ALKALINITY AND ACID NEUTRALIZING CAPACITY (ANC) OF CONSTITUENTS IN BAYER REFINERY NEUTRALIZED WASTEWATER

Alkalinity is considered one of the best indicators of Bayer liquor neutralization with seawater. For most aqueous systems, alkalinity is controlled by carbonate chemistry. However, there are non-carbonate contributors to alkalinity and acid neutralizing capacity (ANC); some of these are constituents in Bayer liquor and products of wastewater neutralization. The effects of vanadate, aluminate, hydrotalcite and organic anions on seawater alkalinity and ANC are presented in this paper.

The alkalinity and ANC of seawater was not affected by vanadate. This was also true for when 2.86 to 30.3 μg/L Al was added to seawater. However, alkalinity decreased in the presence of 69.3 μg/L Al. This may be attributed to hydrotalcite (HT) formation causing an increase in ANC. Alkalinity and ANC increased with 147 μg/L Al.

Hydrotalcite greatly increased ANC. The results showed a linear correlation between HT and ANC. A slight increase in alkalinity was also observed, which was attributed to HT nanoparticles and release of loosely sorbed carbonate/ noncarbonate contributors.

When present in high concentrations (≥1809mg/L), oxalate greatly increased alkalinity. However, at lower concentrations (≤1206mg/L), the effect of oxalate on seawater alkalinity was limited by the precipitation of non-titratable calcium oxalate; and did not contribute to ANC. Formation of non-titratable calcium oxalate was verified by experiments in deionized water, which also showed that alkalinity varied linearly with dissolved oxalate concentration.

Gran function plots of titrations down to pH 2.5 were used to determine alkalinity and ANC throughout this study. These were compared to those calculated using a set endpoint of pH 4.5. The results indicate that the endpoint method is adequate for typical seawater. However, the shortcomings of this method are revealed when HT and oxalate are present. In the case of HT, its incomplete dissolution at pH 4.5 results in an underestimation of ANC. However the contribution of oxalate to seawater alkalinity are completely undetected by titrations to pH 4.5, which is above the pKa values of this organic anion.