By Ron Tenny, Eaglebrook, Inc., and Jack Adams, Applied Biosciences Corp.
The use of ferric salts has a proven record of accomplishment for arsenic removal in the treatment of effluent from mines. And now, biological processes are becoming an accepted alternative for low-level arsenic removal. By combining and optimizing these treatment processes, the future holds much promise for reduced costs, lower sludge volumes and decreased effluent arsenic levels. This type of combined treatment has the potential to decrease the amount of coagulant used, treat a broader spectrum of sites and waters and ultimately lower the overall cost of treatment. Remediation of abandoned and existing mine sites, and the treatment of waters surrounding these sites with combined chemical-biological processes, holds considerable potential.
The source of the problem
In Canada, for each ton of average grade copper extracted, there will be 99 tons of waste material comprised of soil, waste rock, and finely ground tailings. Waste rock, from mining operations that might contain acid-generating sulphides, heavy metals and other contaminants, is usually stored above ground in large free-draining piles. This waste rock and the exposed bedrock walls, from which it is excavated, are the source of most of the metals pollution caused by mining in British Columbia.
Mine tailings often contain the same toxic heavy metals and acid-forming minerals that waste rock does, and can contain chemical agents used to process the ores, such as cyanide or sulphuric acid. Tailings are usually stored above ground in containment ponds. In underground operations, they may be pumped as backfill into the excavated space from which they are mined. If improperly secured, contaminants in mine waste can leach out into the surface and groundwater, causing serious pollution that can last for many generations.
The tailing pond of a mine site provides storage capacity for all process effluents and site run-off. In many cases, the tailing pond supernatant is recycled back to the plant for reuse as process water to minimize fresh water consumption. Over time, untreated effluent in ponds generally acidifies and leaches metals.
Chemical treatment with ferric salts for arsenic removal
Trace metals are effectively removed from mining effluent by the addition of ferric salts. Through precipitation, arsenic is removed as either calcium or ferric arsenate. Arsenites can also be precipitated, but they are generally more soluble and less stable then arsenates. Arsenite-containing effluent is generally oxidized prior to precipitation to ensure that the arsenate predominates. Wastewater from the processing of arsenic bearing ores may contain varying amounts of arsenic (III) and (V) oxyanions, arsenites and arsenate. The presence of such metal ions as copper, lead, nickel, and zinc limit the solubility of arsenic because of the formation of sparingly soluble metal arsenates.
The stability and solubility of these arsenates depends on the ratio of iron to arsenic. The larger the ratio, the more insoluble and stable the precipitate. Thus, where ferric arsenate is relatively soluble, the basic arsenates with an iron-to-arsenic molar ratio of eight or more are orders of magnitude less soluble in the pH range of approximately 2 to 8. Dissolved arsenic concentrations of 0.5 mg/L or less can be obtained by precipitation with ferric iron.
The precipitation of insoluble ferric arsenates is very likely accompanied by co-precipitation of other metals such as selenium; that involves interactions between the various metals species and the ferric hydroxide precipitate. This makes ferric a very effective scavenger for the removal of trace contaminants. Thus, arsenic and many other elements such as antimony and molybdenum can be reduced to levels of less than 0.5 mg/L by contact with ferric hydroxide. The process normally involves the addition of a soluble ferric salt to the wastewater followed by the addition of sufficient base to induce the formation of insoluble ferric hydroxide. In many situations, the wastewater contains adequate iron, thus only the addition of a base is required to induce the precipitation of ferric hydroxide.
Biological process for removing heavy metals
Biological materials are effective at binding or absorbing metals, including arsenic, present in various solutions. These biological materials range from shrubs and grasses to mosses, fungi, algae and bacteria. Additionally, both chemical reactions and microbial metabolism are responsible for formation of the mineral and petroleum deposits currently being mined. Bacteria such as sulphate-reducing bacteria (SRB) reduce sulphate to sulphide as a part of their metabolic cycle. The reaction is as follows:
8Fe2+ + SO42- + 20 H2O · 8Fe(OH)3 + 14H+ + H2S
Arsenic is then precipitated by the sulphide as shown below.
2H3AsO4 + 5HS- · As2S5 + 3H2O + 5OH
Arsenic-containing wastewaters are amended with low levels of an economical nutrient mix containing FeSO4 * 7H2O and treated using retention times of 6 to 18 hours. Wastewater pH can be within 4.5 to 9.0 for treatment, but has an optimum pH range from 6.5 to 8.0. For in situ(treatment in tailing ponds) the stability of the arsenic sulphide complexes must be considered, and increases as the pH decreases.
Arsenic sulphide precipitation using hydrogen sulphide gas is not as effective and requires pHs in the range of 2.5 to 3.0. Hydrogen sulphide production and subsequent arsenic reduction/precipitation using SRB and other arsenic reducing bacteria can be accomplished usually without pH adjustment.
Other bacteria like Pseudomona and Alcalagines sp.are capable of direct arsenic reduction using arsenate and arsenite as terminal electron acceptors instead of oxygen. These bacteria are also capable of performing arsenic-reduction/precipitating over a broad pH range.
Biological arsenic removal utilizes simple reactors and produces hundreds to thousands times less sludge than conventional arsenic precipitation processes, such as ferric arsenic precipitation. Sludge reduction levels achievable, and the full-scale capital and operating costs, are site and wastewater dependent. The system uses anaerobic (SRB) and other direct arsenic reducing bacteria to precipitate arsenic from solution as insoluble arsenic-sulphide complexes. The precipitate is then removed from solution using conventional solid/liquid separation techniques.
Bioremediation of arsenic contaminated wastewater uses a solid matrix material to support high-density bacterial growth. Contaminated solution is pumped up-flow through the reactor, and arsenic-sulphide precipitates accumulate in the matrix. When the column is saturated, the arsenic is stripped and concentrated by filtration. This system is applicable also to semi-passive remediation of a pond.
There are clear benefits using biological processes for removing heavy metals. These include:
The new technology
Chemical-biological treatment of wastewater holds the promise of being more economical than either chemical or biological treatment, alone. Combined treatments have been approached from two directions:
A chemical precipitation followed by biological treatment is also possible in a staged reactor system.
By combining and optimizing chemical-biological processes for treatment of mining effluents, the future holds much promise for reduced costs, lower sludge volumes and decreased effluent arsenic levels.
For references, contact: Jack Adams, Applied Biosciences Corp., E-mail: djadams@bioprocess.com.
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