Dissolution of microcline and labradorite in a forest O horizon extract: the effect of naturally occurring organic acids

Water extracts of O horizon soil (mor) containing realistic soil solution levels of organic acids and cations were used to dissolve microcline, labradorite and a natural soil (C-horizon) in fluidized bed reactors. Experiments were also conducted with inorganic solutions containing the same concentration of cations (inorganic eluent) as in the mor extract until reaction rates were stabilized. All experiments were performed at pH 3.5 and 5. After 2000 h, the dissolution rate for the microcline experiments was in the range 0.6–6×10−16 mol mineral cm−2 s−1. In the mor eluent, the rate was largely unaffected at pH 3.5 but at pH 5, the increase on average was by a factor of 3 compared to the inorganic eluent. The increase of the dissolution rate for labradorite by the mor eluent of pH 3.5 was small (1.3 times), but at pH 5, the rate tripled (range 0.2–3×10−15 mol mineral cm−2 s−1) compared to inorganic control. The largest increase in the dissolution rate (comparing the inorganic and mor eluent) was observed for natural soil which at pH 5 increased by a factor of 6. The presence of secondary minerals, which dissolve and affect the dissolution rate, might be a possible cause for this observation. The relative release ratio for Al/Si (RRRAl) was below 1 for all microcline runs, but higher for runs with the mor eluent. Labradorite experiments showed RRRAl values of 1.5–2.3 in the mor eluent and a stochiometric Al release in the inorganic eluent.