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The Effect of Oxyanions on the Oxalate‐Promoted Dissolution of Goethite
Author(s) -
Eick Matthew J.,
Peak John D.,
Brady Warren D.
Publication year - 1999
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1999.6351133x
Subject(s) - goethite , arsenate , oxyanion , chemistry , dissolution , oxalate , adsorption , inorganic chemistry , chromate conversion coating , arsenic , organic chemistry , catalysis , coating
Organic ligand–promoted dissolution of oxide minerals can be enhanced or inhibited in the presence of specifically adsorbed oxyanions. It has been proposed that an oxyanion inhibits or enhances dissolution depending on the type of surface complex formed and the strength of the bond. Mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution. Recent spectroscopic evidence indicates that chromate and arsenate form different surface complexes depending on surface coverages. This study examined the influence of chromate and arsenate on the oxalate promoted dissolution of goethite. Based on a previous spectroscopic study, oxyanion surface coverages were varied to generate both mononuclear and binuclear surface complexes. Chromate and arsenate inhibited the oxalate promoted dissolution of goethite at all surface coverages investigated except at pH 6 ( Γ arsenate= 1 . 70 × 10 − 6 and Γ chromate= 1 . 66 × 10 − 6 mol m − 2 ) It is proposed that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. At pH 6 the adsorption of chromate and arsenate increases the negative charge of the goethite surface which in turn increases proton adsorption. Since proton adsorption is a necessary step for oxalate‐promoted dissolution of goethite, and since proton activity at pH 6 is low, an increase in the negative charge of goethite upon adsorption of the oxyanions accelerates dissolution.