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Interaction of Hg(II) with kaolin-humic acid complexes

¹ Area de Edafoloxía e Química Agrícola, Facultade de Ciencias de Ourense, Universidade de Vigo, 32004 Ourense, ² Departamento de Edafoloxía e Química Agrícola, Facultade de Farmacia, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, ³ Departamento de Química Física, Facultade de Ciencias de Ourense, Universidade de Vigo, 32004 Ourense, Spain

^* E-mail: mastevez{at}uvigo.es

(Received 10 February 2003; revised 3 October 2003)

The adsorption and desorption of Hg(II) by humic acid (HA) previously adsorbed on kaolin was studied. In the range of HA concentration investigated (0.0–26.9 mg g^–1), the Hg(II) adsorption capacity of kaolin at pH 4 is enhanced by the presence of HA. For the complexes with the highest HA concentration and for low Hg(II) initial concentrations, adsorption was lower, i.e. as HA concentration on the complexes increases, Hg(II) equilibrium concentration also increases. This behaviour is due to the increasing presence of dissolved organic matter as the HA concentration on the complexes increases. The dissolved organic matter is able to form a soluble complex with Hg, thus decreasing adsorption. Hg(II) adsorption from a 2.5 x 10^–5 MHg(II) solution was influenced by pH. For kaolin, a pH_max (pH where maximum adsorption occurs) of 4.5 was observed. At pH values >pH_max retention decreased with increasing pH. This same behaviour was observed for the kaolin-HA complex containing the lowest HA concentration (6.6 mg g^–1). For the other kaolin-HA complexes there was little effect of pH on Hg(II) adsorption between pH 2.5 and pH 6.5. The presence of HA increased the adsorption of Hg(II) on kaolin all along the pH range studied. Desorption experiments showed that the amount of Hg(II) desorbed was quite low (<1%) for all the HA and Hg(II) concentration range studied, except for the kaolin at acid pH (pH 2.5) where the Hg(II) released was >50% of Hg(II) previously adsorbed. The presence of HA dramatically reduced this percentage of desorption to values of <3%, indicating reduced risk of toxicity problems in surface and subsurface waters. The addition of Cu(II) did not favour any Hg(II) desorption, even though Cu exhibits a strong affinity for organic matter.

KEYWORDS: mercury, humic acid, kaolin, adsorption, desorption