Speciation of metals in sediment and water in a river underlain by limestone: role of carbonate species for purification capacity of rivers

Abstract

Rivers whose basins are underlain by limestone possess a high buffering capacity for discharged pollutants. During the discharge of metals in the aquatic environment, metals are partitioned between the sediment and the water column phases. Further partitioning of metals occurs within the sediment chemical fractions and metal speciation in water column, thus leading to the possible reduction of the toxic free hydrated metal ion. The present study focuses on one of Lebanon's rivers; the Nahr-Ibrahim whose basin is underlain by limestone and its river water is dominated by carbonate species due to the high pH and alkalinity values. The objectives of this study were: first, to determine the metal speciation (Fe, Zn, Pb and Cd) in the (operationally defined) sediment chemical fractions and metal speciation in river water; second, to evaluate the role of carbonate species in the self-purification process from metal pollutant inputs into the river. Bed sediments and water were collected from eight locations in one dry season (September, 1997), and a sequential chemical fractionation scheme was applied to the <75 μm sieved sediment fraction. The data show that the highest percentages of total metal content in sediment are for: Fe in the residual fraction followed by moderately reducible fraction, Zn and Pb in the carbonate and in the moderately reducible fractions and Cd primarily in the carbonate fraction. Aqueous metal speciation was predicted using AQUACHEM software interfaced to PHREEQC geochemical computer model. The water speciation data predicted that a high percentage of Pb and Zn were present as carbonate species, very low percentages as free hydrated ion species; whereas Cd exhibited high percentage occurrence as a free hydrated metal ion species. Iron was present in water mainly as ferric hydroxide ion pair species. This study has evaluated the role of carbonate species for self-purification process by the reported high percentage occurrence of metals in the carbonate sediment fraction and the interaction of metals with the carbonate water species.