Jarno Mäkinen

Microbial Community Structure and Functions in Ethanol-Fed Sulfate Removal Bioreactors for Treatment of Mine Water

Sulfate-rich mine water must be treated before it is released into natural water bodies. We tested ethanol as substrate in bioreactors designed for biological sulfate removal from mine water containing up to 9 g L−1 sulfate, using granular sludge from an industrial waste water treatment plant as inoculum. The pH, redox potential, and sulfate and sulfide concentrations were measured twice a week over a maximum of 171 days. The microbial communities in the bioreactors were characterized by qPCR and high throughput amplicon sequencing. The pH in the bioreactors fluctuated between 5.0 and 7.7 with the highest amount of up to 50% sulfate removed measured around pH 6. Dissimilatory sulfate reducing bacteria (SRB) constituted only between 1% and 15% of the bacterial communities. Predicted bacterial metagenomes indicated a high prevalence of assimilatory sulfate reduction proceeding to formation of l-cystein and acetate, assimilatory and dissimilatory nitrate reduction, denitrification, and oxidation of ethanol to acetaldehyde with further conversion to ethanolamine, but not to acetate. Despite efforts to maintain optimal conditions for biological sulfate reduction in the bioreactors, only a small part of the microorganisms were SRB. The microbial communities were highly diverse, containing bacteria, archaea, and fungi, all of which affected the overall microbial processes in the bioreactors. While it is important to monitor specific physicochemical parameters in bioreactors, molecular assessment of the microbial communities may serve as a tool to identify biological factors affecting bioreactor functions and to optimize physicochemical attributes for ideal bioreactor performance.

Rare Earth Elements Recovery and Sulphate Removal from Phosphogypsum Waste Waters with Sulphate Reducing Bacteria

Phosphogypsum waste, originating from phosphoric acid production from apatite ores, is well known for its high production rate and possible release of sulphate-rich seepage waters. In addition to negative environmental impacts, phosphogypsum waste heaps are also remarkable secondary sources of Rare Earth Elements (REE); in the phosphoric acid production process a majority of REE, occurring in apatite, are precipitated to the phosphogypsum waste. Therefore, a method treating both sulphate-rich waters and recovering REE from phosphogypsum heaps and seepage waters would offer both economic and environmental benefits. In this ongoing study, seepage waters from a phosphogypsum heap are treated with Sulphate Reducing Bacteria (SRB) and ethanol as a substrate. Sulphate is first reduced to hydrogen sulphide, which then precipitates REE as sulphides. The main challenge, low concentration of REE in seepage waters (e.g. 2.87 μg/l La, 5.13 μg/l Ce, 0.67 μg/l Y and 3.32 μg/l Nd), is overcome by utilizing continuous mode, semi-passive and cost effective column apparatus, requiring no agitation and performing both sulphate reduction and REE recovery in a single reactor. The SRB method results in a sulphate reduction rate of 40-80 % (from app. 1400 mg/l to 276-844 mg/l sulphate in the effluent) and efficient REE recovery from seepage water. The concentrate obtained from the column consists of a mixture of anaerobic sludge and precipitated REE, with respective REE concentrations of 202 mg/kg La, 477 mg/kg Ce, 49 mg/kg Y and 295 mg/kg Nd.

Comparison of Reductive and Oxidative Bioleaching of Jarosite for Valuable Metals Recovery

Jarosite is a typical stream of zinc refineries, with high production rates and possible release of metal-contaminated seepage waters during long-term storage in respective disposal sites. Jarosite contains remarkable concentrations of valuable metals, like several weight percentages of zinc and lead, in addition to lower concentrations of copper, silver, germanium, gallium and indium. In this study, jarosite was treated with reductive and oxidative bioleaching for valuable metals recovery. The reductive bioleaching was seen to enhance iron liberation, by transforming the dissolved Fe(III) to Fe(II), while in the oxidative bioleaching iron liberation was lower. Zinc, copper, indium, gallium and germanium dissolution rates were rather identical with both methods. In reactor experiments, the zinc and copper yields were higher than in flask experiments resulting at best in the leaching yield of 35% and 38% for zinc and copper, respectively. Indium and gallium yields were between 5-8%, but approximately 40% of germanium was leached.

Sulphate removal from mine water with chemical, biological and membrane technologies

Chemical, physical and biological technologies for removal of sulphate from mine tailings pond water (8 g SO42-/L) were investigated. Sulphate concentrations of approximately 1,400, 700, 350 and 20 mg/L were obtained using gypsum precipitation, and ettringite precipitation, biological sulphate reduction or reverse osmosis (RO) after gypsum pre-treatment, respectively. Gypsum precipitation can be widely utilized as a pre-treatment method, as was shown in this study. Clearly the lowest sulphate concentrations were obtained using RO. However, RO cannot be the only water purification technology, because the concentrate needs to be treated. There would be advantages using biological sulphate reduction, when elemental sulphur could be produced as a sellable end product. Reagent and energy costs for 200 m3/h tailings pond water feed based on laboratory studies and process modelling were 1.1, 3.1, 1.2 and 2.7 MEur/year for gypsum precipitation, ettringite precipitation, RO and biological treatment after gypsum precipitation, respectively. The most appropriate technology or combination of technologies should be selected for every industrial site case by case.

Pilot-Scale Bioleaching of Metals from Pyritic Ashes

Solid waste from sulfuric acid production may contain relatively high levels of metals such as Fe, Zn, Co, Cu and As that are harmful if inappropriately disposed of in the environment, but may be a valuable resource if metals can be recovered. The objective of this research was to investigate the pilot-scale acid bioleaching of metals from pyritic ashes, originating from the roasting of pyrite ores for sulfuric acid production and consisting mainly of hematite. Bioleaching was carried out at 25 °C in pilot-scale continuously stirred tank reactors (CSTR), with 50 L working volume in mineral salts medium supplemented with trace elements, 1 % (w/v) elemental sulfur and with pyritic ash pulp densities 10 % and 20 %. The reactors were inoculated with a mixed culture of iron- and sulfur-oxidising acidophiles containing Acidithiobacillus (At.) ferrooxidans, At. thiooxidans/albertensis, At. caldus, Leptospirillum ferrooxidans, Sulfobacillus (Sb.) thermosulfidooxidans, Sb. thermotolerans and some members of Alicyclobacillus genus. Metal leaching yields from pyritic ashes in the CSTR after 32 days were 54.6-56.7 % Cu, 41.7-43.2 % Zn, 1.7-1.8 % Co, 3.0-5.4 % As and 0.3-0.5 % Fe. Solution pH decreased during the experiment from 2.9 to 1.9-2.2. Elemental analysis using X-ray fluorescence showed that the contents of metals, except for As, in the leach residue were below the higher guideline values given in the Government decree on the assessment of the soil contamination and remediation needs by the Ministry of the Environment, Finland. Bioleaching facilitated the extraction of metals from pyritic ashes and the mitigation of environmental risks related to the residue disposal for other metals except for As.

Water Conscious Mining (WASCIOUS)

The main objective of the NordMin WASCIOUS project was to develop a technology concept for water conscious mining, where innovative water and tailings treatment technologies provide good-quality wa ...

Optimisation of Acid Bioleaching of Metals from Pyrithic Ashes

Solid waste from sulphuric acid production contains high concentrations of metals that are harmful if released to the environment. The purpose of this study was to evaluate the acid bioleaching of metals from a sample of pyritic ashes, consisting mainly of hematite. Bioleaching was tested in shake flasks and continuously stirred tank reactors (CSTR) inoculated with iron and sulphur oxidising acidophiles. Solubilisation of metals was mainly achieved through acid attack due to the formation of sulphuric acid by sulphur oxidising bacteria.