The Biochemistry of Stagnant Ponds
The chemistry of mud-dyeing has never been properly analysed and is likely to be complex. Nevertheless it is possible to gain some insight into the processes involved.
We first need to understand how ferrous iron is produced in stagnant rivers and ponds. Iron is the fourth most abundant element in the Earth's crust and leaches into freshwater rivers, lakes and ponds from the soil and weathered rocks. However iron is extremely insoluble in water so its concentration in most natural freshwaters is very low, not exceeding 1mg per litre (Wang and Dou 1988). Iron is mainly present in the form of particulate matter – the ratio of dissolved iron to particulate iron is around 1:500 (Davidson and DeVitre 1992).
Both the concentration and the speciation (either ferrous or ferric) of iron in water depend on many factors such as water flow, the pH of the water, its chemical composition, incident light and the amount and type of dissolved organic matter (Vuori 1995). In so-called oxic waters (where oxygen is present) with neutral pH, ferrous iron Fe(II) is rapidly oxidised into much more stable ferric iron Fe(III), which precipitates as highly insoluble oxides and hydroxides (Xing and Liu 2011). This oxidation is very pH sensitive and accelerates with increasing pH (alkalinity).
In anoxic waters, devoid of oxygen, iron is mainly found in the ferrous state Fe(II), usually in the form of dissolved ions - although it will combine to form insoluble salts if there are high concentrations of carbonate, sulphide or orthophosphate ions. Under the right conditions there are various ways that ferric iron can be naturally reduced to ferrous iron, although the mechanisms are still poorly understood (Vuori 1995). It can be achieved by the action of bacteria, dissolved sulphides and certain organic compounds, by photo-reduction, or by the reaction of enzymes (Davidson and DeVitre 1992).
In oxic water aerobic bacteria survive by consuming oxygen, ultimately ‘reducing’ it to water and carbon dioxide. The solubility of oxygen in water at 20°C is only 9mg per litre and less at higher temperatures. In a warm stagnant tropical pond the rate of diffusion of oxygen from the atmosphere is slow relative to the speed of microbial metabolism. Consequently oxygen is quickly depleted, not only by the action of bacteria but also by the biological degradation of organic matter.
When oxygen is depleted, aerobic bacteria can no longer survive and anaerobic bacteria take over. Anaerobic bacteria obtain their energy from oxidants other than oxygen. In order of decreasing energy production these are nitrate, manganese, ferric iron, sulphate and, ultimately, carbohydrate and carbon dioxide. The aerobic bacteria are first replaced by nitrate-reducing bacteria, which convert nitrates into gaseous nitrogen. When the nitrates are exhausted, the nitrate-reducing bacteria are replaced by ferric oxide-reducing bacteria, which convert ferric iron into ferrous iron. The process continues until finally the methanogenic bacteria take over and convert carbonates into methane (Lovley and Klug 1983; Ward and Winfrey 1985).
This explains why mud from stagnant water sources contains ferrous iron. The reason that ferrous iron is so important is because it is far more reactive than ferric iron.
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