Local Structure and Crystallization Transformation of Hydrous Ferric Arsenate in Acidic H2O-Fe(III)-As(V)-SO42- Systems: Implications for Acid Mine Drainage and Arsenic Geochemical Cycling.

Hydrous ferric arsenate (HFA) is a common thermodynamically metastable phase in acid mine drainage (AMD). However, little is known regarding the structural forms and transformation mechanism of HFA. We investigated the local atomic structures and the crystallization transformation of HFA at various Fe(III)/As(V) ratios (2, 1, 0.5, 0.33, and 0.25) in acidic solutions (pH 1.2 and 1.8). The results show that the Fe(III)/As(V) in HFA decreases with decreasing initial Fe(III)/As(V) at acidic pHs. The degree of protonation of As(V) in HFA increases with increasing As(V) concentrations. The Fe K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure results reveal that each FeO6 is linked to more than two AsO4 in HFA precipitated at Fe(III)/As(V) < 1. Furthermore, the formation of scorodite (FeAsO4·2H2O) is greatly accelerated by decreasing the initial Fe(III)/As(V). The release of As(V) from HFA is observed during its crystallization transformation process to scorodite at Fe(III)/As(V) < 1, which is different from that at Fe(III)/As(V) ≥ 1. Scanning electron microscopy results show that Oswald ripening is responsible for the coarsening of scorodite regardless of the initial Fe(III)/As(V) or pH. Moreover, the formation of crystalline ferric dihydrogen arsenate as an intermediate phase at Fe(III)/As(V) < 1 is responsible for the enhanced transformation rate from HFA to scorodite. This work provides new insights into the local atomic structure of HFA and its crystallization transformation that may occur in AMD and has important implications for arsenic geochemical cycling.

Direct immobilization of Se(IV) from acidic Se(IV)-rich wastewater via ferric selenite Co-precipitation.

The attenuation of acidic Se(IV)-rich wastewater, including those associated with acid mine drainage (AMD) and nonferrous metallurgical wastewater (NMW), presents a serious environmental challenge. This study investigates the effects of diverse factors from pH values to Se(IV)/Fe(III) molar ratios, initial Se(IV) concentrations, and alkali neutralization agents on the direct co-precipitation of ferric selenites in AMD and NMW systems involving different orders of Fe(III) and alkali addition. Our results show that amorphous sulfate-substituted ferric (hydrogen) selenite and Se(IV)-bearing ferrihydrite-schwertmannite are the major Se(IV)-attenuation solids except that gypsum is an additional phase in the NMW system with Ca(OH)2 neutralization. Produced ferric selenites achieve 98-99.8% of Se(IV) immobilization under optimal conditions of pH 4.5, Se(IV)/Fe(III) molar ratios of 0.0625-0.5, and initial Se(IV) concentrations of 0.15-1.3 mmol·L-1. Moreover, completing FeSO4+ and FeHSeO32+/FeSeO3+ complexes as well as different ferric selenite co-precipitates are shown to collectively control aqueous Se(IV) remaining. Specifically, three distinct trends of aqueous Se(IV) concentrations separately correspond to changes in the four factors. The co-precipitation in the NMW system via pH adjustment followed by Fe(III) addition is more efficient for Se(IV) fixation than that in the AMD system because of minimal complexation, concurrent Fe(III) hydrolysis, and enhanced ferric selenite co-precipitation in the former.

Effects of nitrate and Fe/As molar ratio on direct iron(III)-arsenite precipitation in high-sulfate-chloride wastewaters.

The addition of an arsenite-chloride solution into an arsenite-sulfate solution is extremely beneficial for the removal of As(III) via Fe(III) salt precipitation at pH 2.3. However, the applicability of this method to complicated high-As(III) metallurgical wastewaters still requires further verification. This work investigated the effects of nitrate and Fe/As molar ratio on As(III) immobilization using Fe(III) in three acid radical media including sulfate, chloride, and nitrate at pH 2.3. Our results indicated that 72.1‒93.5% of As(III) was precipitated, which was 5‒10% less than those obtained in the nitrate-free systems. The Fe/As molar ratio of 4 was the optimal condition with an average of 93% As(III) removal based on a broad sulfate/chloride molar ratio range (1:1‒16). However, a maximum of 96% As(III) removal was observed under the Fe/As molar ratio of 1.5 and the sulfate/chloride condition of 1:16. The negative correlation between complexation and precipitation was attributed to the enhanced initial complexation by the synergistic effect of the mononitratoiron complex and FeH2AsO32+. The variation of Fe/As molar ratios resulted in the diverse solid species, thus further affecting the As(III) removal efficiency. Despite producing tooeleite as a major As(III) host phase, ferrihydrite and poorly crystalline ferric arsenite hydroxysulfate formed simultaneously at the Fe/As molar ratio of 4 participated in As(III) immobilization compared with the solid products at Fe/As molar ratios ≤ 2.

Enhanced removal of high-As(III) from Cl(-I)-diluted SO4(-II)-rich wastewater at pH 2.3 via mixed tooeleite and (Cl(-I)-free) ferric arsenite hydroxychloride formation.

An advanced cost-saving method of removal of high-As(III) from SO4(-II)-rich metallurgical wastewater has been developed by diluting the SO4(-II) content with As(III)-Cl(-I)-rich metallurgical wastewater and then by the direct precipitation of As(III) with Fe(III) at pH 2.3. As(III) removal at various SO4(-II)/Cl(-I) molar ratios and temperatures was investigated. The results showed that 65.2‒98.2% of As(III) immobilization into solids occurred at the SO4(-II)/Cl(-I) molar ratios of 1:1‒32 and 15‒60 °C in 3 days, which were far higher than those in aqueous sole SO4(-II) or Cl(-I) media at the equimolar SO4(-II) or Cl(-I) and the same temperature. SO4(-II)/Cl(-I) molar ratio of 1:4 and 25 °C were optimal conditions to reach the As removal maximum. Mixed aqueous SO4(-II) and Cl(-I) played a synergetic role in the main tooeleite formation together with (Cl(-I)-free) ferric arsenite hydroxychloride (FAHC) involving the substitution of AsO33- for Cl(-I) for enhanced As fixation. The competitive complexation among FeH2AsO32+, FeSO4+ and FeCl2+ complexes was the main mechanism for the maximum As(III) precipitation at the SO4(-II)/Cl(-I) molar ratio of 1:4. Low As(III) immobilization at high temperature with increased Fe(III) hydrolysis was due to the formation of As(III)-bearing ferrihydrite with the relatively high Fe/As molar ratio at acidic pH.

Iron(II)-activated phase transformation of Cd-bearing ferrihydrite: Implications for cadmium mobility and fate under anaerobic conditions.

The factors and mechanisms affecting the fate of the associated Cd during the Fe(II)-activated Cd-bearing ferrihydrite transformation remain poorly understood. Herein we have conducted a series of batch reactions containing ferrihydrite with diverse pH values and initial Fe(II) and Cd concentrations coupled with chemical analyses and spectroscopic examination on the transformation products to probe the mechanisms of the Cd partitioning and the processes of Fe(II)-activated Cd-bearing ferrihydrite transformation under anaerobic conditions. Chemical analyses, Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (PXRD) results show that the initial Fe(II) and Cd concentrations as well as pH values all have significant effects on the rates and pathways of ferrihydrite transformation. Increasing Cd loading enhances the inhibition of the Fe(II)-activated ferrihydrite transformation rates. High Cd loading alters the Fe(II)-activated ferrihydrite transformation pathways by hindering the recrystallization of both ferrihydrite to more stable iron minerals and the newly formed lepidocrocite to goethite. Chemical analyses show that the release of Cd to solutions during ferrihydrite transformation is accompanied by a reduction in the 0.4 M HCl extractable Cd fraction and that a significant amount of the released Cd is transformed to a 0.4 M HCl unextractable form. Moreover, enhanced Cd release during the Fe(II)-activated ferrihydrite transformation is observed by reducing the pH value or increasing the initial Cd concentration. Results from synchrotron X-ray absorption spectroscopy (XAS) confirm that the majority of the 0.4 M HCl unextractable Cd form is associated with structural incorporation into the recrystallized iron (hydr)oxides via isomorphous substitution for Fe(III). These findings not only provide molecular-level understanding on the behavior of Cd under natural anoxic environments, but also are useful in predicting the geochemical cycling of Cd and developing long-term Cd contaminant management strategies.

The fate of co-existent cadmium and arsenic during Fe(II)-induced transformation of As(V)/Cd(II)-bearing ferrihydrite.

Ubiquitous co-existence of arsenic (As) and cadmium (Cd) in smelting operations and mine drainage presents a major challenge to the environment. Fe(II)-induced ferrihydrite transformation into secondary, more crystalline minerals often controls the geochemical behavior of associated contaminants including arsenate (As(V)) and Cd(II) in natural and contaminated environments. However, the fate of co-existent As(V) and Cd(II) and the underlying mechanism during this transformation process remain unclear. In this contribution, ferrihydrite containing co-precipitated Cd(II) and As(V) with Fe(II) under diverse pH conditions has been investigated. Results from powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectra show that the co-existence of As(V) and Cd(II) significantly retards the transformation rates of As(V)/Cd(II)-bearing ferrihydrite to more stable iron oxides and reduces that from the newly formed lepidocrocite to goethite. Compared to Cd(II), the co-existent As(V) has stronger influence on the compositions of the transformation products. Chemical analysis shows that phosphate-unextractable As(V) and 0.4 M HCl unextractable Cd(II) both increase as the reaction proceeds during the recrystallization of As(V)/Cd(II)-bearing ferrihydrite, indicating that both As(V) and Cd(II) partially transform to a more stable phase. The co-existent Cd(II) has negligible effects on the As(V) redistribution, but the co-existent As(V) at high loadings has a significant modification in the distribution of Cd(II) during the transformation, which reduces the liberation of Cd(II) into solution, thus decreasing the mobility of Cd(II). These findings have important implications for understanding the mobility and fate of the co-existent As(V) and Cd(II) under natural anoxic environments, remediating the co-existent contaminants, and predicting the long-term behavior of As(V) and Cd(II) in natural and contaminated environments.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products.

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min-1. NaOH or Ca(OH)2 as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O2 for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min-1 were optimal for As removal.

Divergent synthesis of oxazolidines and morpholines via PhI(OAc)2-mediated difunctionalization of alkenes.

Herein we describe the PhI(OAc)2-mediated 1,1- and 1,2-difunctionalization of alkenes with N-tosyl amino alcohols to form oxazolidine and morpholine derivatives. This transformation was realized under mild reaction conditions and allows application to various substrates furnishing the multi-substituted oxazolidines and morpholines with yields up to 98%. A deuterium-labeling experiment was carried out and the result indicated that a phenyl group migration occurred to generate oxazolidine products.

Digestive solubilization of Cd in highly-contaminated sediment by marine deposit feeders: The roles of intestinal surfactants in Cd mobilization and Re-Adsorption processes.

Marine deposit feeders are of ecological significance in transferring sedimentary Cd along aquatic food chains. A key process for this transfer is these organisms' dietary uptake of Cd via solubilization of Cd present in ingested contaminated sediment. To better understand the bioavailability of sedimentary Cd to deposit feeders, the present study used in vitro extraction experiments to explore the contribution of different digestive agents (proteins, amino acids and surfactants) to the solubilization of Cd from sediment collected in a highly-contaminated Chinese bay. This was done for various commercially-available mimetic digestive agents (the protein BSA, a mixture of amino acids, and the surfactants rhamnolipid and SDS), and for proteins and surfactants collected from the gut juice of a sipunculan worm. The Cd mobilization capacity of BSA was significantly higher than that of the amino acids and the commercial surfactants. In the presence of BSA, > 70% of the released Cd became associated with this protein. In contrast, the digestive proteins from the sipunculan had a lower Cd mobilization capacity than was the case for the other digestive agents and the majority of the released Cd (∼80%) was associated with small molecular weight fractions. The differences in Cd mobilization between the BSA and the digestive proteins were attributed to differences in their sediment-adsorption tendencies and their Cd-complexing capacities. While the digestive surfactants had minor effects on the release of sedimentary Cd, they significantly enhanced Cd mobilization by the digestive proteins when both were present simultaneously. Our results suggest that the characteristics of proteins should be considered when using commercially-available mimetic digestive agents to explore Cd bioavailability in sediments. Furthermore, digestive surfactants seem to have important effects on the solubilization of Cd during gut passage by reducing the adsorption of the digestive proteins to the sediments.

Alternative Method for the Treatment of Hydrometallurgical Arsenic-Calcium Residues: The Immobilization of Arsenic as Scorodite.

Arsenic-calcium residue (ACR) is one of the major hazardous solid wastes produced by the metallurgical industry that poses a serious threat to the environment. However, a suitable method for the effective treatment of ACR is still lacking. In this study, an alternative treatment method for ACRs via the immobilization of As as scorodite was proposed with the use of two types of ACRs (ACRreal directly collected from a Pb refinery and ACRlab precipitated from waste sulfuric acid in the lab). The treatment of ACR included preparing the As-enriched solution via H2SO4 dissolution-neutralization of ACR at pH < 2, As(III) was oxidized by H2O2, and As(V) was immobilized as scorodite. The results showed that gypsum produced from ACRlab in the dissolution-neutralization process contained 68 mg/kg of As, far below the Chinese national standard for hazardous solid wastes (<0.1 wt %, GB5085.62007). The gypsum produced from ACRreal contained 5400 mg/kg of As due to the presence of original high-As gypsum (1.6 wt %) in ACRreal. These results showed that the preliminary removal of SO4 2- from waste sulfuric acid by lime neutralization-precipitation at pH ∼ 2 could produce pure-phase gypsum by avoiding the HAsO4 2- isomorphic substitution for SO4 2-. The scorodite produced from both ACRs displayed good As stability at pH 4.95 (0.9 and 0.5 mg/L) via the toxicity characteristic leaching procedure (TCLP) method and at pH 3-7 (0.4-3.0 mg/L) via a 15 day short-term stability test.

Detoxification and reclamation of hydrometallurgical arsenic- and trace metals-bearing gypsum via hydrothermal recrystallization in acid solution.

Arsenic- and trace metals-bearing gypsum (As-gypsum) is one of the major hazardous solid wastes produced from metallurgical industry that poses a serious threat to the environment. However, the method for effective extraction of As and trace metals from As-gypsum is still lacking. In this study, simultaneous extraction of As and trace metals from a hydrometallurgical As-gypsum via hydrothermal recrystallization in acid solution was investigated. The effects of the type (H2SO4 vs HCl) and concentration of acid, and temperature on extraction efficiency were assessed. The results showed that 99% As, >92% Cu and >96% Zn could be extracted from the As-gypsum during hydrothermal treatment in 6 mol L-1 H2SO4 at 90 and 120 °C, but Pb and Cd could not be extracted efficiently. The results of hydrothermal treatment in HCl solutions demonstrated that higher HCl concentration and temperature significantly enhanced the extraction efficiency and 100% As, Cu2+, Zn2+, Pb2+ and >90% Cd were removed from the As-gypsum after treatment in 6 mol L-1 HCl, at 120 °C, for 12 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy results revealed that dissolution-recrystallization of gypsum is the key process for the removal of the incorporated As and trace metals. Thermodynamic modelling indicated that the released HAsO42-/Me2+ transformed into H3AsO4/MeCln(2-n) (1 ≤ n ≤ 4) species in HCl solution, hence inhibiting their reincorporation into the recrystallization products via isomorphic substitution for SO42-/Ca2+. This work provides a simple and effective method for detoxification and reclamation of As-gypsum.

Characterization of Fe5(AsO3)3Cl2(OH)4·5H2O, a new ferric arsenite hydroxychloride precipitated from FeCl3-As2O3-HCl solutions relevant to arsenic immobilization.

Tooeleite (Fe6(AsO3)4SO4(OH)4·4H2O) is widely precipitated for direct As(III) removal from sulfate-rich industrial effluents. However, whether or not Fe(III)-As(III)-Cl(-I) precipitate is produced in chloridizing leaching media for As immobilization is almost unknown. This work founded the existence of ferric arsenite (hydroxy)chloride as a new mineral for As(III) removal. Its chemical composition and solid characterization were subsequently studied by using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), infrared (FT-IR), Raman spectroscopy and thermogravimetric (TG) curve. The results showed the formation of a yellow precipitate after 3-days reaction of Fe(III)/As(III) with molar ratio ≈ 1.7 in chloride solution at pH 2.3 neutralized with NaOH. Compared with tooeleite, chemical analysis and solid characterization indicated that Cl(-I) replaces SO4(-II) producing ferric arsenite hydroxychloride with formula Fe5(AsO3)3Cl2(OH)4·5H2O. This new plate shaped solid showed better crytallinity than tooeleite, although it has similar morphology and characteristic bands to tooeleite. The FT-IR bands at 628, 964 cm-1 and the Raman bands at 448, 610, 961 cm-1 were assigned to Fe-O or As(III)-O-Fe or As(III)-O bending/stretching vibration, indicating that both arsenite and chloride substituted for the position of sulfate for ferric arsenite hydroxychloride produced due to the lack of the SO42- vibrations. Cl-(I) also contributed to increase As removal efficiency in aqueous sulfate media under acidic pH conditions via the probable formation of sulfate-chloride ferric arsenite.

Digestive solubilization of particle-associated arsenate by deposit-feeders: The roles of proteinaceous and surfactant materials.

Solubilization of arsenate in guts of deposit-feeders is a key process for their dietary uptake of arsenate from contaminated sediments. The present study explored this digestive solubilization with in vitro extraction experiments that quantified arsenic (As) release from substrates (natural sediment and As-enriched iron oxides) in the presence of various digestive agents (proteins, amino acids and surfactants collected from gut fluid of a sipunculan worm). To investigate potential mechanisms for the influence of digestive agents, analyses determined correlations between As and Fe concentrations, the size distribution of the As bound to the digestive agents, and the adsorption of the digestive agents on the substrates. Both the digestive surfactants and proteinaceous materials increased arsenate mobilization, with the surfactants enhancing the effects of the proteinaceous materials. Arsenate reduction and reductive dissolution of iron oxides were not observed and correlations between the concentrations of released As and Fe were weak. These findings indicate that dissolution release of Fe did not appear to be the main route by which the digestive agents mobilized particle-associated As. Most of the released As (>70%) was distributed in the <10 kDa fraction of the digestive agents, showing that the As mobilization was also not caused by complexation with proteins in the digestive agents. In contrast, adsorption of the digestive agents occurred along with the release of arsenate from the arsenate-rich substrates, suggesting that competitive adsorption was the mechanism by which the digestive agents mobilized sedimentary arsenate. Our work demonstrated that the presence of digestive surfactants significantly enhances arsenate availability during deposit feeding.

The stability of Fe(III)-As(V) co-precipitate in the presence of ascorbic acid: Effect of pH and Fe/As molar ratio.

The potential hazards of Fe(III)-As(V) co-precipitate under reducing conditions are incompletely known. This work investigated the effect of Fe(III) reduction by ascorbic acid (AH2) on the stability of Fe(III)-As(V) co-precipitate at different pHs and Fe/As molar ratios. The results showed that As (14-98.9%) and Fe (27.9-99.3%) were significantly released into solution by 79.9-97.5% Fe(III) reduction of the co-precipitate (Fe/As molar ratios of 3 and 5) at pH 5-9. More As release was observed with the increase of pH (6-9) or decrease in Fe/As molar ratio (from 5 to 3). This could be attributed by oxalate, the final product of AH2 decomposition, which strongly competed with As(V) for Fe(II) at higher pH or lower Fe/As molar ratio, inhibiting parasymplesite accumulation and then causing more As mobilization. The stability of Fe(III)-As(V) co-precipitate with AH2 upon Fe(III) reduction was lower than that in oxic environment. Compared with produced Fe(II,III) (hydr)oxides in the presence of hydroquinone (QH2), humboldtine was formed during the long-term reactions of Fe(III)-As(V) co-precipitate with AH2. The findings of this study implied that parasymplesite and humboldtine as secondary solid products were environmental relevant and mainly responsible for As(V) and Fe(II) immobilization.

Effect of iron reduction by enolic hydroxyl groups on the stability of scorodite in hydrometallurgical industries and arsenic mobilization.

Scorodite (FeAsO4·2H2O) is an important arsenic-bearing solid waste in hydrometallurgical industries, but its stability in reducing environments is not well understood. This study investigated the effect of Fe(III) reduction by enolic hydroxyl groups on the stability of scorodite and arsenic mobilization at various pH values and ascorbic acid/scorodite molar ratios (AH2/Sc). The results showed that 47-89% Fe(III) reduction by ascorbic acid caused approximately 10-69% (~ 37-260 mg L-1) arsenic release and 4.5-63% (~ 13-176 mg L-1) Fe(II) release at pH 5-8. The releases of arsenic and Fe(II) increased with increasing AH2/Sc, whereas they decreased as pH increased. The results of the solid characterization and chemical analysis indicated that the mixture of poorly crystalline parasymplesite and probably amorphous FeHAsO4⋅xH2O was the new arsenic sink. The high solubility of this ferrous arsenate with the Fe(II)/As(V) molar ratio > 1 was deemed to be a major contributor to the relatively high arsenic release. This work differed from our previous finding that almost all arsenic was retained in the solid phase after similar Fe(III) reduction in scorodite with hydroquinone. Phenolic hydroxyl groups complexed with aqueous Fe(II), unlike enolic hydroxyl groups, was possibly the dominant reason for the formation of different secondary minerals, which strongly influenced arsenic redistribution between aqueous and solid phases.

Speciation change and redistribution of arsenic in soil under anaerobic microbial activities.

Arsenic speciation and behavior in soil are strongly affected by redox conditions. This work investigated speciation transformation and redistribution of arsenic in soil under anaerobic conditions. The effect of microbial sulfidogenesis on these processes was examined by addition of sulfate to the incubation systems. As(III) was found to be the dominant arsenic species in solution during the process of anaerobic incubation. The change of dissolved As concentration with incubation time showed "M" shaped profiles, e.g. the curves displaying two peaks at approximately 24 h and 240 h for the system with added sulfate. Arsenic was released and reduced to As(III) in the early stage of the incubation, and then resequestered into the solid phase. After excess sulfide was generated, the resequestered arsenic was released again (probably due to the dissolution of arsenic sulfide by dissolved sulfide ions) via the formation of thioarsenite. At the end of the incubation process, most of the dissolved arsenic was removed again from solution. The findings may have important implications to the fate of arsenic in flooded sulfur-rich soils.

Incorporation of arsenic into gypsum: Relevant to arsenic removal and immobilization process in hydrometallurgical industry.

Gypsum precipitates as a major secondary mineral during the iron-arsenate coprecipitation process for the removal of arsenic from hydrometallurgical effluents. However, its role in the fixation of arsenic is still unknown. This work investigated the incorporation of arsenic into gypsum quantitatively during the crystallization process at various pHs and the initial arsenic concentrations. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray absorption near edge spectroscopy (XANES) and scanning electron microscopy (SEM) were employed to characterize the coprecipitated solids. The results showed that arsenate was measurably removed from solution during gypsum crystallization and the removal increased with increasing pH. At lower pH where the system was undersaturated with respect to calcium arsenate, arsenate ions were incorporated into gypsum structure, whereas at higher pH, calcium arsenate was formed and constituted the major arsenate bearing species in the precipitated solids. The findings may have important implications for arsenic speciation and stability of the hydrometallurgical solid wastes.

Effects of nutrient and sulfate additions on As mobility in contaminated soils: a laboratory column study.

The effect of nutrient and sulfate additions on As mobility in contaminated soils was investigated under advective-flow anoxic columns in this study. The mobility of As in surface contaminated soils was investigated by the leaching of de-ionized water (DI-water), artificial ground water (AG-water) and AG-water+sulfate (Sulfate). After 144 d of experiments, compared to the DI-water column, the total As exported from the columns AG-water and Sulfate was enhanced by seven and eightfold, respectively. The results indicated that the nutrient and sulfate addition significantly enhanced the As mobility in contaminated soils. In low-sulfate soils (DI-water and AG-water systems), As mobilization was primarily attributed to As reduction and to the transformation of amorphous Fe(III) (oxy)hydroxides. In soil with sulfate addition (Sulfate system), besides As reduction and Fe(III) (oxy)hydroxides transformation, the dissolution of As sulfides and the formation of thioarsenic species under sulfidogenic condition were possibly important processes accelerating As release. In conclusion, the addition of the nutrient solution and sulfates may increase the mobility of As in contaminated soils, posing a potential threat to groundwater.

[A study of the prevalence of obstructive sleep apnea hypopnea syndrome in drivers].

OBJECTIVE: To study the prevalence of snoring and obstructive sleep apnea hypopnea syndrome (OSAHS) in drivers in Chengde city. METHODS: The prevalence of snoring and OSAHS in drivers was investigated by a questionnaire, and some of the drivers with more severe snoring were also evaluated by polysomnographic study. RESULTS: Among the 718 investigated drivers, never snoring was reported in 68.3% (490/718), mild snoring in 31.7% (228/718), and moderate and severe snoring in 16.6% (119/718). The prevalence of snoring increased with age. The prevalence of snoring was higher in males (33.6%) than in females (15.3%). When their knowledge of snoring was evaluated, 43.2% (310/718) thought snoring was not a disease, 16.7% (120/718) considered snoring as a disease but needed no treatment, 40.1% (228/718) considered it as a disease and needed treatment but only 1 had ever received treatment. If AHI > or = 5/h was used as the diagnostic criteria, the prevalence of OSAHS in the drivers was 12.3% (28/228); and if Epworth Sleepiness Scoring (ESS > or = 9) was combined, the prevalence of OSAHS was 4.1% (9/228). CONCLUSION: The prevalence of snoring in drivers was 31.7%, and the prevalence of OSAHS was 4.1%. Drivers need to be informed of the potential harm of OSAHS on their work.