Natural ferrihydrite as an agent for reducing turbidity caused by suspended clays.

Biologically impaired waters are often caused by the turbidity associated with elevated suspended sediment concentrations. Turbidity can be reduced by the addition of positively charged compounds that coagulate negatively charged particles in suspension, causing them to flocculate. This research was conducted to determine the effectiveness of ferrihydrite, a poorly crystalline Fe oxide, as a flocculating agent for suspended clays similar to those found in high-turbidity waters of the Mississippi delta. Clay concentrations of 100 mg L(-1) from a Dubbs silt loam (fine silty, mixed, active, thermic Typic Hapludalfs), a Forestdale silty clay loam (fine, smectitic, thermic Typic Hapludalfs), and a Sharkey clay (very fine, smectitic, thermic Chromic Epiaquerts) were suspended in 0.0005 mol L(-1) CaCl(2) solutions at pH 5, 6, 7, or 8. Natural ferrihydrite with a zero point of charge at pH 5.8 was acquired from a drinking water treatment facility and mixed with the suspension at concentrations of 0, 10, 25, and 50 mg L(-1). After settling periods of 24 and 48 h, percent transmittance was measured at a wavelength of 420 nm using a 3-mL sample collected at a depth of 2 cm. The greatest reductions in turbidity after 24-h equilibration were recorded for the pH 5 suspensions of the Dubbs (31%) and Forestdale (37%) clays at a ferrihydrite concentration of 10 mg L(-1) and for the Sharkey clay at a ferrihydrite concentration of 25 mg L(-1) (relative to the 0 ferrihydrite treatment). Water clarity for all samples further increased after 48 h. These results indicate that the effectiveness of ferrihydrite, as a means of reducing turbidity associated with suspended clays, is greatest at pH values below its zero point of charge.

Influence of soil pH and application rate on the oxidation of calcium sulfite derived from flue gas desulfurization.

Calcium sulfite hemihydrate (CaSO(3).0.5H2O), a common byproduct of coal-fired utilities, is fairly insoluble and can decompose to release toxic SO2 under highly acidic soil conditions; however, it can also oxidize to form gypsum. The objective of this study was to examine the effects of application rate and soil pH on the oxidation of calcium sulfite under laboratory conditions. Oxidation rates measured by release of SO4-S to solution decreased with increasing application rate. Leachate SO4-S from soils amended with 1.0 to 3.0 g kg-1 CaSO3 increased over a 21 to 28 d period before reaching a plateau. At 4 g kg-1, maximum SO4-S release was delayed until Week 7. Oxidation and release of SO4-S from soil amended with 3.0 g kg-1 calcium sulfite increased markedly with decreasing soil pH. After only 3 d incubation, the concentrations of SO4-S in aqueous leachates were 77, 122, 170, 220, and 229 mg L-1 for initial soil pH values of 7.8, 6.5, 5.5, 5.1, and 4.0, respectively. At an initial soil pH value of 4.0, oxidation/dissolution did not increase much after 3 d. At higher pH values, oxidation was maximized after 21 d. These results suggest that autumn surface applications of calcium sulfite in no-till systems should permit ample time for oxidation/dissolution reactions to occur without introducing biocidal effects related to oxygen scavenging. Soil and annual crops can thus benefit from additions of soluble Ca and SO4 if calcium sulfite is applied in advance of spring planting.

Phosphate adsorption by ferrihydrite-amended soils.

New technology and approaches for reducing P in runoff from high sediment yield areas are essential due to implementation of increasingly rigorous water quality standards. The objectives of this research were to characterize ferrihydrite (Fe(5)HO(8).4H(2)O) in terms of its ability to adsorb P from soil solutions and relate its P adsorptive capacity to several soil properties that influence P mobility. A naturally occurring ferrihydrite, collected as an Fe oxide sludge by-product from a water treatment facility, was equilibrated with soil samples at equivalent rates of 0, 0.34, 3.36, 16.80, and 33.60 Mg ha(-1) for a 60-d period. Individual 2-g subsamples of each soil were then equilibrated with 0, 5, 10, 20, and 40 mg kg(-1) P in 20 mL of 0.01 M CaCl(2) on a reciprocating shaker for 24 h. After 24 h, P in solution was measured by colorimetric methods, and designated as final P concentrations. The data indicated that the unamended soils with a pH of <6.0 adsorbed, in some cases, 50 times more P than soils with a pH of >7.0. The final P concentrations, averaged for all initial P concentrations and ferrihydrite rates, ranged from 0.09 to 4.63 mg kg(-1), and were most highly correlated with pH (r = 0.844; P < or = 0.01), oxalate-extractable Fe (r = -0.699; P < or = 0.10), and dithionite-extractable Fe (r = -0.639; P < or = 0.10) contents of the unamended soils. In terms of individual soils, correlation coefficients (r) for final P concentrations versus ferrihydrite amendment rates indicated a statistically significant (P < or = 0.001) negative relationship at all initial P concentrations for most A horizons. The r values for the high Fe oxide content B horizon soils did not show a statistically significant response to ferrihydrite additions. The results indicate that P adsorption, in soils amended with ferrihydrite, will be greatest under acid pH conditions below the ferrihydrite zero point of charge (pH 5.77), and low incipient Fe oxide contents.

Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues.

Ochreous precipitates containing 5.5-69.8 g/kg As were isolated from mine drainage in Finland and were composed of schwertmannite, ferrihydrite, and goethite. Schwertmannite formation was favored at pH 3-4, but its structure was degraded at high As levels. A series of coprecipitates were therefore prepared from mixed iron arsenate/sulfate solutions to define the limits of schwertmannite stability. Schwertmannite was replaced as the dominant phase by a poorly crystalline ironIII hydroxy arsenate (FeOHAs) when As/Fe mole ratios exceeded 0.15. The FeOHAs gave an X-ray diffraction pattern similar to that obtained from an "amorphous" ironIII arsenate (As/Fe = 1.0) with broad peaks at 0.30 and 0.16 nm. The FeOHAs possessed a magnetic hyperfinefield of 41.9T at 4.2 K that was intermediate to those of schwertmannite (46.1 T) and the ironIII arsenate (24.8 T). These data indicate a strong disruptive effect of arsenate on magnetic ordering and structure development in schwertmannite. Equilibration of 0.01 M arsenate solutions with freshly prepared schwertmannite and 2-line ferrihydrite at pH 3.0 for up to 60 d gave sorbed As contents of 175 and 210 g/kg, respectively. Arsenate sorption degraded the host schwertmannite and ferrihydrite, perhaps due to the formation of an FeOHAs surface phase.

Oxidative dissolution of arsenopyrite by mesophilic and moderately thermophilic acidophiles.

The purpose of this work was to determine solution- and solid-phase changes associated with the oxidative leaching of arsenopyrite (FeAsS) by Thiobacillus ferrooxidans and a moderately thermoacidophilic mixed culture. Jarosite [KFe(3)(SO(4))(2)(OH)(6)], elemental sulfur (S), and amorphous ferric arsenate were detected by X-ray diffraction as solid-phase products. The oxidation was not a strongly acid-producing reaction and was accompanied by a relatively low redox level. The X-ray diffraction lines of jarosite increased considerably when ferrous sulfate was used as an additional substrate for T. ferroxidans. A moderately thermoacidophilic mixed culture oxidized arsenopyrite faster at 45 degrees C than did T. ferroxidans at 22 degrees C, and the oxidation was accompanied by a nearly stoichiometric release of Fe and As. The redox potential was initially low but subsequently increased during arsenopyrite oxidation by the thermoacidophiles. Jarosite, S, and amorphous ferric arsenate were also formed under these conditions.

Mineral Products of Pyrrhotite Oxidation by Thiobacillus ferrooxidans.

The biological leaching of pyrrhotite (Fe(1-x)S) by Thiobacillus ferrooxidans was studied to characterize the oxidation process and to identify the mineral weathering products. The process was biphasic in that an initial phase of acid consumption and decrease in redox potential was followed by an acid-producing phase and an increase in redox potential. Elemental S was one of the first products of pyrrhotite degradation detected by X-ray diffraction. Pyrrhotite oxidation also yielded K-jarosite [KFe(3)(SO(4))(2)(OH)(6)], goethite (alpha-FeOOH), and schwertmannite [Fe(8)O(8)(OH)(6)SO(4)] as solid-phase products. Pyrrhotite was mostly depleted after 14 days, whereas impurities in the form of pyrite (cubic FeS(2)) and marcasite (orthorhombic FeS(2)) accumulated in the leach residue.

Characterization of Jarosite Formed upon Bacterial Oxidation of Ferrous Sulfate in a Packed-Bed Reactor.

A packed-bed bioreactor with activated-carbon particles as a carrier matrix material inoculated with Thiobacillus ferrooxidans was operated at a pH of 1.35 to 1.5 to convert ferrous sulfate to ferric sulfate. Despite the low operating pH, trace amounts of precipitates were produced in both the reactor and the oxidized effluent. X-ray diffraction and chemical analyses indicated that the precipitates were well-ordered potassium jarosite. The chemical analyses also revealed a relative deficiency of Fe and an excess of S in the reactor sample compared with the theoretical composition of potassium jarosite.