Absorption of sulphur dioxide by sodium humates

As part of a programme for assessing the potential of basic humates as stack gas scrubbing media, the reaction of sodium salts of coal derived humic acids (HA) with sulphur dioxide was investigated. The principal absorption mechanism was found to be acid-base:

$\text{Na-HA(aq) + SO}{}_2\text{(g) + H}{}_2\text{O}\text{?}\text{HA(s) + HSO}{}^{\mathrm{t-}}{}_3\text{(aq) + Na}{}^{\mathrm{+}}\text{(aq)}\text{Soluble sodium}\text{Insoluble humic}\text{humate}\text{acid}$

However, formation of a sulphur dioxide-HA ‘complex’ was observed under conditions (low pH, high temperature) which favoured conversion of a significant fraction (> 5%) of dissolved sulphites to the SO₂(aq) ‘H₂SO₃’ form. Greater complexation of SO₂(aq) was observed for HA preparations which had been stored under basic conditions for periods up to two years. Reversibility of absorption and desorption was demonstrated. Similar experiments with hydrogen sulphide revealed no significant reaction with sodium or iron(III) humates.