Preparation of immobilized sulfate reducing bacteria (SRB) granules for effective bioremediation of acid mine drainage and bacterial community analysis

Heavy metal-resistant immobilized sulfate-reducing bacteria (SRB) granules were prepared to treat acid mine drainage (AMD) containing high concentrations of multiple heavy metal ions using an up-flow anaerobic packed-bed bioreactor. The bioreactor demonstrated satisfactory performance at influent pH 2.8 and high concentrations of metals (Fe 463 mg/L, Mn 79 mg/L, Cu 76 mg/L, Cd 58 mg/L and Zn 118 mg/L). The effluent pH ranged from 7.8 to 8.3 and the removal efficiencies of Fe, Cu, Zn and Cd were over 99.9% except for Mn (42.1–99.3%). The bacterial community in the bioreactor was diverse and included fermentative bacteria and SRB (Desulfovibrio desulfiricans) involved in sulfate reduction. The co-existing anaerobic fermentative bacteria (Clostridia bacterium, etc.) with the ability to use lactate as electron donor could explain the differences between actual lactate consumption and what would be expected based solely on sulfate reduction.     

Uranium recovery from strong acidic solutions by solvent extraction with Cyanex 923 and a modifier

Uranium stripping with strong acid solution is always highly desired due to its simple operation and less pollution. However, intensive acid neutralisation for uranium precipitation in the subsequent step limited its application. A new solvent extraction process has been developed to transfer uranium from strong to weak sulphuric acid solutions suitable for uranium precipitation without intensive neutralisation. An organic system consisting of 10% Cyanex 923 and 10% isodecanol as the modifier in ShellSol D70 was optimised for the process. It was found that uranium was extracted efficiently from 4 to 6 M H₂SO₄ solutions with the organic system, and it could be efficiently stripped with 0.2–0.5 M H₂SO₄ solutions. Both extraction and stripping kinetics of uranium were very fast, reaching the equilibrium within 0.5 min. Temperature between 30 and 60 °C has slight effect on uranium extraction and stripping. Four theoretical stages could effectively extract more than 98% uranium from a solution containing 17.5 g/L U and 6.0 M H₂SO₄ at an A/O ratio of 1:1.5, and it could generate a loaded organic solution containing about 12 g/L U. More than 99% U could be stripped from the loaded organic solution containing 14.6 g/L U with 0.5 M H₂SO₄ using five stages at an A/O ratio of 1:3. As a result, the loaded strip liquor containing more than 40 g/L U would be obtained which is suitable for uranium recovery by precipitation using hydrogen peroxide. A conceptual process has been proposed for uranium transfer from strong to weak sulphuric acid solutions for its recovery.     

The effect of [Fe]TOT on the dissolution of synthetic Pb-doped UO2 and Th-doped UO2

The dissolution of synthetic Pb-doped UO₂ and Th-doped UO₂ was systematically studied to determine the influence of leach parameters [Fe]_TOT and ORP under standard leach conditions of: T = 50 °C, [H₂SO₄] = 15 g/L (0.15 M), and UO₂ = 100 mg/L. Results demonstrated reduced uranium dissolution in both systems compared to pure UO₂. This effect was greatest for Th-doped UO₂. The decrease in uranium dissolution between the doped systems and pure UO₂ was attributed to the formation of precipitate layers at the surface of the solid, slowing down or blocking uranium release. In the case of Pb-doped UO₂, the formation of a Pb sulphate phase was directly detected but in the case of Th-doped UO₂, no layer was found. For the latter system it was postulated that passivation of the Th-doped UO₂ surface occurs due to the formation of oxidised Th-rich phases Th(OH)₄, ThO₂ and ThO₂·nH₂O at the surface of grains preventing uranium release. In tests varying the ORP, there was an approximately linear dependence of the dissolution rate on [Fe]_TOT for both systems however the rate orders indicated a step change between an ORP of 420 and 460 mV. The specific influence of Fe^II showed that both Pb–UO₂ and Th–UO₂ exhibited two distinct regions of dissolution rate dependency similar to that previously noted for pure UO₂.     

Immobilization and ferrous iron bio-oxidation studies of a Leptospirillum sp. mixed-cell culture

Immobilization of a mixed bacterial culture (predominantly Leptospirillum sp.) on mechanically modified graphite surfaces and different types of activated carbon fiber supports (felt and textile; both silicated and non-silicated) was studied experimentally. Maximum cell coverage on graphite samples occurred on a surface roughness of 2.08 μm (3.9 × 10⁴ cells/mm²). In non-silicated samples the activated carbon fiber support with the greatest surface area per gram (felt) lead to the greatest number of immobilized microorganisms over a 10 h period (2.2 × 10⁴ cells/mm²). The silication significantly increased surface area in the fibrous matrix voids and thereby increased the number of immobilized microorganisms on both modified activated carbon felt and fabric. The silicated felt exhibited the greatest number of immobilized Leptospirillum sp. cells of all activated carbon fiber cathodes studied (2.9 × 10⁴ cells/mm²). Physical property and elemental analyses of silicated samples indicated that other methods of augmenting bacterial immobilization should be explored as silication increased electrical resistance of the samples 100 fold. Leptospirillum sp. immobilized on unmodified activated carbon felt yielded the maximum experimentally observed rate of ferrous iron bio-oxidation (～900 mg/L h).     

A novel low pH sulfidogenic bioreactor using activated sludge as carbon source to treat acid mine drainage (AMD) and recovery metal sulfides: Pilot scale study

In this work a pilot scale sulfidogenic bioreactor was used to treat acid mine drainage (AMD) from Zijinshang copper mine. In this process, S²⁻ produced in the Up-flow Anaerobic Sludge Bed (UASB) reactor were recycled in the two precipitation tanks for copper and iron precipitation, activated sludge from local waste water treatment plant was used as the carbon source. The reactor were steady operated in acid condition (with no pH control) for 4 month, AMD with a copper concentration of 100–120 mg/L, iron concentration of 170–200 mg/L, sulfate concentration of 2000–2500 mg/L and pH of 2.34–2.56, were feeding into the reactor under a feed rate of 1 m³/days and HRT of 3 days, copper and iron removal were 60.95%, 97.83% respectively. The precipitant in the precipitation tank containing 15.7% Cu and 22.66% Fe, thus indicating a recovery possibility of copper by pyrometallurgy process. From these results we can conclude that an SRB process would be a viable method of treating Zijinshan AMD.     

Effect of thiocyanate on BIOX® organisms: Inhibition and adaptation

Stringent environmental legislation and the desire to become zero discharge have motivated mining operations to treat and recycle process water. Cyanidation tailings effluent contains elevated concentrations of cyanide and thiocyanate (SCN⁻), precluding recycling to the BIOX® process without prior treatment to reduce SCN⁻ to below 1 mg/l. The current study investigated the effect of SCN⁻ on individual microbial species. Iron oxidation by Leptospirillum ferriphilum was not affected by SCN⁻ concentrations below 0.5 mg/l, with concentration dependent inhibition observed between 0.75 and 1.25 mg/l and complete inhibition of iron oxidation above 1.25 mg/l. Sulphur oxidation by Acidithiobacillus caldus showed a similar trend, with limited inhibition below 1.25 mg/l and almost complete inhibition above 1.25 mg/l. Repeated sub-culturing at low concentrations induced adaptation, with adapted cultures currently growing at SCN⁻ concentrations of 7 mg/l. The phenomenon of inhibition at low concentration, with subsequent adaptation was repeated in stirred tank reactors, leaching a pyrite/arsenopyrite concentrate.     

Leaching behaviour of natural and heat-treated brannerite-containing uranium ores in sulphate solutions with iron(III)

Uranium leaching tests were conducted on two naturally occurring, highly metamict brannerite ores from the Crockers Well and Roxby Downs deposits, South Australia. The ores were leached over a range of temperatures and Fe^(III) and H₂SO₄ concentrations. As well, samples of the ores were calcined at 1200 °C in air to investigate the effect of thermally induced recrystallisation on uranium dissolution. For the unheated samples, a maximum of ∼80% U dissolution was obtained using an Fe^(III) concentration of 12 g/L, an acid concentration of 150 g/L H₂SO₄ and a temperature of 95 °C. The heat treated samples performed poorly under identical conditions, with maximum uranium dissolution of <10% recorded. High uranium dissolution from natural brannerite can be achieved providing; (i) acid strength, oxidant strength and temperatures are maintained at elevated levels (compared to those traditionally used for uraninite leaching), and, (ii) the brannerite has not undergone any significant recrystallisation (e.g. through metamorphism).     

Recovery of lanthanum(III) from aqueous solution using biosorbents of plant and animal origin: Batch and column studies

The present investigation attempts to recover lanthanum(III) from aqueous environments using biosorbents of animal (fish scales) and plant origin (neem saw dust). Optimization experiments were carried out using a log transformed 5-level Box–Behnken design. Maximum La(III) uptake was noted to be 200.0 mg/g in the case of fish scales (FS) and 160.2 mg/g in the case of neem sawdust (NS) under optimum conditions (pH: 6.0, biomass dosage: 0.3 g/L; 0.2 g/L, initial metal concentration: 300 mg/L; 250 mg/L, temperature: 50 °C, time: 4 h; 3 h). A low p-value of <0.0001 validated the significance of the model. The process was found to follow a homogeneous chemical mode in the case of FS whereas heterogeneity and physisorption was noted in the case of NS. This was further confirmed by scanning electron microscopy (SEM). High uptake values in the case of FS could be attributed to the involvement of both intraparticle and film diffusion. Thermodynamic studies showed that the process is endothermic and spontaneous in the case of both the adsorbents. FTIR analysis confirmed a major involvement of the participation of amide, amines, alkines, aliphatic compounds and alkyl groups during La(III) biosorption. Maximum adsorption efficiency and recovery of La(II) from ceramic industrial effluent using FS were noted as 88.5% and 84.6% which were obtained in column mode at a flow rate of 1 ml/min, bed height of 12 cm and 0% dilution. Regeneration studies suggested that the biosorbent FS could be consistently reused up to 6 cycles.     

Recovery of boron from borax sludge of boron industry

A two-step process for boron recovery from borax sludge is proposed in the present work. The borax sludge was leached with sulphuric acid solution. Then, for the removal of alkaline species from the leachate, calcium and magnesium were precipitated by adjusting the pH of leachate using 1.5 M NaOH and 1.5 M HCl solutions. The effects of pH, temperature, concentration and time on the precipitation process were investigated. It was determined that the calcium and magnesium concentrations in the leachate were reduced from 121 mg/L to 2.6 mg/L and from 145 mg/L to 4.0 mg/L, respectively, at a pH value of 12, a temperature of 70 °C, an initial boron concentration of 500 mg/L and a precipitation time of 3 h. Under these optimum conditions, it was observed that the boron concentration in the solution decreased very slightly. In this process, the alkaline species were successfully separated from the boron.Finally, borax pentahydrate (Na₂B₄O₇·5H₂O) was produced by the evaporation of the final solution obtained after precipitation process.     

Biosorption of arsenic(V) with Lessonia nigrescens

Conventional treatment methods for arsenic removal from copper smelting wastewaters create sludge that is difficult to handle. Biosorption of arsenic using algae as sorbent is an interesting alternative to the conventional methods.This work shows results from biosorption of arsenic(V) by Lessonia nigrescens at pH = 2.5, 4.5 and 6.5. The adsorption of arsenic could be explained satisfactorily both by the Freundlich and the Langmuir isotherms. Maximum adsorption capacities were estimated to 45.2 mg/g (pH = 2.5), 33.3 mg/g (pH = 4.5), and 28.2 mg/g (pH = 6.5) indicating better adsorption at the lower pH. These values are high in comparison with other arsenic adsorbents reported.The sorption kinetics of arsenic by L. nigrescens could be modelled well by Lagergren’s first-order rate equation. The kinetics were observed to be independent of pH during the first 120 min of adsorption with the Lagergren first-order rate constant of around 1.07 × 10⁻³ min⁻¹.     

Thorium and uranium extraction from rare earth elements in monazite sulfuric acid liquor through solvent extraction

This paper describes the process of extraction of thorium and uranium from the sulfuric liquor generated in the chemical monazite treatment through a solvent extraction technique. The influence of the extractant type and concentration, contact time between phases, type and concentration of the stripping solution and aqueous/organic volumetric ratio were investigated. The results indicated the possibility of extracting, simultaneously, thorium and uranium from a solvent containing a mixture of Primene JM-T and Alamine 336. The stripping was carried out with a hydrochloric acid solution. After selecting the best conditions for the process, a continuous experiment was carried out in a mixer-settler circuit using four stages in the extraction step, five stages of stripping and one stage of the solvent regeneration. A loaded stripping solution containing 29.3 g/L of ThO₂ and 1.27 g/L of U₃O₈ was obtained. The metals content in the raffinate was below 0.001 g/L, indicating a thorium extraction of over 99.9% and a uranium extraction of 99.4%. The rare earths content in the raffinate was 38 g/L of RE₂O₃.     

Oxidation of cyanide in effluents by Caro’s Acid

Caro’s Acid (peroxymonosulphuric acid: H₂SO₅) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 2³ factorial designs using three factors: initial [CN⁻]: 100–400 mg/L; H₂SO₅:CN⁻ molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H₂SO₄:H₂O₂ molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN⁻] and the H₂SO₅:CN⁻ molar ratio. The highest reaction rates were obtained for the following conditions: H₂SO₅:CN⁻ molar ratio = 4.5:1; pH = 9; and Caro’s Acid strength produced from the mixture of 3 mol of H₂SO₄ with 1 mol of H₂O₂. These conditions led to a reduction of [CN⁻] from an initial value of 400 mg/L to [CN⁻] = 1.0 mg/L after 10 min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN⁻]/dt = k [CN⁻]^1.8 [H₂SO₅]^1.1 [H⁺]^0.06, with k = 3.8 (±2.7) × 10⁻⁶ L/mg min, at 25 °C.     

Electrocoagulation as a remediation tool for wastewaters containing arsenic

This work shows results of electrocoagulation of solutions containing arsenic. The continuous flow treatment consisted of an electrocoagulation reactor with two parallel iron plates and a sedimentation basin.The results showed that the electrocoagulation process of a 100 mg/L As(V) solution could decrease the arsenic concentration to less than 2 mg/L in the effluent with a current density of 1.2 A/dm² and a residence time of around 9 min. Liquid flow was 3 L/h, and the DC current was reversed each 2 min.Increasing the current density from 0.8 to 1.2 A/dm², the Fe³⁺ and OH⁻ dosages increase too, and thereby favouring the As removal. On the other hand, it seems that increasing the current density beyond a maximum value, the electrocoagulation process would not improve further. This could probably be explained by passivation of the anode.     

Biosorption of Cr(VI) from aqueous solutions by nonliving green algae Cladophora albida

Biosorption of Cr(VI) from aqueous solutions by nonliving green algae Cladophora albida was investigated in batch experiments. The influence of pH, algal dosage, initial Cr(VI) concentration, temperature and co-existing anions on removal efficiencies of C. albida was studied. Cr(VI) removal process was influenced significantly by the variation of pH, and the optimum pH was chosen at a range of 1.0–3.0. The optimum algal dosage 2 g/L was used in the experiment. The removal rate of Cr(VI) was relatively rapid in the first 60 min, but then the rate decreased gradually. Removal mechanism was studied by analyzing Cr(VI) and total Cr in the solution. Biosorption and bioreduction were involved in the Cr(VI) removal. Biosorption of Cr(VI) was the first step, followed by Cr(VI) bioreduction and Cr(III) biosorption on the algal biomass. Actual industrial wastewater was used to evaluate the practicality of the biomass C. albida. From a practical viewpoint, the abundant and economic biomass C. albida could be used for removal of Cr(VI) from wastewater by the reduction of toxic Cr(VI) to less toxic Cr(III).     

Stripping conditions to prevent the accumulation of rare earth elements and iron on the organic phase in the solvent extraction circuit at Skorpion Zinc

The concentrations of the rare earth elements (particularly Y, Yb, Er and Sc) in the zinc-stripped organic phase streams in the solvent extraction process at Skorpion Zinc mine have increased gradually over the past four years. Iron is the only other impurity present in notable quantities in the organic phase after washing and scrubbing prior to zinc stripping. This project aimed to evaluate the effects that rare earth elements and iron in the organic phase have on the zinc solvent extraction process and to subsequently find appropriate stripping conditions for the removal of these elements from the zinc-stripped organic phase.Results obtained by performing laboratory scale batch tests indicated that the viscosity of the organic phase doubled and the phase disengagement time increased from 100 s to 700 s when the total rare earth elements and iron concentration in the organic phase was increased from 3100 mg/L to 6350 mg/L. The zinc loading capacity of the organic phase after two extractions furthermore decreased by a value between 1 g/L and 3 g/L, depending on the composition of the pregnant leach solution. The stripping of low concentrations of rare earth elements and iron from 40% di-2-ethylhexyl phosphoric acid (D2EHPA) diluted in kerosene was evaluated using two different stripping agents (H₂SO₄ and HCl) with concentrations between 1 M and 7 M, organic-to-aqueous ratios between 0.5 and 4, and temperatures between 30 °C and 50 °C. The highest stripping percentages were achieved at acid concentrations greater than 5 M, organic-to-aqueous ratios of less than 0.5 and high temperatures.     

Heavy metals removal from solution by palygorskite clay

The possible use of palygorskite clay, mined in the Dwaalboom area of the Northern Province of South Africa, as an adsorbent for the removal of metal ions such as lead, nickel, chromium and copper from aqueous solution, was investigated. In this work, adsorption of these metals onto palygorskite has been studied by using a batch method at room temperature. The results of adsorption were fitted to both the Langmuir and Freundlich models. Satisfactory agreement between experimental data and the model-predicted values was expressed by the correlation coefficient (R²). The Langmuir model represented the sorption process better than the Freundlich one, with correlation coefficient (R²) values ranging from 0.953 to 0.994. The adsorption capacity (Q₀) calculated from the Langmuir isotherm was 62.1 mg Pb(II) g⁻¹, 33.4 mg Ni(II) g⁻¹, 58.5 mg Cr(VI) g⁻¹ and 30.7 mg Cu(II) g⁻¹ at a pH of 7.0 at 25 ± 1 °C for a clay particle size of 125 μm. Kinetic investigations were performed to investigate the rate of adsorption of metal ions. The Lagergren’s first-order rate constants were calculated for different initial concentrations of metal ions. In batch mode adsorption studies, removal increased with an increase of contact time, adsorbent amount and solution pH. Adsorption of metals from the single-metal solutions was in the order: Pb > Cr > Ni > Cu. Data from this study proved that metal cations from aqueous solution can be adsorbed successfully in significant amounts by palygorskite. This opens up new possibilities and potential commercial uses in the palygorskite market.     

Bioleaching of anilite using pure and mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus

Anilite oxidation was evaluated with two acidophilic thiobacilli that are important in bioleaching processes. The experiments were carried out in shake flasks in the absence and presence of energy sources such as 2 g/L powdered sulphur and 10 g/L Fe²⁺ (as ferrous sulphate) at pH 2.0, 150 rpm, 35 °C. Tests showed that copper extraction in a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus was higher than in pure cultures with added sulphur in the presence of anilite. The effect of supplemental iron clearly improved Cu leaching by the A. ferrooxidans culture and the mixed culture. The oxidation of anilite by A. caldus was negligible and this bacterium seemed to have no ability to initiate anilite solubilization. On the other hand, an important potential of A. caldus to leaching anilite was indicated. It can decrease pH of the medium and supply a suitable bioleaching environment.     

Formation of jarosite during Fe2+ oxidation by Acidithiobacillus ferrooxidans

Jarosite precipitation is a very important phenomenon that is observed in many bacterial cultures. In many applications involving Acidithiobacillus ferrooxidans, like coal desulphurization and bioleaching, it is crucial to minimize jarosite formation in order to increase efficiency. The formation of jarosite during the oxidation of ferrous iron by free suspended cells of A. ferrooxidans was studied. The process was studied as a function of time, pH and temperature. The main parameter affecting the jarosite formation was pH. Several experiments yielded results showing oxidation rates as high as 0.181–0.194 g/L h, with low jarosite precipitation of 0.0125–0.0209 g at conditions of pH 1.6–1.7 with an operating temperature of 35 °C.     

Long-term efficiency and kinetic evaluation of ZVI barriers during clean-up of copper containing solutions

Sixteen continuous column experiments were carried out under dynamic flow conditions in order to study the efficiency of zero-valent iron (ZVI) permeable reactive barriers (PRBs) to remove copper from solutions. The effect of various operational parameters, such as pH of the feed solution (2.5 and 4.5), initial copper concentration (5 and 50 mg/L), pore water velocity (30.48 and 152.40 cm/day) and residence time on iron corrosion and degree of copper removal was evaluated. Breakthrough curves showed that higher initial copper concentration and pore water velocity accelerated iron corrosion and slowed down the rate of copper removal due to the formation of mineral precipitates. The maximum removal rates recorded for initial copper concentrations of 5 and 50 mg/L were 2.68 and 13.33 mg/g of ZVI/sand mixture, after 440 and 227 pore volumes, respectively. The reduction of copper follows a pseudo-first-order kinetics while the rate constants (k_obs) decrease over time from 0.60 to 0.10 h⁻¹, indicating thus progressive passivation of the ZVI/sand reactive bed. Mineralogical studies of the exhausted reactive mixture, using XRD and SEM/EDX, confirmed the deposition of copper on the ZVI surface and the formation of iron (oxy)hydroxides.