The dynamics of alkalinity generation by an anoxic sediment exposed to acid water

Abstract

The time dependence of the rate of generation of alkalinity by productive sediments treated with acid waters was investigated for various conditions using continuously stirred flowing reactors. Complete mass balances allowed the generated alkalinity to be related to specific processes, none of which were affected by light. The high rate of oxidation of organic material was sufficient to consume all the oxygen, nitrate and sulphate supplied to the vessels during the first few days of incubation, but by 15 days sulphate reduction had ceased and oxygen and nitrate reduction were incomplete. In incubations supplied with anoxic waters there was an initial small release of calcium, but by 10–15 days the generation of alkalinity could be accounted for by nitrate reduction (23%), sulphate reduction (47%), Fe(II) production (23%) and NH 4 + production (7%). Iron(II) was exhausted when only 1% of the total amount of iron in the sediment had been released. In incubations supplied with oxygenated waters sulphate was only reduced during the first 10 days while oxygen was completely consumed. By 15 days there was incomplete consumption of oxygen and the generation of alkalinity was accounted for by nitrate reduction (36%), calcium release (53%) and NH 4 + production (11%). Comparisons of reaction rates indicate that calcium is released by being replaced by ammonium ions which are generated by decomposition of organic matter. Although, in the longer term, more base is generated by sediment incubated anoxically, sediments incubated with oxygenated water generate base more rapidly for the first few days until the most readily oxidized organic matter is consumed.