Calcium Arsenate Research Articles

Theoretical Assessment of Calcium Arsenates Stability: Application in the Treatment of Arsenic Contaminated Waste

Several approaches for immobilization of arsenic (As) based on the transformation of its soluble forms (compounds) into highly insoluble arsenate apatite Ca5(AsO4)3OH have been proposed. These immobilization techniques are successfully applied in treatment of industrial waste containing As. Quite the contrary, treatment of soil contaminated with As by apatite amendments, instead of immobilization of this toxic element, increases its mobility and bioavailability. The mechanism underlying these opposite effects still remains elusive. Here, the stability analysis of different calcium arsenates: Ca5(AsO4)3OH, Ca4(AsO4)2(OH)2, Ca3(AsO4)2 Ca5H2(AsO4)2 and CaHAsO4 was performed, which is based on the calculation of the ion-ion interaction potential (IIIP). It has been demonstrated earlier that IIIP, representing the main term of the cohesive energy, is a suitable parameter for assessment of mineral stability. According to the results of this analysis, arsenate apatite with IIIP value of -0.578 Ry represents the most stable chemical form among analyzed compounds. Based on this finding, we proposed a mechanism of formation of arsenate apatite in the presence of hydroxyapatite. This mechanism can explain the suitability of this approach for the treatment of industrial waste and its limitations for in situ treatment of soil and water contaminated with As.

Dynamics of arsenic in the mining sites of Pine Creek Geosyncline, Northern Australia

The transportation and fixation of arsenic (As) in soil and sediments from five mine sites within the Pine Creek Geosyncline, Northern Territory, were examined based on measurements of operationally-defined fractions of As in soils, sediment and evaporates. Arsenic was mainly retained in sediments in the form iron arsenate (Fe-As). In wetland systems, As was retained as Fe-As together with calcium arsenate (Ca-As) from alkaline groundwater and organic-bound As from detrital material. In retention ponds As was retained as Fe-As, Ca-As and residual As (Res-As) up to 1700 mg/kg. Sediment traps can retain As from alkaline and acidic source seepages. The retention of Res-As and other mineral particulates during erosional or controlled process water discharges was associated with high Fe-As and organic-bound As in sediment. Arsenic was retained as Fe-As, Ca-As and residual As in 100 year old tailings at Millar's Battery, Union Reefs mine nearby McKinlay River and the small copper mine lease MLN 95 adjacent Copperfield Creek nearby Pine Creek. Natural geo-mobilisation of As was observed in upstream sediments at Copperfield Creek (5-8 mg/kg), Mt. Bundey Creek (10-12 mg/kg), upstream Ryan's Creek (10-12 mg/kg) and downstream East branch Ryan's Creek (7 mg/kg). Erosion of As-containing mineralisation was observed in the McKinlay River upstream and downstream (23-26 mg/kg) and upstream Ryan's Creek boundary of the Goodall mine lease MLN 1049 (24-40 mg/kg). Overall, As was mainly retained in sediments in the form Fe-As. The concentration data for As were used to propose mechanisms of As dispersion and retention occurring at the various mine sites that can be utilised for future mine water management design to minimise As dispersion.

Simultaneous Removal of SO2 and Trace As2O3 from Flue Gas: Mechanism, Kinetics Study, and Effect of Main Gases on Arsenic Capture

Sulfur dioxide (SO2) and trace elements are pollutants derived from coal combustion. This study focuses on the simultaneous removal of S02 and trace arsenic oxide (As2O3) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range. Experiments have been performed on a thermogravimetric analyzer (TGA). The interaction mechanism between As2O3 and CaO is studied via XRD detection. Calcium arsenate [Ca3(AsO4)2] is found to be the reaction product in the range of 600-1000 degrees C. The ability of CaO to absorb As2O3 increases with the increasing temperature over the range of 400-1000 degrees C. Through kinetics analysis, it has been found that the rate constant of arsenate reaction is much higher than that of sulfate reaction. SO2 presence does not affect the trace arsenic capture either in the initial reaction stage when CaO conversion is relatively low or in the later stage when CaO conversion is very high. The product of sulfate reaction, CaS04, is proven to be able to absorb As2O3. The coexisting CO2 does not weaken the trace arsenic capture either.

Cobaltaustinite, CaCo(AsO4)(OH)

This study presents the first structural report of a natural cobaltaustinite sample (calcium cobalt arsenate hydroxide) based on single-crystal X-ray diffraction data. Cobaltaustinite, with the ideal formula CaCo(AsO4)(OH), belongs to the adelite mineral group. The CoO6 octa­hedra share edges to form chains extending parallel to the c axis, which are cross-linked by Ca2+ ions and by sharing vertices with isolated AsO4 tetra­hedra. The Ca2+ ions are situated in square anti­prisms formed by eight O atoms. The major structural difference between the five calcium arsenates in the adelite group is shown in the bonding environments around the octa­hedrally coordinated M2+ cations (M = Cu, Zn, Co, Ni and Mg), with the average M—O distance decreasing from Cu—O in conichalcite, CaCu(AsO4)(OH) to Zn—O in austinite, CaZn(AsO4)(OH), Co—O in cobaltaustinite, Ni—O in nickelaustinite, CaNi(AsO4)(OH), and Mg—O in adelite, CaMg(AsO4)(OH). The donor–acceptor O—H⋯O distance [2.721 (7) Å] in cobaltaustinite is similar to those in austinite and nickelaustinite, but different from those in adelite and conichalcite.

The form, distribution and mobility of arsenic in soils contaminated by arsenic trioxide, at sites in southeast USA

Soils from many industrial sites in southeastern USA are contaminated with As because of the application of herbicide containing As 2O 3. Among those contaminated sites, two industrial sites, FW and BH, which are currently active and of most serious environmental concerns, were selected to characterize the occurrence of As in the contaminated soils and to evaluate its environmental leachability. The soils are both sandy loams with varying mineralogical and organic matter contents. Microwave-assisted acid digestion (EPA method 3051) of the contaminated soils indicated As levels of up to 325 mg/kg and 900 mg/kg (dry weight basis) for FW and BH soils, respectively. However, bulk X-ray powder diffraction (XRD) analysis failed to find any detectable As-bearing phases in either of the studied soil samples. Most of the soil As was observed by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM/EDX), to be disseminated on the surfaces of fine-grained soil particles in close association with Al and Fe. A few As-bearing particles were detected in BH soil using electron microprobe analysis (EMPA). Synchrotron micro-XRD and X-ray absorption near-edge structure (XANES) analyses indicated that these As-rich particles were possibly phaunouxite, a mineral similar to calcium arsenate, which could have been formed by natural weathering after the application of As 2O 3. However, the scarcity of those particles eliminated them from playing any important role in As sequestration. Synthetic acid rain sequential batch leaching experiments showed distinct As leaching behaviors of the two studied soil samples: BH soil, which has the higher As content, showed a slow, steady release of As, while FW soil, with a lower As content, showed a much quicker release and lower overall retention of As upon leaching. Sequential chemical extraction experiments were carried out using a simplified 4-step sequential chemical extraction procedure (SCEP) previously developed to characterize the fractionation of As and better understand the different leaching behaviors of the two studied soils. It was shown that only about 50% of the total extractable As was removed by the first two extraction steps, which represented the most weakly bonded and readily available As for environmental leaching. Compared with the sequential leaching experiments, it was further indicated that only half of the As associated with phases extracted by the second SCEP step was mobilized by SPLP leaching. Although microwave-assisted acid digestion results showed similar Al and Fe contents in both soils, the sequential chemical extraction experiments indicated that BH soil has a much higher content of amorphous Al and Fe phases and that a comparably higher portion of soil As was associated with those materials. The experimental results suggest that remediation efforts for the contaminated sites can be directed towards enhancing the formation of more stable As-bearing compounds in the soils to reduce the environmental leachability of As.

The Apple Bites Back: Claiming Old Orchards for Residential Development

As the U.S. population continues to grow, increasing demand for housing and related community resources means more land is being converted from agricultural uses to residential applications. According to the revised 1997 National Resources Inventory conducted by the USDA Natural Resources Conservation Service, more than 6 million acres of American farmland were converted to developed uses between 1992 and 1997. That is an annual conversion rate of roughly 1.2 million acres per year—a 51% increase over the average annual rate reported for the preceding decade. Naturally, many of these areas were routinely treated with pesticides and other chemicals during their agricultural lifetimes. Although this legacy has been problematic in a wide variety of land conversion scenarios, one in particular seems to have attracted the attention and concern of environmental officials and property buyers in several states across the country: the residential development of historic orchard properties. In state after state, these old orchards (which most often produced apples, but also peaches, cherries, pears, and other tree crops) are metamorphosing into highly desirable subdivisions—desirable, that is, until it emerges that the soil beneath the feet of the proud new residents may be contaminated with lead and arsenic. These toxic by-products are left from the days before DDT and before organophosphates, when arsenical pesticides, particularly lead arsenate (LA), were the treatment of choice to prevent the ravages of insect damage.

Solubility and Stability of Calcium Arsenates at 25∘C

The solubility and stability of calcium arsenates at 25 ∘C was determined by both precipitation and dissolution experiments. Ca3(AsO4)2⋅ 3H2O(c), Ca3(AsO4)2⋅ 21/4H2O(c), Ca5(AsO4)3(OH)(c) and Ca4(OH)2(AsO4)2⋅ 4H2O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ∼ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca3(AsO4)2⋅ xH2O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca3(AsO4)2⋅ 3H2O(c), Ca3(AsO4)2⋅ 21/4H2O(c), Ca5(AsO4)3(OH)(c) and Ca4(OH)2(AsO4)2⋅ 4H2O(c) were calculated as Ksp of 10− 21.14(10− 20.01 ∼ 10− 22.02), 10− 21.40(10− 20.08 ∼ 10− 21.98), 10− 40.12(10− 37.53 ∼ 10− 42.72) and 10− 27.49(10− 26.10 ∼ 10− 28.91), respectively. Correspondingly, the free energies of forming (Δ Gfo) of these calcium arsenates were calculated to be −3787.87 kJ/mol, −3611.50 kJ/mol, −5096.47 kJ/mol and −4928.86 kJ/mol.

Transformation of arsenic in Yima coal during fluidized-bed pyrolysis

Arsenic is one of the most concerned trace elements in coal because of its toxicity and environmental persistence. Little information about the transformation behavior of arsenic under pyrolysis conditions was reported. In this paper, the volatilization behavior of arsenic and the influence of coexisting mineral matters and additive CaO were investigated during pyrolysis of Yima coal in a fluidized-bed reactor at temperatures ranging from 350 to 900 °C. The modes of occurrence of arsenic in Yima raw coal and its two char samples are determined using four methods including density fractionation, demineralization, X-ray diffraction (XRD) and sequential chemical extraction. It is found that arsenic in Yima coal is mainly associated with pyrite, and changes greatly during coal pyrolysis. Staged volatility behavior of arsenic is observed during Yima coal pyrolysis. From 350 to 500 °C, arsenic volatility increases with increasing temperature, while it changes little from 500 to 800 °C. Arsenic behavior in YimaD (demineralized sample of Yima) and CaO additive reveal that the vaporization of arsenic in Yima coal during pyrolysis is retarded by its coexisting mineral matters or added CaO to form thermally stable forms of arsenic. These forms might be generated through the formation of As–Ca complexes such as calcium arsenate, or through absorption and capture reactions with aluminosilicate mineral matters and trapped into their lattices.

Immobilization mechanisms of arsenate in iron hydroxide sludge stabilized with cement.

Leaching tests, Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS) spectroscopy, and thermodynamic modeling were performed to investigate arsenate [As(V)] immobilization mechanisms in iron hydroxide sludge stabilized with cement. The sludge from a groundwater remediation site in Tacoma, WA was mixed and immobilized with premixed cement to reach cement-to-sludge ratios of 2.5, 3.3, 5, 10, and 20 (wt premixed cement/wt dry sludge). The EXAFS analysis determined that As(V) formed bidentate mononuclear complexes on the iron hydroxide surface in the sludge. The adsorbed As(V) had a characteristic FTIR band at 830 cm(-1). Cement treatment converted the adsorbed As(V) to calcium arsenate precipitate with a FTIR peak at 860 cm(-1). The chemical forms of the As(V) were incorporated in an adsorption triple layer model (TLM) to describe the leaching behavior of As(V) in a pH range between 3 and 12. Cement treatment significantly reduced arsenic mobility because of the formation of the sparingly soluble calcium arsenate.

Geochemistry of arsenic in uranium mine mill tailings, Saskatchewan, Canada

The Rabbit Lake U mine in-pit tailings management facility (TMF) (425 m long×300 m wide×91 m deep) is located in northern Saskatchewan, Canada. The objectives of this study were to quantify the distribution of As phases in the tailings and evaluate the present-day geochemical controls on dissolved As. These objectives were met by analyzing pore fluid samples collected from the tailings body for dissolved constituents, measuring Eh, pH, and temperature of tailings core and pore fluid samples, conducting sequential extractions on solid samples, conducting geochemical modeling of pore fluid chemistry using available thermodynamic data, and by reviewing historical chemical mill process records. Dissolved As concentrations in 5 monitoring wells installed within the tailings body ranged from 9.6 to 71 mg/l. Pore fluid in the wells had a pH between 9.3 and 10.3 and Eh between +58 and +213 mV. Sequential extraction analyses of tailings samples showed that the composition of the solid phase As changed at a depth of 34 m. The As above 34 m was primarily associated with amorphous Fe and metal hydroxides while the As below 34 m was associated with Ca, likely as amorphous poorly ordered calcium arsenate precipitates. The change in the dominant As solid phases at this depth was attributed to the differences in the molar ratio of Fe to As in the mill tailings. Below 34 m it was <2 whereas above 34 m it was >4. The high Ca/As ratio during tailings neutralization would likely precipitate Ca 4(OH) 2(AsO 4) 2:4H 2O type Ca arsenate minerals. Geochemical modeling suggested that if the pore fluids were brought to equilibrium with this Ca-arsenate, the long-term dissolved As concentrations would range between 13 and 126 mg/l.

A comparison of sequential extraction techniques for determining arsenic fractionation in synthetic mineral mixtures.

The sequential extraction methods according to Tessier et al., Borovec et al., Zhang and Moore and Hall et al. have been tested for their suitability for arsenic fractionation in samples of artificially prepared mineral mixtures. Mixtures containing different amounts of As-containing phases were prepared so that their compositions corresponded to weathering products on As-bearing ore deposits. A comparison of different procedures on simple mineral mixtures containing calcium arsenate (CaHAsO(4).H(2)O), As-bearing goethite (FeOOH) and arsenopyrite (FeAsS) showed that only the results of the Hall method satisfactorily correspond to the expected arsenic distribution. A detailed verification of the Hall method was subsequently carried out on most complex synthetic mineral mixtures with varying amounts of As-containing kaolinite and carbonate, calcium arsenate, As-bearing goethite and arsenopyrite. The results confirm that the Hall method cannot be fully employed for an accurate As speciation but may be applied for a route identification of As distribution between "labile", "medium-labile" and "residual" forms in heavily polluted soils.

Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates

In coal combustion systems, the partitioning of arsenic between the vapor and solid phases is determined by the interaction of arsenic vapors with fly ash compounds under post-combustion conditions. This partitioning is affected by gas–solid reactions between the calcium components of the ash particles and arsenic vapors. In this study, bench scale experiments were conducted with calcium compounds typical of coal-derived fly ash to determine product formation, the extent of reaction and reaction rates when contacted by arsenic oxide vapors. Experiments conducted with arsenic trioxide (As 4O 6(g)) vapors in contact with calcium oxide, di-calcium silicate and mono-calcium silicate over the temperature range 600–1000 °C indicated that these solids were capable of reacting with arsenic vapor species in both air and nitrogen. Calcium arsenate was the observed reaction product in all the samples analyzed. Maximum capture of arsenic occurred at 1000 °C with calcium oxide being the most effective of the three solids over the range of temperatures studied. Using a shrinking core model for a first order reaction and the results from intrinsic kinetic experiments conducted in air, the reaction rate constants were found to be 1.4×10 −3exp(−2776/ T) m/s for calcium oxide particles, 7.2×10 −3exp(−3367/ T) m/s for di-calcium silicate particles and 5.5×10 −3exp(−3607/ T) m/s for mono-calcium silicate particles. These results therefore suggest that any calcium present in fly ash can react with arsenic vapor and capture the metal in water-insoluble forms of the less hazardous As(V) oxidation state.

Paget’s disease of bone in Lancashire and arsenic pesticide in cotton mill wastewater: a speculative hypothesis

A 1974 survey of Paget’s disease of bone in 31 UK towns identified a cluster of six Lancashire former cotton mill towns with the highest prevalence rates of the disease and noted a link with the cotton industry. The two highest-prevalence towns are situated on estuaries and are jointly the smallest of the six by the size of their former cotton industry. The two lowest-prevalence towns of the six are highest above sea level and jointly the largest by the size of their former cotton industry. This pattern suggests a waterborne agent. A 1993 repeat survey of 10 of the 31 towns identified a general decline in prevalence, greatest in the three cotton towns included from the six in the earlier survey, further suggesting a link to the cotton industry. Cotton mills and bleach/dyeworks produced a large volume of wastewater that was commonly discharged into adjacent waterways. Wastewater contained process chemicals and imported organisms and pesticides carried in cotton bales. The process chemicals and imported organisms could be discounted as a probable factor in Paget’s disease; however, pesticides could not be discounted. Lancashire cotton came predominantly from the American cotton belt. From 1917 to 1945, calcium arsenate pesticide was used intensively to combat the boll weevil and was imported to Lancashire in cotton bales. The calcium arsenate era is consistent with the high-prevalence findings in 1974 and the decline in 1993, allowing for time lags. Geochemical arsenate is widespread in the environment and may account for geographic variations in Paget’s prevalence, although data are inadequate for correlation studies. Noncytotoxic doses of arsenic affect all the steroid receptors and some signal transduction pathways, which may lead to dysfunctional osteoclast signaling and differentiation. Human cells are more susceptible than animal cells to arsenic, based on studies of nonbone cells. It is speculative that arsenic may be a factor in the abnormal variation in prevalence rates in Lancashire. It is not a general theory of Paget’s disease.

The Western Maryland coal combustion byproducts/acid mine drainage initiative: the Winding Ridge demonstration project

Small-scale combustion experiments were conducted to study the vaporization of trace elements during the combustion of pulverized coal. The combustion process was sampled at high temperatures (1423 K) corresponding to in-flame conditions. Rapid quenching and dilution of the combustion products were followed by a cascade impactor to collect size-segregated ash samples. Based on the comparison of concentrations of refractory elements in size-segregated ash with their bulk concentrations, we conclude that the ash having diameters less than approximately 0.4 micrometers represents the material which vaporizes during combustion in our combustion furnace. Good mass balance closure (90% to 140%) was obtained overall for ash in the sampling system. The volatile elements As, Sb, Se, and Zn showed mass balance closures significantly less than 100%. Thermochemical equilibrium predictions reported in the literature indicate that these elements should be completely volatile in the flame, although As is predicted to form condensed calcium arsenate at the sampling temperature in our system. Little chromium was found in the vaporization mode in our experiments. The uniformly low volatility of chromium suggests that chromium may react with oxides in the bulk ash. An upper bound on the amount of the more volatile elements which vaporized was computed. This upper bound accounted for the amount of the element in the finest ash and the material “losses” indicated by the mass balance closure. The results of these experiments suggest almost complete vaporization of certain trace elements (Se, Zn) from coal combustion in the flame zone, in accordance with theoretical equilibrium predictions. Other elements (As, Sb) appeared considerably less volatile and may react with constituents in the bulk ash at combustion temperatures.

Risk of arsenic transfer to a semi-confined aquifer and the effect of water level fluctuation in North Mortagne, France at a former industrial site

Groundwater contamination by arsenic was studied in the area of a former larger zinc refinery in France. Maximum contamination was observed under the former sulfuric acid factory, while the overall waste storage area was less contaminated. Arsenic concentrations there were controlled by the solubility of 3:2 calcium arsenate mineral Ca 3(AsO 4) 2(s) and probably a gypsum/calcium arsenate CaSO 4 (s)/Ca 3(AsO 4) 2 (s) solid solution. The speciation of As below the former sulfuric acid factory indicates an overall predominance of As(III) species. The sorption by the clay aquitard was complete for As(V), but limited to approximately 30% for As(III) under our experimental conditions. A potential risk exists, although very limited in area, of contamination of the underlying sandy aquifer and drinking water wells.

Transfer of toxicants from exposed nymphs of the drywood termite Incisitermes snyderi (Isoptera: Kalotermitidae) to unexposed nestmates.

Two formulations of spinosad (NAF-85 and NAF-371) were evaluated to determine the effect of concentration, deposit condition (dried, wet, or topical), and exposure time (0.1-10 h) for toxicant transfer among nestmates in the drywood termite Incisitermes snyderi (Light). Spinosad treatments were compared with two formulations of disodium octaborate tetrahydrate (DOT) (10% aqueous dilution, 98% dust) and with 35% calcium arsenate dust. Termites were dyed and individually exposed to different treatments for 0.1, 1.5,1.0, 5.0, 10.0 h, or 0.1 and 1.0 h (DOT and calcium arsenate dust) and then placed with 10 unmarked, untreated nestmates in a petri dish. Spinosad formulations also were evaluated by topical application to dyed termites. Transfer of lethal doses of toxicants between termites was indicated by significant mortality of untreated termites in 25 of 28 treatments by 28 d after introduction of dyed, treated termites. Only three treatments, one spinosad treatment (NAF-371, wet, 1 h) and both DOT 10% solution treatments, resulted in mortality of untreated termites that was not significantly different than that of water-treated controls. Two spinosad treatments and both calcium arsenate dust treatments resulted in >90% (94-98%) mortality of untreated termites by 28 d after introduction. Mortality of untreated termites was significantly different from controls for the two spinosad formulations, depending on condition of deposit and duration of dyed termite exposure to treatments.

Arsenic Disposal Practices in the Metallurgical Industry

The literature on the arsenic disposal practices of the metallurgical industry and the long term stability of the disposed arsenic compounds were reviewed from a Canadian perspective. The review was complemented with visits to selected operating metallurgical sites to obtain information on their specific arsenic disposal practices and the behaviour of their impounded residues. The arsenic disposal procedure currently favoured by the industry involves the formation of an insoluble ferric arsenate compound which is allowed to sediment at the bottom of tailings or residue ponds. It has been shown recently that this poorly formed crystalline compound is similar to “garsenical ferrihydrite” which is ferrihydrite containing strongly adsorbed arsenate anions. Despite concerns about its long term stability on thermodynamic grounds, arsenical ferrihydrite appears to be stable for many years in the proper environment which includes a slightly acidic pH and oxidizing conditions. A high Fe/As ratio and the presence of heavy metals appear to increase the stability of arsenical ferrihydrite. High temperature operations, such as those encountered in the autoclave treatment of refractory gold ores, are conducive to the formation of scorodite, FeAsO4.2H2O, and/or a series of ferric arseno-hydroxy-sulphate compounds depending on the solution composition. Scorodite has several advantages over arsenical ferrihydrite as a disposal compound including a lower iron demand, a higher density and a greater thermodynamic stability. New procedures have been developed to generate scorodite at ambient pressure. These procedures would offer significant reductions in capital cost because of the elimination of the need for autoclaves. Lime precipitation which was widely used in the past is being abandoned as a result of strong evidence showing that calcium arsenate compounds decompose slowly in contact with atmospheric CO2 to form calcium carbonate and soluble arsenic acid. In all cases, the long term stability of the disposed arsenic compounds depends on a number of factors including disposal site characteristics, particle crystallinity and size distribution, the presence of complexing agents and the effect of bacterial activity.

Speciation of Combustion-Derived Particulate Phase Arsenic

The speciation of inorganic arsenic compounds produced in pulverized coal combustion was investigated in a bench-scale study conducted under fuel-lean combustion conditions. Results suggest that at temperatures of 850–1100 K, and in the absence of particulate, arsenic initially condenses as As2O3 rather than the thermodynamically favored As2O5. Synthetic fly ash in the form of uniform 0.25 μm spherical silica particles produced by sol-gel methods, added to the experimental system to simulate the presence of combustion-derived fly ash, did not affect arsenic speciation, nor did doping of the silica particles with small amounts of calcium. As(V) was only observed at these temperatures when calcium was added to the system as calcium acetate, producing a reactive calcium oxide fume and leading to the formation of calcium arsenate. From these results, it was concluded that the formation of arsenic(V) in combustion systems requires either the oxidation of As2O3, which appears to be a kinetically limited process, or the presence of reactive calcium compounds. Additional experiments conducted with calcium silicate in an isothermal tube furnace suggested that calcium in silicates is capable of reacting with arsenic under long (order minutes) reaction time conditions. The apparent ability of calcium silicate particles to capture arsenic under idealized conditions suggests that calcium embedded in silicate fly ash may be capable of forming calcium arsenate, but the extent of this reaction on the time scales of a pulverized coal combustor or waste incinerator remains unclear. Key words: arsenic; speciation; combustion; air pollution; fly ash; particulate matter; air toxics

Pollution prevention in the semiconductor industry through recovery and recycling of gallium and arsenic from GaAs polishing wastes

A process was developed for the recovery of both arsenic and gallium from gallium arsenide polishing wastes. The economics associated with the current disposal techniques utilizing ferric hydroxide precipitation dictate that sequential recovery of toxic arsenic and valuable gallium, with subsequent purification and in-house reuse of both, is to the benefit of the gallium arsenide crystal grower. The developed process involves first the removal of the majority of the arsenic and suspended polish as a mixed precipitate of calcium arsenate and polish. This first process step is performed at ambient temperatures and at a pH>11 using NaOH. At these pH regimes, gallium is retained in solution as a sodium gallate species. Precipitation of virtually pure gallium hydroxide is then accomplished in the next process step through pH adjustment to between 6 and 8 with waste acids. The commonly used ferric hydroxide coprecipitation step is retained as a final treatment step, but because of the removal of the majority of the arsenic, gallium, and polish in the two prior steps, far less waste is land disposed. A patent application has been filed with the United States Patent Office.

Laboratory study of trace element vaporization from combustion of pulverized coal

Small-scale combustion experiments were conducted to study the vaporization of trace elements during the combustion of pulverized coal. The combustion process was sampled at high temperatures (1423 K) corresponding to in-flame conditions. Rapid quenching and dilution of the combustion products were followed by a cascade impactor to collect size-segregated ash samples. Based on the comparison of concentrations of refractory elements in size-segregated ash with their bulk concentrations, we conclude that the ash having diameters less than approximately 0.4 micrometers represents the material which vaporizes during combustion in our combustion furnace. Good mass balance closure (90% to 140%) was obtained overall for ash in the sampling system. The volatile elements As, Sb, Se, and Zn showed mass balance closures significantly less than 100%. Thermochemical equilibrium predictions reported in the literature indicate that these elements should be completely volatile in the flame, although As is predicted to form condensed calcium arsenate at the sampling temperature in our system. Little chromium was found in the vaporization mode in our experiments. The uniformly low volatility of chromium suggests that chromium may react with oxides in the bulk ash. An upper bound on the amount of the more volatile elements which vaporized was computed. This upper bound accounted for the amount of the element in the finest ash and the material “losses” indicated by the mass balance closure. The results of these experiments suggest almost complete vaporization of certain trace elements (Se, Zn) from coal combustion in the flame zone, in accordance with theoretical equilibrium predictions. Other elements (As, Sb) appeared considerably less volatile and may react with constituents in the bulk ash at combustion temperatures.