Physicochemical characterization, elemental speciation and hydrogeochemical modeling of river and peloid sediments used for therapeutic uses

• Physicochemical properties and elemental content of two sediments were determined.
• The analysis involved origin, elemental geoavailability and quality of sediments.
• Sequential extraction and hydrogeochemical model were used for elemental speciation.
• San Diego de los Baños peloid is a mud or fangi, with low contamination factors.
• Most metals found in the river sediment and the final peloid are strongly retained.

The present study was conducted to characterize the river sediment and the final peloid from San Diego de los Baños Thermal Center (Pinar del Rio, Cuba), based on its original sedimentary geochemistry and composition, in order to establish the physicochemical and geochemical properties for its inorganic quality assessment in therapeutic uses. The original sediment was extracted from the estuary of San Diego River, and then maturated with thermal waters, yielding a peloid with known anti-inflammatory and dermatological properties. A comparative study of total content and geochemical speciation of seven transition metals (Cr, Cu, Fe, Ni, Mn, Pb and Zn) in the sediments was performed using a sequential extraction procedure and Inductively Couple Plasma Emission (ICP) techniques, as well as hydrogeochemical modeling to predict and correlate species under the physicochemical conditions measured for the river and peloid sediments. The results showed that the main differences originated from maturation process are closely related to the changes in electric conductivity (EC) and redox potential (Eh). These variations are reflected in the composition of major elements, and at a lesser extent, in the total content of the transition metals. Most of the elements studied in this investigation appeared in the less mobile fractions, which suggested low availability in the sediments, under the studied conditions. In the case of Mn, species are mainly located in the most leachable fractions, which together with its relatively high content, indicates a need for regular monitoring of this element in the peloids used in the Thermal Center. The findings were useful to predict the behavior of these transition metals regarding solubility, potential motility and availability in the river and final peloid sediments, and led to classify the San Diego de los Baños peloid as mud or fangi. It was concluded that most of the metals are strongly retained in the peloid through the maturation process, as many factors contribute to the low mobility, such as the nearly neutral pH, the presence of organic matter, redox conditions, and the presence of carbonates and salts. The sediments were finally characterized regarding possible contamination, according to USEPA and background criteria, and were considered as non-polluted/non-contaminated except for Mn, which showed a moderate contamination factor. The results from the present work show the relevance of physicochemical and elemental characterization to peloid classification and quality assessment. Moreover, it evidences the importance of performing geochemical speciation using both experimental and theoretical techniques, for proper assessment of mobility and distribution mechanisms of soluble and solid mineral species present in these sediments.