Acid Mine Drainage

a metal-bearing acidic solution formed from the oxidation of sulfides e.g. iron pyrite

• many igneous metal ores are sulfides • in spoil heaps chemical reactions take place between the small amounts of remaining sulfide ores and oxygen from the air spaces between the waste particles • this may leave metal oxides and oxides of sulfur which dissolve in drainage water to produce sulfuric acid = ACID MINE DRAINAGE

• elements such as lead, zinc and copper are all stably bound in minerals when they are locked undergrounded • mining • metals are exposed to the air and they become mobile as a result of a chemical reaction • they are converted by presence of O2 from sulfides to sulfates • as the acidic waters (often as low as pH2/3) rise through the adits, they pick up these dissolved metals and transport them into nearby rivers • sulfuric acid created as a result of the chemical reaction can oxidise other metal sulfides (e.g. chalcopyrite) and/or leach furhter metals from the surrounding rocks 6/7. the result is a high and toxic concentrations of metal ions

ACIDIC WATER - acidic wastes reduce water pH below biologically viable limits (>4) - acidic water flow into nearby rivers, bringing large volumes of heavy metals - suspended and dissolved, increasing turbidity and preventing respiration of both plants and animals METALS - at pH values 4-7, the ferric ions released in acid mine reactions will tend to precipitate out - negative effects of iron and aluminium solids - heavy metals also often toxic to organisms

Fine solids of iron and aluminium compounds leached from soils: - coat plants and block up stomata, preventing resp - clogs gills and lungs, suffocating animals - prevents the spawning of fish - dissolved metal ions are more readily absorbed and accumulated by plants and animal life and are, therefore more toxic than when they are in solid form

• they can combine with enzymes so that they cannot function • they can combine chemically with the constituents of cells, disrupting cellular functions and being potentially neurotoxic/carcinogenic

ACTIVE (CHEMICAL) TREATMENT - involves the use of a minewater treatment plant to flocculates the iron e.g. Wheal Jane in Cornwall PASSIVE TREATMENT - the use of plants, such as reeds, to encourage metals to precipitate out of solution - phytoremediation e.g. pilot plant in Wheal Jane CRUSHED LIMESTONE

• RAW MINEWATER - stage 1: mine water enters the tank and added sludge begins the initial process of settling of solids • LIME IS ADDED - stage 2: lime is added to raise the pH and causes precipitation of the metal by dioxides • FLOCCULANT IS ADDED - stage 3: a flocculant is added to coagulate the metal solids and sludge settles out in the clarifier tank • Clean water is discharged to the river, and the recirculated sludge is added back into more raw minewater

• involves creating areas of natural wetland which traps and precipitates the metals • utilises natural processes and it is sustainable • some phytoremediation e.g. cannabis • usually involve the construction of a series of shallow, lined lagoons or cells 2 MAIN STYLES: 1. Aerobic cells - using reed beds/trickle beds to remove heavy metals 2. Anaerobic cells - underground tunnels

• the pH of acid mine drainage water can be increased by passing it through crushed limestone to neutralise the acids. This will also reduce the solubility of many toxic metals

PHYTOREMEDIATION: a plant’s ability to remove or render industrial pollutants harmless Cannabis in Johannesburg - 380 mine residue areas with heavy metal conc hazardous to human health - Randfontein’s Robinson Lake uranium levels 40,000x above nat occurring rates - cannabis = mop crop: rapid growth rate superseded only by bamboo, a high stress tolerance, 2.5m root system probing deep into contaminated soils - sequesters 22 tonnes of CO2 per hectare, better than any forest/commercial crop - heavy metal hyper-accumulator: a plant absorbing toxins at higher concentrations than its growing medium - the metal is pulled out in quantities not toxic to humans, so we can lock it into products made form plants e.g. bioplastics, hempcrete (construction alternative) - also used in Chernobyl

• traditional world-leading methods use water hydraulics • transports slurry through polyethylene lined pipes to tailings dams • heavy metals mostly inert and mostly at conc of minimum human safety consumption standards • the slime dams contain full spectrum of heavy metals e.g. copper, mercury, gold, zinc • BUT risk of leaching and after the mine closes there is no money to keep the maintenance up • phytoremediation costs range from less than half to 20% when compared with other techniues

STIGMA - getting people on board with it - normally used in medicine or recreation (though honestly can’t see the issue…)