. Magnetite nanoparticles are unique because of their affinity for both arsenate and arsenite and increased adsorption capacity from their bulk counterparts. This article shows the effect of various magnetite nanoparticle concentrations on arsenic adsorption kinetics. The adsorption data show the ability of the magnetite nanoparticles to remove arsenate and arsenite from solution in both synthetic and natural waters, and the data fit a first-order rate equation. Because of the increased surface area of these particles, less than 1 g/L of magnetite nanoparticles was needed. The results suggest that arsenic adsorption to the nanoparticles was not significantly affected by the pH, ionic strength and temperature in the ranges tested, which are typical of most potable water sources.

Release of adsorbed polycyclic aromatic hydrocarbons under cosolvent treatment: implications for availability and fate

During laboratory and field studies, a fraction of contaminants in soils or sediments often is observed to be highly resistant to desorption. This desorption-resistant fraction may have significant effects on long-term fate and exposure of soil/ sediment-bound contaminants in particular, causing much reduced availability and contaminant persistence. Previous work by many research groups has indicated that this nonideal desorption behavior could be better predicted with biphasic desorption models. The present study further investigated the release of naphthalene and phenanthrene from sediments during and after cosolvent treatment. Experimental results indicate that release of these two compounds under cosolvent conditions can be accurately predicted with a previously developed, biphasic desorption model when the solubility enhancement effect of cosolvent is accounted for using standard activity coefficient ratios. In addition, desorption of the residual contaminants after cosolvent treatment follows the original biphasic desorption model very well, suggesting that cosolvent treatment increases only the aqueous solubility and has little effect on the nature of the desorption-resistant fraction and that cosolvent desorption is a valuable analytical tool for quickly measuring the magnitude of the desorption-resistant fraction. The present findings might have important implications for the mechanisms controlling resistant desorption of hydrophobic organic compounds and for predicting the availability and long-term fate of contaminants in soils and sediments.

Resistant desorption of hydrophobic organic contaminants in typical chinese soils: implications for long-term fate and soil quality standards

Soil contamination is an enormous problem in China and severely threatens environmental quality and food safety. Establishing realistic soil quality standards is important to the management and remediation of contaminated sites and must be based on thorough understanding of contaminant desorption from soil. In the present study, we evaluated sorption and desorption behaviors of naphthalene, phenanthrene, atrazine, and lindane (four common soil contaminants in China) in two of the most common Chinese soils. The desorption of these compounds exhibited clear biphasic pattern-a fraction of contaminants in soil was much less available to desorption and persisted much longer than what was predicted with the conventional desorption models. The unique thermodynamic characteristics associated with the resistant-desorption fraction likely have important implications for the mechanism(s) controlling resistant desorption. Experimental observations in the present study are consistent with our previous work with chlorinated compounds and different adsorbents and could be well modeled with a biphasic desorption isotherm. We therefore suggest that more accurate biphasic desorption models should be used to replace the conventional linear sorption/desorption model that is still widely adopted worldwide in contaminant fate prediction and soil quality standard calculations.

Adsorption and precipitation of an aminoalkylphosphonate onto calcite

The mechanism of nitrilotris(methylenephosphonic acid) (H6NTMP)/calcite reaction was studied with a large number of batch experiments where phosphonic acid was neutralized with 0 to 5 equivalents of NaOH per phosphonic acid and the concentration ranged from about 10 nmol/L to 1 mol/L. It is proposed that the phosphonate/calcite reactions are characterized in three steps. At low phosphonate concentration (<1 micromol/L NTMP concentration), the phosphonate/calcite reaction can be characterized as a Langmuir isotherm. At saturation, only approximately 7% of the calcite surface is covered with phosphonate; presumably these are the kinks, step edges, or other imperfect sites. At higher phosphonate concentrations, the attachment is characterized by calcium phosphonate crystal growth to a maximum of four to five surface layer thick, with solid phase stoichiometry of Ca(2.5)HNTMP and a constant solubility product of 10(-24.11). After multiple layers of phosphonate are formed on the calcite surface, the solution is no longer at equilibrium with calcite. Further phosphonate retention is probably due to mixed calcium phosphonate solid phase formation at lower pH and depleted solution phase Ca conditions. The proposed mechanism is consistent with phosphate/calcite reaction and can be used to explain the fate of phosphonate in brines from oil producing wells and the results are compared with two oil wells.

Adsorption of cadmium on anatase nanoparticles-effect of crystal size and pH

The adsorption and desorption of Cd(2+) to large and nanometer-scale anatase crystals have been studied to determine the relationship between heavy metal adsorption properties and anatase particle size. A solvothermal method was used to synthesize very fine anatase nanocrystals with average grain sizes ranging from 8 to 20 nm. On a surface area basis, it was found that large and nanometer-scale anatase particles had similar maximum Cd(2+) adsorption capacities, while their adsorption slopes differed by more than 1 order of magnitude. The particle-size effect on adsorption is constant over a pH range of 4-7.5. The desorption of Cd(2+) from both particle sizes is completely reversible. The adsorption data have been modeled by the Basic Stern model using three monodentate surface complexes. It is proposed that intraparticle electrostatic repulsion may reduce the adsorption free energy significantly for nanometer-sized particles.

A program for evaluating dual-equilibrium desorption effects on remediation

Desorption is one of the most critical processes affecting the effectiveness of soil and ground water remediation. None of the currently adopted desorption models can accurately quantify desorption of low-hydrophobicity organic chemicals, and thus could potentially mislead remediation design and decision-making. A recently developed dual-equilibrium desorption (DED) model was found to be much more accurate in quantifying desorption. A screening-level transport model, DED-Transport, was developed to simulate the DED effect on behaviors of organic contaminant plumes during remediation. DED-Transport requires only simple parameters, but is applicable to many remediation scenarios. DED-Transport can be used as a decision-support tool in site remediation to more precisely predict the time required for cleanup.

Critical evaluation of desorption phenomena of heavy metals from natural sediments

In natural sediments, the majority of heavy metal ions are generally associated with the solid phase. To become bioavailable, the metal ions must desorb from the solid. Numerous studies of heavy metals in sediments have suggested that sorption and desorption exhibit hysteresis (i.e., the two processes are not reversible), while other studies have suggested that desorption hysteresis does not exist. In this study, sorption/desorption hysteresis of lead (Pb) and cadmium (Cd) was evaluated over the following range of conditions: (i) desorption induced by replacing the supernatant liquid with contaminant-free electrolyte solution; (ii) desorption induced by lowering the solution pH with mineral acid; and (iii) desorption induced by sequestration with EDTA. Given the importance of dissolved organic and inorganic ligands in regulating heavy metal behavior in nature sediments, sorption/desorption experiments were conducted on both untreated and prewashed sediments. Prewashing treatment increases the sorption potential of Cd but not Pb. Desorption hysteresis is observed in both the untreated and the prewashed sediments using the replaced supernatant method, and the desorption hysteresis appears to increase with aging time. Hysteresis is not observed when desorption is initiated by lowering the solution pH. A large fraction of the sorbed heavy metal ions can be easily desorbed by EDTA; between 0.04 and 1.2 mmol/kg Cd and Pb ions are resistant to desorption.

More realistic soil cleanup standards with dual-equilibrium desorption

The desorption of contaminants from soils/sediments is one of the most important processes controlling contaminant transport and environmental risks. None of the currently adopted desorption models can accurately quantify desorption at relatively low concentrations; these models often overestimate the desorption and thus the risks of hydrophobic organic chemicals, such as benzene and chlorinated solvents. In reality, desorption is generally found to be biphasic, with two soil-phase compartments. A new dual-equilibrium desorption (DED) model has been developed to account for the biphasic desorption. This model has been tested using a wide range of laboratory and field data and has been used to explain key observations related to underground storage tank plumes. The DED model relates the amount of a chemical sorbed to the aqueous concentration, with simple parameters including octanol-water partition coefficient, solubility, and fractional organic carbon; thus, it is the only biphasic model, to date, that is based on readily available parameters. The DED model can be easily incorporated into standard risk and transport models. According to this model, many regulatory standards of soils and sediments could be increased without increasing the risks.

Desorption kinetics of neutral hydrophobic organic compounds from field-contaminated sediment

The chemical release rates from a field-contaminated sediment (Lake Charles, LA) using Tenax desorption were studied. Two dichlorobenzenes (m-, p-), hexachlorobutadiene, and hexachlorobenzene were investigated. Contrary to reports that sorption rates are inversely related to K(OW), the slow desorption rates were found to be similar for the four compounds. The data were modeled by a two-compartment irreversible adsorption and radial diffusion model. Desorption kinetics from the first irreversible compartment can be modeled by radial diffusion and assume an irreversible adsorption constant and soil tortuosity of 4.3. The desorption half-life is approximately 2-7 days. Desorption from the second irreversible compartment is very slow (half-life of approximately 0.32-8.62 years) presumably caused by entrapment in soil organic matter that increases the constrictivity of the solid phase to chemical diffusion. From the kinetic data, it is deduced that the diffusion pore diameter of the second irreversible compartment is approximately equal to the critical molecular diameter. The mass of chemicals in this highly constrictive irreversible compartment is approximately one-fourth of the maximum irreversible, or resistant, compartment. The slow kinetics observed in this study add additional support to the notion that the irreversibly sorbed chemicals are 'benign' to the environment.

Scale Control Aids Gas Recovery

Summary

The East Hitchcock field has produced natural gas since 1958. Toward the end of the primary production phase, the wells watered out and formed excessive amounts of calcite scale. Acidizing was required frequently. In recent years, some of these wells were recompleted for coproduction. Because large volumes of brine are produced, scale control is necessary. Prets Unit No. 1 was successfully treated by an inhibitor squeeze, while Thompson Trustee Unit No. 1 was completed with two treat strings installed at calculated depths to administer scale inhibitor downhole. An inhibitor squeeze protects the formation and the perforations as well as the production tubing, surface equipment, and disposal system, Periodic interruption of production, however, is required to repeat the squeeze, and the concentration of inhibitor in the produced brine is controlled by the chemistry of the inhibitor salt. Installation of treat strings requires that the tubing be pulled and reinstalled with the treat strings clamped to the tubing OD. The concentration of inhibitor in the effluent brine is controlled by a surface injection pump connected at the top of the treat string.

Introduction

Gas reservoirs are generally considered "watered out" when they begin to produce a few hundred barrels of brine per day per well. At this point, the reservoir typically contains 40% or more of the original gas in place. Coproduction is the process of recovering a major fraction of the residual gas in place by simultaneously producing several thousand barrels of brine per day per well from several wells, thereby reducing the reservoir pressure and mobilizing additional gas. Over a period of about 18 months, the gas/brine ratio in the East Hitchcock field has steadily increased from 120 to 160 scf [3.44 to 4.58 std m3] of natural gas per barrel of produced brine. Progress has been made toward controlling scale formation from brines often associated with geopressured energy production, coproduction wells, and oil wells that produce large amounts of water. As brine flows out of the formation and up the well, the pressure drops, causing dissolved CO2) to go out of the solution and thus increasing the solution pH. The pH rise causes aqueous bicarbonate, HCO3, to be converted to carbonate, CO3, which tends to initiate precipitation of calcium carbonate, CaCO3, in the formation pore throats near the wellbore, on the production tubing walls, or in surface handling equipment. Scale-control options include (1) limiting production so that the pressure drop is not sufficient to induce precipitation, (2) injecting a trace concentration of inhibitors into the surface equipment, (3) injecting trace concentrations of inhibitors downhole through a small-diameter treat string, and (4) squeezing inhibitor into the formation in such a manner that the inhibitor will be slowly released when production begins. Option 1, reduced production. generally entails an unacceptable revenue loss. Option 2, injection of inhibitors into the surface equipment, does not protect the production tubing, and Option 3, installation of a downhole treat string, is often prohibitively expensive. Option 4, however, an inhibitor squeeze, can protect the near-wellbore formation. the production tubing, and the surface equipment. Successful inhibitor squeeze jobs were carried out on some coproduction wells in the Hitchcock field near Galveston, TX. Previous laboratory work led to the development of a method to predict when scale will begin to form and how much inhibitor will be needed to prevent scale. This paper first discusses the field applications and results of inhibitor squeezes to prevent formation of CaCO3 scale. This is followed by a description of laboratory experiments and theoretical considerations that led to the development of the inhibitor-squeeze techniques. The northeast Hitchcock Frio A sandstone, at a depth of 9,100 ft [2774 m], was conventionally produced as a waterdrive gas reservoir from 1958 through 1982. More than 80 Bcf [2265 × 10–6 m3] was produced from 12 wells. Peak gas production (20 to 25 MMcf/D [566 to 707 × 10–3 m3/d]) was between 1966 and 1969. By 1978, gas production had dropped to less than 1 MMcf/D [28.3 × 10–3 m3/d]. New wells placed on production in 1980 and 1982 added to the production from a well that had been on line since 1960, providing a production rate averaging about 2 MMcf/D [56.6 × 10–3 m3/d] before the onset of coproduction. Coproduction began in 1983, and some plugged and abandoned wells were reentered. Wells were plugged and abandoned when they were deemed uneconomical under the low, regulated gas prices in 1983. During the primary production, the dry gas/ brine contact rose from 9,106 to 9,060 ft [2776 to 2761 m] subsea because of the pressure decrease in the gas cap and the waterdrive. Gas-phase reservoir pressure declined from the original 5,750 psia [39.6 MPa] to a minimum of about 3,850 psia [26.5 MPa]. During the brine invasion, gas was trapped in about 30% of the PV in the invaded portion of the original gas cap. This trapped gas, plus remaining "attic" gas in the various fault blocks, are the targets for recovery by gas/brine coproduction. The coproduction process consists of completing wells to maximize production of fluids from the reservoir, thereby decreasing reservoir pressure to a level substantially below the minimum achieved during primary production.

JPT

P. 1080^

Anaerobic inhibition of trace organic compound removal during rapid infiltration of wastewater

When soil columns were operated aerobically on a flooding-drying schedule in a previous study, good removals were observed for several organic compounds at concentrations ranging from 1 to 1,000 micrograms per liter in primary wastewater. In this study, fractional breakthroughs of most compounds increased substantially once operating parameters were modified and the soil became anaerobic. These results imply that microbial removal of trace organic compounds can be inhibited if anaerobic conditions develop during rapid infiltration of wastewater.