THE EFFECTS OF PH AND CHLORIDE CONCENTRATION ON MERCURY SORPTION .1. BY GOETHITE

The effects of mercury concentration, pH, and chloride concentration on mercury adsorption by goethite were measured. With increasing pH, mercury adsorption increased rapidly at first, until a maximum was reached, and then decreased slowly. In the absence of chloride, the maximum was just below pH 4. With increasing chloride concentration, the maximum moved to higher pH values. This behaviour contrasts with that of the other heavy metals for which, beyond a certain pH, adsorption increases rapidly with further increase in pH. The results were described by a model in which HgOH+ and HgCl+ ions were adsorbed and the electric potential of the reacting surfaces decreased with increasing pH. The maxima in the adsorption corresponded to maxima in the concentration of HgOH+. Although a good description of the results was obtained, there were consistent discrepancies. These suggested that the charge component of the model was unable to produce the correct relationship between electric potential and pH. The results indicated that mercury adsorption was controlled by the same principles as adsorption of the other heavy metals. However, mercury's lower pK for dissociation and its higher affinity for chloride mean that the effects of pH on the ions present in solution are different, and this is the cause of the different behaviour.