Acid mine drainage
[citation needed] The same type of chemical reactions and processes may occur through the disturbance of acid sulfate soils formed under coastal or estuarine conditions after the last major sea level rise, and constitutes a similar environmental hazard.
[1] For example, a paper presented in 1991 at a major international conference on this subject was titled: "The Prediction of Acid Rock Drainage – Lessons from the Database".
After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity.
Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment.
These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low.
[6] Although a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor.
Negative pH[11] occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.
[citation needed] When the pH of acid mine drainage is raised past 3, either through contact with fresh water or neutralizing minerals, previously soluble iron(III) ions precipitate as iron(III) hydroxide, a yellow-orange solid colloquially known as yellow boy.
All these precipitates can discolor water and smother plant and animal life on the streambed, disrupting stream ecosystems (a specific offense under the Fisheries Act in Canada).
However, although limestone is an unprocessed raw material available in large quantities and the least expensive neutralisation agent, it can suffer from a number of disadvantages possibly limiting its applications.
Indeed, small calcium carbonate grains of crushed limestone can be prone to the formation of a coating of gypsum (CaSO4·2H2O) surrounded by a thin impermeable and protective film of less soluble Fe-Al hydroxysulfate.
In this application, a slurry of lime (CaO – Ca(OH)2 after hydration) is dispersed into a tank containing acid mine drainage and recycled sludge to increase water pH to about 9.
In that vessel, clean water will overflow for release, whereas settled metal precipitates (sludge) are recycled to the acid mine drainage treatment tank, with a sludge-wasting side stream.
[22] Generally, the products of the HDS process also contain gypsum (CaSO4) and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization.
These systems are far less costly to build, but are also less efficient (longer reaction times are required, and they produce a discharge with higher trace metal concentrations, if present).
[23] A calcium silicate feedstock, made from processed steel slag, can also be used to neutralize active acidity in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH.
Limestone grains become coated by a rind of gypsum encapsulated itself in a thin external film of impermeable and protective Fe-Al hydroxysulfate.
[26] Once the contaminants are adsorbed, the exchange sites on resins must be regenerated, which typically requires acidic and basic reagents and generates a brine that contains the pollutants in a concentrated form.
An example of an effective constructed wetland is on the Afon Pelena in the River Afan valley above Port Talbot where highly ferruginous discharges from the Whitworth mine have been successfully treated.
Solid-liquid separation after reaction would produce a base metal-free effluent that can be discharged or further treated to reduce sulfate, and a metal sulfide concentrate with possible economic value.
In this process, sulfate-reducing bacteria (SRB) oxidize organic matter using sulfate as terminal electron acceptor, instead of oxygen.
Their metabolic products include bicarbonate produced by organic matter oxidation, which can neutralize water acidity, and hydrogen sulfide, which forms highly insoluble precipitates with many toxic metals.
[31] Our knowledge of acidophiles in acid mine drainage remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.
