Effect of Malic Acid on the Adsorption/Precipitation Reaction of Calcite with Fluorine
Low molecular weight organic acids often influence the surface reactions(adsorption/precipitation)of minerals,which in turn affect the stability of elemental transport in environmental geochemical processes.The effect of malic acid on calcite-fluorine adsorption/precipitation reactions was investigated.The results show that ① at the initial p H of 7.7 and lower F concentrations(≤5 mg·L-1),as the concentration of malic acid increases,its surface adsorption capacity becomes stronger and exhibits the inhibitory effects on F removal;at the F concentrations of 25 or 60 mg·L-1,the dominant mechanism of F removal is CaF2 precipitation;the intervention of malic acid shows little inhibition of the CaF2 precipitation reaction;the stronger adsorption reaction of malic acid results in an increase of p H and a decrease in Ca concentration.② At the initial p H of 8.3 and low F concentration(≤20 mg·L-1),malic acid still inhibits F removal through its competitive adsorption;as the concentration of malic acid increases,its effect of complexation with Ca2+results in a constant increase in p H and Ca concentration;at the medium F concentration of 60 mg·L-1,the dominant mechanism of F removal has been shifted to CaF2 precipitation;20 mg·L-1 malic acid has inhibited it,and as the malic acid continues to increase,the p H value decreases and then increases,and the Ca concentration continues to increase,highlighting the effect of the malic acid complexation reaction;At high F concentration of 100 mg·L-1,100 mg·L-1 malic acid greatly inhibits the CaF2 precipitation reaction,corresponding to a p H value that do not exceed that of the malic acid-free condition;the corresponding Ca concentration is still lower than its blank background value,implying the dominance of the CaF2 precipitation reaction.③ At the initial p H of 8.7,malic acid has a weak inhibition to F removal at low F concentration(≤5 mg·L-1);however,as the concentration of malic acid increases,the effect of its complexation reaction becomes much clearer,which results in a significant increase in both p H and Ca concentration;at high F concentration of 240 mg·L-1,the dominant mechanism for F removal is the CaF2 precipitation reaction,which is inhibited with increasing malic acid concentration,with increasing Ca concentration showing the effect of malic acid complexation,and decreasing p H values indicating the dominance of the CaF2 precipitation reaction.This study is important for the stability assessment of calcite minerals in organic acid-rich environments,and also provides a reference for the study of fluorine transport transformation on other minerals.
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