Cacodylic acid demethylation

Examples of the Influence of Adsorption on Kinetics. Three cases of arsenic species transformations influenced by adsorption are considered in this section. The three cases considered are 1) reactant not adsorbed, product adsorbed (e.g. demethylation of cacodylic acid to arsenate), 2) reactant adsorbed, product not adsorbed (e.g. methylation of arsenate to cacodylic acid),... 

Example demethylation of cacodylic acid (weakly adsorbed) to arsenate (strongly adsorbed). 

Demethylation of cacodylic acid to arsenate was chosen as an example of case 1, product adsorbing with reactant not adsorbing, because cacodylic acid is only weakly adsorbed by anaerobic sediments and arsenate is strongly adsorbed. For the same reason, methylation of arsenate to produce cacodylic acid was chosen as an example of case 2, reactant adsorbed and product not adsorbed. Demethylation of methanearsonic acid to arsenate was chosen as an example of case 3, reactant and product both adsorbing. 

The predicted arsenate vs time curves for demethylation of cacodylic acid are shown Tn Figure 5. Both curves are for first order decay with a rate constant of 0.04 day" but with different amounts of adsorbent present. Adsorption of arsenate results in dissolved arsenate concentrations that are lower than total arsenate. An s-shaped curve results because arsenate produced during the first few days is almost entirely adsorbed. 

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