This study assessed the groundwater quality in southeastern Brazzaville, Congo, in order to calculate iron chemical speciation. A total of twenty two groundwater samples were collected in rainy and dry seasons. Physico-chemical parameters namely pH, TDS, EC, T, D.O, TH, Alk, ORP, Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3-, PO43-, F-, Fetot, Fe2+ and Fe3+ were analyzed. The results indicate acidic nature and oxidizing conditions in groundwater, contamination of groundwater by domestic sewage, water septic tanks at the sight of EC values and iron content higher than permissible limit of 0.3 mg/L in the majority of groundwater samples. Modeling of iron chemical speciation in the groundwater samples using Visual Minteq computer program revealed that the predominant species were free ionic form Fe2+ in both season followed by FeSO4(aq) for Fe(II) species. The majority of Fe(III) species were fluoro complexes FeF2+ and (FeF2)+, sulphato complex ion FeSO4+ and free ionic form Fe3+ in rainy season. The hydroxo complexes of Fe(III) are predominant in dry season.
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