Precipitation Of Ferric Hydroxide Research Articles

Optimization of Cu, Hg and Cd removal by Enterobacter cloacae by ferric ammonium citrate precipitation

Iron precipitating organisms play a significant role in the formation of ferric hydroxide precipitate, which acts as strong adsorbent for toxic metal. In this respect four different iron precipitating cultures were isolated from Hutti gold mine surface winze water sample on citrate agar medium. The best isolate was screened out for metal removal study on the basis of fast visual iron precipitation. The selected isolate was identified as Enterobacter sp. based on routine biochemical tests and Biolog GN microplate results and as Enterobacter cloacae subsp. dissolvens by 16S rRNA gene sequence analysis (GenBank accession number EU429448). Influence of medium composition, medium initial pH, the influence of inoculum size, effect of various media and ferric ammonium citrate concentration were studied on metal removal in shake flask experiments. Under the optimized conditions studied, E. cloacae showed <TEX>$94{\pm}2$</TEX>, <TEX>$95{\pm}2$</TEX> and <TEX>$70{\pm}2%$</TEX> of cadmium, copper and mercury removal from a simulated waste in shake flask studies. In lab scale column reactor more than 85% of copper and mercury removal was achieved.

Rapid separation and determination of 107Pd in radioactive waste produced during NPP A-1 decommissioning

Procedure for 107Pd determination in radioactive wastes coming from decommissioning of NPP A-1 is proposed and verified. 107Pd was separated and purified by Pd precipitation with dimethylglyoxime (DMG) and/or ferric hydroxide precipitation. 107Pd yield was determined gravimetrically by weighing the Pd(DMG)2 precipitate. 107Pd counting was performed using liquid scintillation counter (LSC-TDCR) and scintillation cocktail Hionic Fluor. In all analyzed samples the determined activities of 107Pd were lower than MDA. Nevertheless such analyses permit the decision concerning the storage of solidified wastes in the Mochovce regional repository.

X-ray and electron microscopy studies on the biodistribution and biomodification of iron oxide nanoparticles in Daphnia magna

Biodistribution and biomodification of iron oxide (Fe3O4 and α-Fe2O3) nanoparticles (NPs) in a well-known toxicity test organism, Daphnia magna (D. magna), were investigated using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). In addition to the morphological changes in the gut tissues of D. magna, biodistribution and biomodification of iron oxide NPs in the digestive tract of D. magna were also monitored in this study. Upon exposures to both iron oxide NPs, unique morphological changes (e.g., irregular shaped microvilli, epithelial cell protrusion, and dilatation of cytoplasmic inclusion) in the gut tissues of D. magna were observed along with bacterial colonization of the gut lumen. However, despite their heavy accumulations in the digesitive tract, TEM and STXM images confirmed us that both Fe3O4 and α-Fe2O3 NPs were not penetrating into the gut tissues of D. magna. Moreover, for the Fe3O4 NPs in direct contact with the gut microvilli of D. magna, slight but significant spectral changes were observed in their Fe L-edge X-ray absorption near edge structure (XANES) spectra, which indicated that there were biomodifications of Fe3O4 NPs, probably involving oxidative dissolution of Fe3O4 NPs followed by rapid precipitation of ferric oxide or hydroxide. However, no significant changes were observed in the Fe L-edge XANES spectra of the α-Fe2O3 NPs present in the gut lumen of D. magna. These X-ray and electron microscopic observations confirmed us that, despite similarities in core sizes and chemical compositions, NPs with different crystalline phase and dissolution rates can interact quite differently with their local environment, may result in different biodistribution and cause completely dissimilar toxicities.

Intensification of a solar photo-Fenton reaction at near neutral pH with ferrioxalate complexes: A case study on diclofenac removal from aqueous solutions

Abstract The kinetic degradation profile for diclofenac (DCF) antibiotic in aqueous solutions using the conventional photo-Fenton reaction is characterized by a very slow reaction rate mainly associated to DCF precipitation-redissolution-degradation mechanism at acidic conditions and DCF degradation via a ligand-to-metal charge mechanism that occurs on the surface of the iron precipitates, under neutral pH conditions. This work evaluates the intensification of a solar photo-Fenton reaction by using ferrioxalate complexes (Fe3+/H2O2/oxalic acid/UVA–vis) in the degradation of diclofenac (DCF) in aqueous solution at near neutral pH values, which avoids the precipitation of DCF and ferric hydroxide, and iron concentrations below the discharge limits into water bodies ([Fe] ⩽ 2.0 mg/L). The efficiency of the photo-Fenton process was evaluated at different iron/oxalate molar ratios (1:3; 1:6 and 1:9; [Fe3+] = 2 mg/L), under controlled conditions of pH (5.0 and 6.0), using a lab-scale photoreactor irradiated with artificial sunlight. At pH 6.0 ([DCF]0 = 20 mg/L), using an iron/oxalate molar ratio of 1:9 it was achieved complete degradation of DCF (below the detection limit) and 63.0% mineralization in 90 min, consuming 1.9 mM H2O2. A lower dose of oxalic acid (1:3 iron/oxalate molar ratio) was needed to attain similar results at pH 5.0. DCF degradation at pH 5.0, for an iron/oxalate molar ratio of 1:3, was also evaluated at different temperatures (15–45 °C), UVA irradiances (27.8–59.9 WUV/m2), DCF and iron concentrations ([DCF] = 2–20 mg/L, [Fe3+] = 1–2 mg/L), presence of inorganic ions (0.1 g/L; Cl−, SO42−, NO3−, HCO3−, NH4+) and radical scavengers (sodium azide 10 mM and d -mannitol 50 mM). DCF degradation kinetics at pilot plant scale under natural solar radiation, using the best conditions obtained at lab-scale (pH 5.0, iron/oxalate molar ratio of 1:3), showed similar results to those obtained in the lab-scale photoreactor. The nitrogen and chlorine atoms in the DCF molecule structure were easily released to the solution as chloride, ammonium and nitrite ions. Acetic acid was the main low-molecular-weight carboxylic acid detected during the reaction, stopping the photo-Fenton process due to the low photoactivity of ferric-acetate complexes and its recalcitrant character to further mineralization by hydroxyl radical attack.

Iron- and calcium-based biogrouts for porous soils

Chemical grouts are often used to reduce the hydraulic conductivity of soil for seepage control purposes. However, chemical grouts can be expensive and environmentally unfriendly. Therefore, two new biogrouts or microbial-based grouting materials were developed. The first was a calcium-based biogrout, which contained urease-producing bacteria, calcium chloride and urea for the crystallisation of calcite due to enzymatic hydrolysis of urea. The second was an iron-based biogrout, which consisted of urease-producing bacteria, ferric chelate, and urea for the precipitation of ferric hydroxide due to enzymatic hydrolysis of urea. Both biogrouts reduced the hydraulic conductivity of loose clean sand from 5 × 10−5 to 1·4 × 10−7 m/s when the precipitated metal content was at 1·5% w/w or lower. The unconfined compressive strength of the treated specimens also increased. However, at a low metal content, the amount of improvement was still much less than that obtained by chemical grouting. Therefore, suitable geotechnical applications of the calcium-based and iron-based biogrouts at a precipitated metal content of 1·5% w/w or less would be mainly the bioclogging of porous soils for the sealing of aquaculture or wastewater treatment ponds and landfill sites, and for plugging of the piping in dams and retaining walls in sandy soil.

Ecological relationships of wild rice, Zizania spp. 11. Electron microscopy study of iron plaques on the roots of northern wild rice (Zizania palustris)

The deposition, composition, and structure of iron plaques formed on the roots of northern wild rice (Zizania palustris L.) from five wetlands across Ontario, Canada, were examined through a scanning electron microscope and energy-dispersive X-ray analyzer. Iron was oxidized within the rhizosphere resulting in the precipitation of ferric hydroxide onto root surfaces, visible as an orange-brown plaque. The northern wild rice samples analyzed were found to form substantial iron root plaques in a variety of surface water and sediment chemistries. Iron plaques ranged structurally from thin to crust plaques (&lt;1 μm to 14 μm thick) and were composed mainly of Fe, O, Al, and K. Iron plaque was found within and on root epidermal cells, occasionally filling epidermal cells and penetrating into the root cortex. Rounded grooves were observed in the plaque surface of two samples and were reasoned to have been caused by iron-reducing bacteria. Observations on the anatomy of northern wild rice roots are presented along with a model of iron plaque development.

Study of arsenic(III) and arsenic(V) removal from waters using ferric hydroxide supported on silica gel prepared at low pH

Removal of As(III) and As(V) species using ferric hydroxide supported on silica gel was studied. Laboratory reagent quality silica gel was used as to avoid uncertainties that may be caused by impurities. Ferric hydroxide precipitation was realized at various pH values and a relatively low pH 6.0 was chosen because, at this pH, the highest arsenic removal capacity and removal efficiency were obtained and clear supernatant solution was observed. It was also shown by arsenic speciation analysis at trace level that As(III) is adsorbed onto ferric hydroxide partly without oxidation to As(V); this has been a controversial point in the literature. The effects on arsenic removal of some parameters such as pH, flow rate and matrix ions were investigated. In the batch method, initial pH change of the solution did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range of 3.1–9.7. This was attributed to the decreases of the initial pH values to around 5 at equilibrium. The column capacities of 1.32 mg As(III)/g sorbent and 1.21 mg As(V)/g sorbent were found for initial concentration of 1.00 mg/L arsenic. Batch capacities were 16.2 mg As(III)/g sorbent and 17.7 mg As(V)/g sorbent for initial arsenic concentration of 100 mg/L. The method was applied successfully to the removal of As(III) and As(V) from drinking water, geothermal water and mineral water.

Aquifer geochemistry at potential aquifer storage and recovery sites in coastal plain aquifers in the New York city area, USA

Abstract The effects of injecting oxic water from the New York city (NYC) drinking-water supply and distribution system into a nearby anoxic coastal plain aquifer for later recovery during periods of water shortage (aquifer storage and recovery, or ASR) were simulated by a 3-dimensional, reactive-solute transport model. The Cretaceous aquifer system in the NYC area of New York and New Jersey, USA contains pyrite, goethite, locally occurring siderite, lignite, and locally varying amounts of dissolved Fe and salinity. Sediment from cores drilled on Staten Island and western Long Island had high extractable concentrations of Fe, Mn, and acid volatile sulfides (AVS) plus chromium-reducible sulfides (CRS) and low concentrations of As, Pb, Cd, Cr, Cu and U. Similarly, water samples from the Lloyd aquifer (Cretaceous) in western Long Island generally contained high concentrations of Fe and Mn and low concentrations of other trace elements such as As, Pb, Cd, Cr, Cu and U, all of which were below US Environmental Protection Agency (USEPA) and NY maximum contaminant levels (MCLs). In such aquifer settings, ASR operations can be complicated by the oxidative dissolution of pyrite, low pH, and high concentrations of dissolved Fe in extracted water. The simulated injection of buffered, oxic city water into a hypothetical ASR well increased the hydraulic head at the well, displaced the ambient groundwater, and formed a spheroid of injected water with lower concentrations of Fe, Mn and major ions in water surrounding the ASR well, than in ambient water. Both the dissolved O 2 concentrations and the pH of water near the well generally increased in magnitude during the simulated 5-a injection phase. The resultant oxidation of Fe 2+ and attendant precipitation of goethite during injection provided a substrate for sorption of dissolved Fe during the 8-a extraction phase. The baseline scenario with a low (0.001 M) concentration of pyrite in aquifer sediments, indicated that nearly 190% more water with acceptably low concentrations of dissolved Fe could be extracted than was injected. Scenarios with larger amounts of pyrite in aquifer sediments generally resulted in less goethite precipitation, increased acidity, and increased concentrations of dissolved Fe in extracted water. In these pyritic scenarios, the lower amounts of goethite precipitated and the lower pH during the extraction phase resulted in decreased sorption of Fe 2+ and a decreased amount of extractable water with acceptably low concentrations of dissolved Fe (5.4 × 10 −6 M). A linear decrease in recovery efficiency with respect to dissolved Fe concentrations is caused by pyrite dissolution and the associated depletion of dissolved O 2 (DO) and increase in acidity. Simulations with more than 0.0037 M of pyrite, which is the maximum amount dissolved in the baseline scenario, had just over a 50% recovery efficiency. The precipitation of ferric hydroxide minerals (goethite) at the well screen, and a possible associated decrease in specific capacity of the ASR well, was not apparent during the extraction phase of ASR simulations, but the model does not incorporate the microbial effects and biofouling associated with ferric hydroxide precipitation. The host groundwater chemistry in calcite-poor Cretaceous aquifers of the NYC area consists of low alkalinity and moderate to low pH. The dissolution of goethite in scenarios with unbuffered injectate indicates that corrosion of the well could occur if the injectate is not buffered. Simulations with buffered injectate resulted in greater precipitation of goethite, and lower concentrations of dissolved Fe, in the extracted water. Dissolved Fe concentrations in extracted water were highest in simulations of aquifers (1) in which pyrite and siderite in the aquifer were in equilibrium, and (2) in coastal areas affected by saltwater intrusion, where high dissolved-cation concentrations provide a greater exchange of Fe 2+ (FeX 2 ). Results indicate that ASR in pyrite-bearing, Cretaceous aquifers of the NYC area can be a cost effective way to both store drinking water supplies and significantly reduce ambient concentrations of dissolved Fe.

Attenuation of dissolved metals in neutral mine drainage in the Zambian Copperbelt

Behaviour of metals like Cu and Co was studied in nearly neutral (pH ≥ 6.4) mine drainage seepage in a stream downgradient of a tailing dam at Chambishi site in the Copperbelt of Zambia. They are attenuated by precipitation of ferruginous ochres that incorporate significant quantities of metals. Using chemical analysis, X-ray powder diffraction and Mössbauer spectroscopy, we show that the ochres are composed mostly of amorphous ferric hydroxide. Close to the seepage face, the total Fe content of ochres increases due to precipitation of amorphous ferric hydroxide, but total Fe in sediment decreases further downstream. The stream then flows through wetland (dambo) where the remaining fraction of metals is removed. During rainy periods, increased flow rate may result in re-suspension of ochres, increasing thus the mobility of metals. Major ions like sulphate are conservative at the start of the dry period (May), but gypsum may probably precipitate later at the end of the dry period. Sequential extractions of bulk sediments indicate that Mn behaves differently to Fe, with a trend of increasing Mn with distance from the tailing dam. There is much more Fe than Mn in residual (Aqua Regia) fraction, indicating that amorphous ferric hydroxides are transformed to more crystalline phases deeper in sediment. Environmental impact of mine drainage is relatively limited due to its neutral character.

Treatment of membrane concentrates: phosphate removal and reduction of scaling potential

The widespread application of nanofiltration (NF) and reverse osmosis (RO) membranes in wastewater reuse inevitably generates a concentrate stream. Due to high concentrations of phosphate and salts, disposal of membrane concentrates is a problem which seriously constrains the application of this technology, especially in inland applications. There is a need for technologies which facilitate an affordable and environmentally-safe disposal of membrane concentrates. The objectives of this study are to investigate appropriate treatment techniques to (1) increase the recovery of the membrane filtration thus minimising the volume of the concentrate stream, and (2) increase the concentrate quality to enable discharge into surface water bodies. The results show that both adsorption onto granular ferric hydroxide (GFH) and chemical precipitation are generally effective for phosphate removal from NF concentrates. Chemical precipitation by dosing of sodium hydroxide solution is rapid and removes more than 90% of phosphate and calcium ions. By the removal of calcium ions, chemical precipitation can significantly reduce the scaling potential of NF and RO concentrates. This may allow higher recoveries in the NF/RO process.

Release of heavy metals from sulfide flotation tailings under deepwater discharge environments

Deepwater discharges of sulfide flotation tailings have been practiced for several decades. Oxygen penetration and organic matter content in such environments are the two major factors that determine their reduction-oxidation condition and hence the rate of sulfide mineral dissolution under such discharge environments. In this study, the effects of solution redox potential and pH on the dissolution of some heavy metals from sulfide flotation tailings were examined. Zn, Pb and Cu concentrations in solution were all higher at a higher redox potential and lower pH. Dissolved Zn concentrations reached 100 ppm even at a low redox potential of 105 mV and near neutral pH after 33 days. Pb concentrations were below 2 ppm at 105 mV and near neutral pH after 165 days. At 105 mV, the Cu concentrations were below 0.2 ppm at near neutral pH after 165 days. At pH 2.3, the Fe concentrations were higher at 420 mV than at 105 mV. But at pH 6.8, the Fe concentrations were lower at 420 mV than at 105 mV, due to the oxidation of Fe2+ to Fe3+ and the subsequent precipitation of ferric hydroxide. Arsenic concentrations in solution were all less than 0.002 ppm. For Zn, Cu and Fe, no significant differences were observed between the tests in solutions prepared using seawater and those using distilled water. However, Pb concentrations reached higher levels in solutions prepared using seawater.

Indecisiveness of electrophoretic mobility determination in evaluating Fe(III) coagulation performance

The magnitude of electrophoretic mobility (EM) is usually used to represent the repulsion force between colloid particles in an aqueous system. A generally accepted point is that particles will coagulate well when the suspension EM is close to zero, which is often realized by adding trivalent cations such as Fe(III). With regard to Fe(III) coagulation, however, we found that particles were primarily enmeshed by voluminous ferric hydroxide precipitate or flocs, and the enmeshment of particles by flocs was independent of the suspension EM during Fe(III) coagulation. A suspension with either highly negative or positive EM could be well coagulated and clarified if the solution pH and Fe(III) concentration favored the formation and fast growth of ferric hydroxide precipitate. For an effective Fe(III) coagulation, the EM was not necessarily zero.

Evaluation of the impacts of mine drainage from a coal waste pile on the surrounding environment at Smolnica, southern Poland

Mine drainage impacts from a coal waste pile at Smolnica, Poland have been monitored. Groundwater in an unconfined aquifer downgradient from the pile has near-neutral pH, but high concentrations of sulfate (up to 3,827 mg/l), chloride (up to 903 mg/l), and sodium (up to 2,606 mg/l). Concentrations of iron and manganese are elevated only locally, and concentrations of other metals are low. The behavior of sulfate seems to be conservative in the downgradient aquifer, and gypsum may only be precipitating locally. Concentrations of iron and manganese seem to be controlled by the precipitation of ferric oxide and hydroxides and rhodochrosite, respectively. Complete neutralization of mine drainage by carbonates is consistent with high concentrations of calcium (up to 470 mg/l) and magnesium (up to 563 mg/l) and also with high strontium concentrations of up to 3.08 mg/l, observed in groundwater downgradient from the pile. Hydraulic head profiles at two sites within the river bottom sediments indicate upward flow toward the river with large local differences in groundwater recharge. Water chemistry profiles in the river bottom sediments and geochemical modeling suggest conservative behavior of Na, Cl, and SO(4) and precipitation of Fe and Mn at the groundwater/river water interface. Mine drainage enters the Bierawka River and causes increasing sulfate concentrations. In contrast, concentrations of sodium and chloride in the Bierawka River decrease downgradient from the pile because water in the river upgradient from the pile is already highly contaminated by these species from the discharge of mining waters. Concentrations of Fe and Mn in the river water are low, as a consequence of the precipitation of Fe and Mn oxide and hydroxides. Direct geochemical modeling was able to reproduce measured concentrations of conservative species (e.g., Na, Cl, and SO(4)), but errors for metals and Ba were relatively large. In addition, calculated P(CO2) values in the river water are very high, suggesting that equilibrium with atmospheric P(CO2) and P(O2) has not been reached, and at least some reactions should be modeled as kinetic processes. High concentrations of Na, Cl, and SO(4) contribute to the contamination of the Odra River, which is joined by the Bierawka River farther downgradient, thus limiting the use of river water for recreation and other purposes.

Determination of Hydraulic Residence Times in Several UK Mine Water Treatment Systems and their Relationship to Iron Removal

In the UK, the Coal Authority has more than 40 mine water treatment systems, most of which are wetland systems with settlement lagoon pretreatment. The purpose of treatment in wetlands is the oxidation of ferrous to ferric iron and the subsequent hydrolysis and precipitation of ferric hydroxide within the wetland. It is generally accepted (Hedin et al., Passive treatment of coal mine drainage, 1994, p 35; Skousen and Ziemkiewicz, Acid mine drainage control and treatment, 1996, p 362; Younger et al., Mine water: hydrology, pollution, remediation, 2002, p 442) that this process proceeds by a first-order rate law, although most systems are designed based on an areal removal rate (10 g/m2/day) developed by the U.S. Bureau of Mines (Hedin et al., Passive treatment of coal mine drainage, 1994, p 35); this design guideline inherently assumes a constant removal rate. Given the actual kinetics of iron removal in wetlands, it follows that residence time will control iron removal; given the wide range of system geometries and aspects, it is logical to ascertain the actual hydraulic residence time of wetlands and settlement lagoons and determine the effect this has on iron removal. To make a preliminary assessment of this link, hydraulic residence time of two Coal Authority wetlands (Lambley and Whittle) and two Coal Authority settlement lagoons (Acomb East, Acomb West and Whittle) were measured using bromide tracer tests. Water samples for iron analysis and flow measurements were taken during each tracer test. The Lambley wetland performs well in terms of residence time, and, as reeds become established and adsorptive processes increase, its iron removal performance (currently 58% removal) may improve, but the low influent iron concentration appears to be a significant impediment to meeting the original performance target. In contrast, the hydraulic performance of the Whittle wetland system is poor, which appears to be due to accumulation of dead plant material coupled with a high length to width ratio. However, performance in terms of iron removal is good (92% removal), which appears to be due to the higher influent iron concentration, and especially the fact that the iron enters the wetland largely in particulate form. The longer residence time of water within the Acomb lagoons (≈12 h) resulted in far more effective iron removal (72% in the east lagoon and 85% in the west lagoon) than the shorter residence time at Whittle (24% iron removal, ≈5 h residence time). Performance (in terms of iron removal) of the settlement lagoon systems appears to be far more closely related to the hydraulic residence time (albeit this conclusion must be tentative, given that only three systems have been investigated, and the Acomb system receives chemical addition). Based on this study, treatment system sizing using 100 m2 lagoon area per 1 L/s flow appears to be a more appropriate basis for design rather than an areal iron removal rate.

Silicate HinderingIn SituFormed Ferric Hydroxide Precipitation: Inhibiting Arsenic Removal from Water

The effects and dominant mechanism of silicate hindering arsenic removal with in situ formed ferric hydroxides were investigated. Zeta potential analysis showed that 10 mg/L silicate as Si obviously decreased ζ potential from −11.9 mV (pH 8.61) to −29.9 mV (pH 8.84) for ferric hydroxides; more significant effects was observed at higher pH levels. Photometric dispersion analysis indicated that silicate apparently hindered ferric hydroxides precipitation and aggregation, with more apparent influence at elevated pH values. Furthermore, in high pH ranges silicate led to filtered iron concentration increase. These adverse effects resulted in a significant decrease of arsenic removal with in situ formed ferric hydroxides, especially in higher pH ranges. Sequential filtration analysis showed the presence of aqueous arsenic species in liquid phase, demonstrating that silicate increasing repulsive charges between ferric precipitates and decreasing surface sites available for arsenic, rather than contributing to the formation of tiny ferric colloids with adsorbed arsenic, was more significant in silicate hindering arsenic removal by ferric hydroxides.

Fouling Mechanisms in a Laboratory‐Scale UV Disinfection System

The fouling of quartz sleeves surrounding UV disinfection lamps is a perennial problem affecting both drinking water and wastewater applications. The mechanisms of fouling are not fully understood, but factors promoting fouling are believed to include heat, high hardness and/or high iron concentrations, and hydrodynamic forces. The role of UV radiation itself is unclear. The goal of this paper is to attempt to isolate the fouling mechanisms and to provide key information about those induced by UV radiation, using a unique laboratory-scale continuous-flow UV reactor. Its design allowed for irradiated and nonirradiated zones and control of both temperature and UV intensity at the fouling surface. Synthetic wastewater samples were tested with two levels of calcium, iron, phosphorus, and biochemical oxygen demand (as beef broth), and constant levels of magnesium and nitrogen to assess the effects of the four key variables. Average UV fluence before fouling exceeded 35 mJ/cm2, based on collimated beam tests. Foulant accumulation was monitored by UV intensity measurements and by mass and composition of foulant collected after an average of 56 hours of continuous operation. Tests showed that relative UV intensity dropped by as much as 100% when iron was present. Detailed results were assessed and yielded support for the following three UV-induced fouling mechanisms: (a) precipitation of ferric hydroxide [Fe(OH)3], (b) release of calcium from calcium-organics complexes followed by precipitation of iron-organics complexes, and (c) calcium carbonate precipitation. Other fouling mechanisms, such as sedimentation of preformed particles and sorption of calcium onto preformed colloids of Fe(OH)3, occurred outside the zone of UV radiation. Hence, these could be confused with concurrent UV-induced mechanisms in full-scale reactors. Iron and/or calcium undoubtedly created the most favorable conditions for fouling to occur; in the absence of both, fouling would be unlikely. The rates of fouling were enhanced when organics were also present; however, when phosphorus was present, fouling in the UV section was reduced. Indeed, UV may be viewed as inhibiting the fouling caused by phosphate complexes.

Nickel Stabilization Efficiency of Aluminate and Ferrite Spinels and Their Leaching Behavior

Stabilization efficiencies of spinel-based construction ceramics incorporating simulated nickel-laden waste sludge were evaluated and the leaching behavior of products investigated. To simulate the process of immobilization, nickel oxide was mixed alternatively with gamma-alumina, kaolinite, and hematite. These tailoring precursors are commonly used to prepare construction ceramics in the building industry. After sintering from 600 to 1480 degrees C at 3 h, the nickel aluminate spinel (NiAl204) and the nickel ferrite spinel (NiFe204) crystallized with the ferrite spinel formation commencing about 200-300 degrees C lower than for the aluminate spinel. All the precursors showed high nickel incorporation efficiencies when sintered at temperatures greater than 1250 degrees C. Prolonged leach tests (up to 26 days) of product phases were carried out using a pH 2.9 acetic acid solution, and the spinel products were invariably superior to nickel oxide for immobilization over longer leaching periods. The leaching behavior of NiAl2O4 was consistent with congruent dissolution without significant reprecipitation, but for NiFe2O4, ferric hydroxide precipitation was evident. The major leaching reaction of sintered kaolinite-based products was the dissolution of cristobalite rather than NiAl2O4. This study demonstrated the feasibility of transforming nickel-laden sludge into spinel phases with the use of readily available and inexpensive ceramic raw materials, and the successful reduction of metal mobility under acidic environments.

English

The Great Falls-Lewistown Coal Field (GFLCF) in central Montana contains over 400 abandoned underground coal mines, many of which are discharging acidic mine water with serious environmental consequences. Completely submerged areas of the mines are not strongly acidic, whereas water quality quickly deteriorates in portions that are only partially flooded. In general, the pH of the GLFLC mine waters can decrease or increase after discharging to the surface, depending on the initial ratio of acidity, mainly as dissolved ferrous iron, to alkalinity, mainly as bicarbonate ion. Although the chemistry of many of the adit discharges is nearly constant with time, large diurnal and seasonal changes in the quality of down-gradient waters have been observed. Decreases in concentrations of dissolved Fe and Zn during the day at one location are explained by an increase in the rate of Fe 2+ oxidation and precipitation of ferric hydroxide as the water warms. The precipitation of ferric hydroxide resulted in sorption of Zn 2+ , a reaction that was also thermodynamically favored by an increase in water temperature and pH during the afternoon. Historical efforts to passively treat acid mine drainage in the GFLCF using wetlands, limestone channels, and anoxic drains have been unsuccessful, due to the harsh climate, high metal concentrations, and acidity loads of the mine waters. Alternative mitigation concepts are currently being evaluated that focus on source control rather than treatment.

Chemical phosphorus removal to extremely low levels: experience of two plants in the Washington, DC area

Chemical phosphorus removal using metal (iron and aluminium) salts is frequently used to control effluent soluble phosphorus levels in wastewater treatment plants. In the Washington DC area effluent phosphorus requirements are extremely stringent to protect the Chesapeake Bay. Full-scale data from two plants in the area were analysed to establish phosphate behaviour in the presence of iron. Titration experiments and mathematical modelling were performed to determine the role of ferric phosphate and hydroxide precipitation and other mechanisms that may potentially be involved in phosphorus removal. Iron addition is described in the model using a chemical equilibrium approach extended with surface charges and adsorption. The model verifies key observations from full-scale data: (a) extremely low orthophosphate levels can be achieved over a wide range of pH values, (b) a mixture of ferric phosphate and ferric hydroxide precipitate is forming with the hydroxide acting as sorbent, (c) molar ratios of Fe/P (iron dosed to phosphate removed) vary widely (1.0-3.9) based on the technology used and residual phosphate levels. The model will be a useful tool for engineers to optimise preliminary, simultaneous and tertiary P removal, both for design and plant operation.

Chemical precipitation within pyritic waste rock

Pyritic waste rock piles generate acid rock drainage (ARD). If the chemical precipitation of metals could be induced within waste rock piles by the weathering products of phosphate and carbonate minerals, seepage metal loads–and treatment costs–could be reduced. Seventeen 70-L drums were filled with refractory ore, supposedly non-reactive waste rock, freshly broken low-pyrite waste rock, ‘old’ low-pyrite rock (weathered four years or longer), freshly broken high-pyrite rock or old high-pyrite rock. Control drums were set aside for each category. Phosphate mining wastes, containing both phosphate and calcium carbonates, were added to the remainder. The drums were weathered for 2.7 years, stored for 4.5 years indoors and re-exposed outdoors for 1.3 years. Discharge was collected and analysed. In the first phase of the experiment, acid generation in the waste rock drums containing natural phosphate rock (NPR) was reduced by 85%. In storage, acidity continued to be generated on the waste rock surfaces but was reduced even further in the second phase of the experiment when the rocks were returned outdoors. Although the mechanisms causing this are not fully understood, carbonate neutralization, along with ferric phosphate and iron hydroxide precipitation on the mineral surfaces, as well as microbial biofilm formation, are likely contributory factors. The results suggest that the standard practice of estimating acid generation potential of base-metal waste rock should be reconsidered.