In situ fractionation of water to reduce colloidal influence on Al-speciation and estimated ANC-values

Summary

In fresh water, Al can be present in different physico-chemical forms, varying from simple ions to polymers, colloids and particles. The distribution of Al-species will depend on pH, temperature and Al-complexing ligands such as humic substances. In acidified water the concentration of cationic low molecular mass Al-ions (LMM Ali) can be high, causing toxic effects towards fish. In humic rich water Ali is usually associated with DOC (dissolved organic carbon), and the concentration of LMM Ali, based on chromatography (Amberlite) prior to measurements, can be overestimated due to retention of Al-colloids in the column. The concentration of LMM Ali in unstable water qualities can also be significantly underestimated due to storage effects. Thus, in situ size and charge fractionation techniques are needed. Furthermore, estimated ANC-values (acid neutralization capacity) are applied as an alternative indicator for acidification and effects towards fish. In DOC poor water qualities estimated ANC-values are shown to be useful, while in TOC rich water, humic substances will significantly influence the ANC-estimates. Using in situ hollow fibre ultrafiltration interfaced with chromatography in river systems situated south and southwest in Norway, information of LMM and HMM (high molecular mass) Al-species, DOC-species and base cations was obtained. During flooding events pH decreased, while the concentrations of HMM Ali and HMM DOC increased, due to colloidal and particle transport (Fig. 1A). God correlation (r2=0.7) was obtained for HMM Ali and HMM DOC. Unless in situ fractionation techniques were applied, LMM Ali-species would be significantly overestimated during flooding. Moreover, the ANC-values based on unfiltered water were significantly higher than values based on in situ filtered samples, underestimating the acidification (Fig 1B)