Microbial succession and denitrifying woodchip bioreactor performance at low water temperatures.

Mining activities are increasingly recognized for contributing to nitrogen (N) pollution and possibly also to emissions of the greenhouse gas nitrous oxide (N2O) due to undetonated, N-based explosives. A woodchip denitrifying bioreactor, installed to treat nitrate-rich leachate from waste rock dumps in northern Sweden, was monitored for two years to determine the spatial and temporal distribution of microbial communities, including the genetic potential for different N transformation processes, in pore water and woodchips and how this related to reactor N removal capacity. About 80 and 65 % of the nitrate was removed during the first and second operational year, respectively. There was a succession in the microbial community over time and in space along the reactor length in both pore water and woodchips, which was reflected in reactor performance. Nitrate ammonification likely had minimal impact on N removal efficiency due to the low production of ammonium and low abundance of the key gene nrfA in ammonifiers. Nitrite and N2O were formed in the bioreactor and released in the effluent water, although direct N2O emissions from the surface was low. That these unwanted reactive N species were produced at different times and locations in the reactor indicate that the denitrification pathway was temporally as well as spatially separated along the reactor length. We conclude that the succession of microbial communities in woodchip denitrifying bioreactors treating mining water develops slowly at low temperature, which impacts reactor performance.

How effective is the retention of microplastics in horizontal flow sand filters treating stormwater?

Microplastics accumulate in stormwater and can ultimately enter freshwater recipients, and pose a serious risk to aquatic life. This study investigated the effectiveness of lab-scale horizontal flow sand filters of differing lengths (25, 50 and 100 cm) in retaining four types of thermoplastic microplastics commonly occurring in stormwater runoff (polyamide, polyethylene, polypropylene, and polyethylene terephthalate). Despite the differences in particle shape, size and density, the study revealed that more than 98% of the spiked microplastics were retained in all filters, with a slightly increased removal with increased filter length. At a flow rate of 1 mL/min and after one week of operation, 62-84% of the added microplastics agglomerated in the first 2 cm of the filters. The agglomerated microplastics included 96% of high-density fibers. Larger-sized particles were retained in the sand media, while microplastics smaller than 50 µm were more often detected in the effluent. Microplastics were quantified and identified using imaging based micro Fourier Transform Infrared Spectroscopy. The efficient retention of microplastics in low-flow horizontal sand filters, demonstrated by the results, highlights their potential importance for stormwater management. This retention is facilitated by various factors, including microplastic agglomeration, particle sedimentation of heavy fibers and favorable particle-to-media size ratios.

Microbial controls on net production of nitrous oxide in a denitrifying woodchip bioreactor.

Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO3 - ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO2 - ), nitrous oxide (N2 O), or ammonium (NH4 + ). To identify the factors controlling the relative accumulation of NO2 - , N2 O, and/or NH4 + in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO3 - from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N2 O reduction, and dissimilatory NO3 - reduction to NH4 + were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N2 O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N2 O. Temperature and the total organic carbon/NO3 - ratio were strongly correlated with NH4 + concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO3 - transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6 d demonstrate the temporal development in the microbial community and indicate an increased potential for N2 O emissions with time from the denitrifying woodchip bioreactor.

Denitrification in a low-temperature bioreactor system at two different hydraulic residence times: laboratory column studies.

Nitrate removal rates in a mixture of pine woodchips and sewage sludge were determined in laboratory column studies at 5°C, 12°C, and 22°C, and at two different hydraulic residence times (HRTs; 58.2-64.0 hours and 18.7-20.6 hours). Baffles installed in the flow path were tested as a measure to reduce preferential flow behavior, and to increase the nitrate removal in the columns. The nitrate removal in the columns was simulated at 5°C and 12°C using a combined Arrhenius-Monod equation controlling the removal rate, and a first-order exchange model for incorporation of stagnant zones. Denitrification in the mixture of pine woodchips and sewage sludge reduced nitrate concentrations of 30 mg N L-1 at 5°C to below detection limits at a HRT of 58.2-64.0 hours. At a HRT of 18.7-20.6 hours, nitrate removal was incomplete. The Arrhenius frequency factor and activation energy retrieved from the low HRT data supported a biochemically controlled reaction rate; the same parameters, however, could not be used to simulate the nitrate removal at high HRT. The results show an inversely proportional relationship between the advection velocity and the nitrate removal rate, suggesting that bioreactor performance could be enhanced by promoting low advection velocities.

Nitrogen removal and spatial distribution of denitrifier and anammox communities in a bioreactor for mine drainage treatment.

Mine drainage water may contain high levels of nitrate (NO3(-)) due to undetonated nitrogen-based explosives. The removal of NO3(-) and nitrite (NO2(-)) in cold climates through the microbial process of denitrification was evaluated using a pilot-scale fixed-bed bioreactor (27 m(3)). Surface water was diverted into the above-ground bioreactor filled with sawdust, crushed rock, and sewage sludge. At hydraulic residence times of ca.15 h and with the addition of acetate, NO3(-) and NO2(-) were removed to below detection levels at a NO3(-) removal rate of 5-10 g N m(-3) (bioreactor material) d(-1). The functional groups contributing to nitrogen removal in the bioreactor were studied by quantifying nirS and nirK present in denitrifying bacteria, nosZI and nosZII genes from the nitrous oxide - reducing community, and a taxa-specific part of the16S rRNA gene for the anammox community. The abundances of nirS and nirK were almost 2 orders of magnitude greater than the anammox specific 16S rRNA gene, indicating that denitrification was the main process involved in nitrogen removal. The spatial distribution of the quantified genes was heterogeneous in the bioreactor, with trends observed in gene abundance as a function of depth, distance from the bioreactor inlet, and along specific flowpaths. There was a significant relationship between the abundance of nirS, nirK, and nosZI genes and depth in the bioreactor, such that the abundance of organisms containing these genes may be controlled by oxygen diffusion and substrate supply in the partially or completely water-saturated material. Among the investigated microbial functional groups, nirS and anammox bacterial 16S rRNA genes exhibited a systematic trend of decreasing and increasing abundance, respectively, with distance from the inlet, which suggested that the functional groups respond differently to changing environmental conditions. The greater abundance of nirK along central flowpaths may indicate that the bioreactor design favored preferential flow along these flowpaths, away from the sides of the bioreactor. An improved bioreactor design should consider the role of preferential flowpaths and the heterogeneous distribution of the genetic potential for denitrification, nitrous oxide reduction and anammox on bioreactor function.

Hydration of arsenic oxyacid species.

The bond distances in hydrated arsenic oxyacid species in aqueous solution have been studied by EXAFS spectroscopy and large angle X-ray scattering, LAXS. These results have been compared to structures in the solid state, as found in an extensive survey of available crystal structures. Protonated oxygen atoms can be distinguished with a longer As-O distance for both arsenic(V) and arsenic(III) species in the crystalline state. However, the average As-O distance for the H(n)AsO(4)((3-n)-) species (0 ≤n≤ 3) remains the same. These average values are slightly shorter, ca. 0.02 Å, than in aqueous solution due to the hydration as determined by EXAFS and LAXS. The K absorption edges for arsenic(V) and arsenic(III) species are separated by 4.0 eV, and the shape of the absorption edges differs as well. Small but significant differences in the absorption edge features are seen between the neutral acids and the charged oxyacid species. The most important arsenic species from an environmental point of view is arsenous acid, As(OH)(3). In addition to arsenous acid, we have used orthotelluric acid, Te(OH)(6), for comparison with arsenous acid and for detailed studies of the hydration of covalently bound hydroxo groups. Arsenous acid cannot be studied with the same accuracy as orthotelluric acid, due to a relatively low solubility of As(2)O(3)(s) in neutral to acidic aqueous solution. The results from the DDIR studies support the assignment of As(OH)(3) as a weak structure maker analogous to Te(OH)(6), both being neutral weak oxyacids.

Implications of non-equilibrium transport in heterogeneous reactive barrier systems: evidence from laboratory denitrification experiments.

Organic substrates in reactive barrier systems are often heterogeneous material mixtures with relatively large contrasts in hydraulic conductivity and porosity over short distances. These short-range variations in material properties imply that preferential flow paths and diffusion between regions of higher and lower hydraulic conductivity may be important for treatment efficiency. This paper presents the results of a laboratory column experiment where denitrification is investigated using a heterogeneous reactive substrate (sawdust mixed with sewage sludge). Displacement experiments with a non-reactive solute at three different flow rates are used to estimate transport parameters using a dual porosity non-equilibrium model. Parameter estimation from breakthrough curves produced relatively consistent values for the fraction of the porosity consisting of mobile water (ß) and the mass transfer coefficient (α), with average values of 0.27 and 0.42 d(-1), respectively. The column system removes >95% of the influent nitrate at low and medium flow, but only 50-75% of the influent nitrate at high flow, suggesting that denitrification kinetics and diffusive mass transfer rates are limiting the degree of treatment at lower hydraulic residence times. Reactive barrier systems containing dual porosity media must therefore consider mass transfer times in their design; this is often most easily accommodated by adjusting flowpath length.

Global emission and production of mercury during the pyrometallurgical extraction of nonferrous sulfide ores.

The contribution of the milling, smelting, and refining of sulfide ores to Hg emissions and to Hg byproduction is not adequately quantified in a global context. In this study, we estimate Hg emissions from the pyrometallurgical treatment of Cu, Pb, and Zn sulfide ores. We base our calculations on quantities processed and Hg content in Cu, Pb, and Zn concentrates, derived from unique global databases on smelter feed and production. In 2005, about 275 tons of Hg were emitted globally to the atmosphere from Cu, Pb, and Zn smelters. Nearly one-half was emitted from Zn smelters and the other half equally divided between Cu and Pb smelters. Most Hg was emitted in China, followed by the Russian Federation, India, and South Korea. Global emission factors were 5.81, 15.71, and 12.09 g of Hg ton(-1) of metal for Cu, Pb, and Zn smelters, respectively. Calculations indicate that Hg abatementtechnologies applied to flue gases may have recovered 8.8 tons and 228 tons Hg from Pb and Zn smelters, respectively, most of which was probably sold as a byproduct. In conclusion, Hg emitted from processing copper, lead, and zinc ores has been largely underestimated in Hg emission inventories. Reducing these emissions may be one of the most economical measures to reduce global Hg emissions.

Iron isotope fractionation by biogeochemical processes in mine tailings.

Iron isotope ratios were determined for the pore water, the 1 M HCl/1 M hydroxylamine hydrochloride (HAH)-extractable solid phase, and the total extractable solid phase from sulfidic mine tailings in Impoundment 1, Kristineberg mine, northern Sweden. Within the tailings, pyrite oxidation occurs in a distinct Fe-depleted oxidation zone, and the greatest number of Fe(II)-oxidizing bacteria in the profile occur close to the boundary between oxidized and unoxidized tailings. Above the oxidation front in the oxidized tailings, a large iron isotope fractionation (-1.3 to -2.4% per hundred) is measured between the pore water and the HAH-extractable solid phase. This isotope fractionation is explained by aqueous Fe(II)-Fe(III) equilibrium, microbial Fe(II) oxidation, and Fe(III) oxyhydroxide precipitation. The data suggests that pyrite in the tailings is enriched in 56Fe relative to Fe-rich silicates in the same material, such that pyrite oxidation results in a decrease in the mean delta56Fe value for the bulk tailings in the oxidized zone: a change in isotope composition that is not attributable to isotope fractionation. Iron isotope analyses yield valuable information on iron cycling in mine wastes, and they have the potential for becoming a tool for the prediction and control of acid mine drainage.

Analysis of bacterial diversity in acidic pond water and compost after treatment of artificial acid mine drainage for metal removal.

The microbial population of a sludge amended leaf compost material utilized for treatment of artificial acid mine drainage was studied by culture-independent molecular methods. Iron-rich and sulfurous wastewater (artificial acid mine drainage) was circulated through a column bioreactor for 16 months. After 12 months the column was inoculated with a mixed culture from an acidic pond receiving acid mine drainage from a tailings impoundment at a decommissioned site in Kristineberg, North Sweden. Hydrogen sulfide odor and the formation of black precipitates indicated that sulfate-reduction occurred in the column. 16S rDNA gene analysis by denaturing gradient gel electrophoresis, cloning, and sequencing as well as fluorescent in situ hybridization confirmed the presence of microorganisms closely related to sulfate-reducing bacteria and microorganisms from the genera Pseudoxanthmonas, Dechlorosoma, Desulfovibrio, Agrobacterium, Methylocapsa, Rhodococcus, Sulfobacillus, and some unidentified bacteria. Sulfate-reducing bacteria were found in the column bioreactor 2 weeks after inoculation, but not thereafter. This suggests they were in low abundance, even though sulfate remediation rates were significant. Instead, the population contained species similar to those previously found to utilize humic substances released from the compost material.

Quantification of abiotic reaction rates in mine tailings: evaluation of treatment methods for eliminating iron- and sulfur-oxidizing bacteria.

Effective treatment techniques for eliminating iron-oxidizing (IOB) and sulfur-oxidizing bacteria (SOB) are required for the comparison of abiotic and microbial sulfide oxidation rates and mechanisms in mine tailings. This study evaluates the effect of autoclaving, repeated heating, ethanol treatment, antibiotic treatment, gamma-radiation, and washing with deionized water on tailings characteristics and concentrations of IOB and SOB. Most probable number enumeration indicates that IOB and SOB were present at very low concentrations or below detection limits following treatment with all methods except rinsing and antibiotics treatment, where higher concentrations of IOB and SOB were present. The physical, chemical, and mineralogical characterization of the tailings indicated no changes in bulk mineralogy or bulk chemical composition as a result of treatment. However, an increase in oxidized sulfur species at the tailings surface, as determined by X-ray photoelectron spectroscopy, was observed for the heating, autoclaving, and antibiotics treatments. Batch weathering experiments, used to evaluate the effect of treatment on element release rates, indicated that the final element release rates (after >30 d) were similar between treated and untreated control samples. On the basis of the results of this study, experiments over relatively long periods (>30 d) are to be recommended forthe establishment of microbial and abiotic weathering rates in mill tailings samples. For the determination of abiotic reaction rates, treatment by gamma-radiation is suggested to be the most appropriate method for sulfide-rich tailings.