Sediments of fluvial streams are a sink for pollutants such as heavy metals and organic recalcitrant compounds. In anoxic conditions, sulphide generation and alkalinisation cause precipitation of heavy metals. The redox potential of the system could change to higher values, mainly owing to oxygen exposition generated by anthropogenic disturbances of the sediment such as dredging operations. Knowledge about the mechanisms involved is fundamental to assess the risk of acid generation and rise in heavy metals bioavailability. In this paper, the characterisation and assessment of acidification risk of sediments of a contaminated stream near Buenos Aires was carried out by static and kinetic assays. Speciation of heavy metals was determined after metal sequential extraction procedure. Also, re-suspension of sediments assay in batch mode, with and without Acidithiobacillus ferrooxidans inoculum, was carried out to assess the chemical changes that occur at oxygen exposure, verifying the acidification and heavy metal release processes.

Sediments from polluted urban streams act as a sink of contaminants. The high content of organic matter and sulphides makes the system appropriate for binding heavy metals. However, changes in the redox potential leads to processes in which sediments acts like a low sulphidic ore in an oxidizing environment, and could generate acid drainages. Human and not human disturbances of the sediments could derive in its oxidation catalyzed by sulphur oxidizing bacteria (SOB). This process leads to acidification and metal release. In this study we analyze the acidification potential of anaerobic sediments of polluted streams near Buenos Aires with static and kinetic methods. The results remark the necessity to consider this process before any sediment management action.

Anaerobic sediments of urban watercourses are subjected to industrial pollution and frequently tend to accumulate heavy metals. The biocatalyzed oxidation and reduction of sulphur compounds that occur within the sediment are key reactions that determine mobility of metals such as that occurred in mine acidic drainage reactions. The aim of this work was to study the application of these processes using heap leaching technology for the remediation of anaerobic contaminated sediments from Reconquista River basin. The bioleaching potentiality for remediation was demonstrated through batch tests in shake flasks with different pulp densities of anaerobic sediment containing 338 mg kg of Zn and 117 mg kg of Cu. Subsequently, bioleaching heap systems were compiled into columns of 12-cm height and 6-cm diameter, fitted with perlite to improve drainage. In order to assess the effect of elementary sulphur over the mobility of metals from the bioheap to the aqueous solution, increasing concentrations of elementary sulphur (1, 2, 5 % w/w) were added. After 3 months of acidification generated by periodic watering, the extraction of 70 % of the initial Zn and 43 % of the initial Cu was achieved. Polluted sediments from waterways as Reconquista River should not be indiscriminately manipulated if acid drainage is possible. Remediation by a simple and economically viable strategy like heap leaching is feasible.

In this work, we report on the structural and textural changes in fluvial sediments from Reconquista River´s basin, Argentina, due to processes of contamination with organic matter and remediation by bioleaching. The original uncontaminated matrix showed quartz and phyllosilicates as the main primary mineral constituents and phases of interstratified illite-montmorillonite as secondary minerals. It was found that in contaminated sediments, the presence of organic matter in high concentration causes changes in the specific surface area, particle size distribution, size and distribution of micro and meso, and the morphology of the particles with respect to the uncontaminated sediment. After the bioleaching process, there were even greater changes in these parameters at the level of secondary mineral formation and the appearance of nanoparticles, which were confirmed by SEM. Especially, we found the formation of cementing substances such as gypsum, promoting the formation of macroporous aggregates and the weathering of clay components. Our results indicate that the bioleaching not only decreases the content of metals but also favors the formation of a material with improved characteristics for potential future applications.

The physicochemical and textural characteristics of river sediments and, essentially, their clays, are at the center of a network of biological and geochemical factors that are mutually modifying. Therefore, the contamination, the characteristics of the clays, and the associated microorganisms strongly influence each other. In this work, sediments from two sites of the urban Reconquista River, near Buenos Aires City, Argentina, exposed to different environmental contexts were characterized. The huge differences in the organic matter content in the vertical profile between both sediments strongly evidenced the polluted status of San Francisco (SF) site as opposed to the Dique Roggero (DR) site. Thorough physicochemical and textural characterization of the sediments and their clay fraction performed by pH, Oxidation-reduction potential (ORP), spectrophotometry, XRD, laser diffraction, N2 adsorption–desorption isotherms, EDS, and SEM measurements revealed that organic matter (DR: 41 ± 5 g kg−1; SF: 150 ± 30 g kg−1) intervened in the retention of heavy metals (DR: 5.6 mg kg−1 Zn, 7 mg kg−1 Cu, 3.1 kg−1 Cr; SF: 240 mg kg−1 Zn, 60 mg kg−1 Cu, 270 mg kg−1 Cr) and affected the level of association and the formation of mineral–organic aggregates (DR: 15 ± 3 μm; SF: 23 ± 4 μm). This can be decisive in the surface interaction required for the establishment of bacterial assemblages, which determine the biogeochemical processes occurring in sediments and have a key role in the fate of contaminants in situ and in the remediation processes that need to be applied to restore the anoxic contaminated sediments.