Selective recovery of calcium from estuarine sediments of Raša Bay

Abstract

Abstract The historic mining of Raša coal on the Istrian Peninsula, Croatia for about quadricentennial, has left significant impacts on the soil chemistry in the surrounding areas. Geochemical examination of such soils commonly demonstrated notable leaching of hazardous trace metals into the soil, thus rendering them inappropriate for utilization. Interestingly, the abundance of calcium in the karstic soil of Croatia and its known versatile commercial application is noteworthy. Here, a thorough investigation was performed to delineate the occurrence and association of Ca in soils from Istrian peninsula along with its selective precipitation. Comprehensive mineralogical analysis using X-ray fluorescence and ICP-OES revealed the presence of Ca in noticeable quantity (~ 15 wt%) along with clay minerals (Si, Al, Fe) and alkaline metals (Na and K). Evaluation of the soil trace elements demonstrated the presence of both elements of environmental concern (Sr, Cr Zn and V) as well as of economic importance (Sc, Ga and Ge). The high concentration of Sr and Cr (~ 300 ppm) was considered to be an implication of intrusion of coal waste into the selected area. The presence of NaCl corroborated the intrusion of sea water into the soil sediment. The results obtained from the five step sequential leaching demonstrated Ca to be primarily present as dolomite mineral (CaMg(CO3)2). Optimization of the leaching parameters (temperature, acid concentration and duration of leaching) of Ca using HCl was performed, followed by its selective precipitation and calcination which resulted in isolation of CaO with 98.7% purity and 85% cumulative yield. Statistical correlations further confirmed the linear association of CaO and MgO (r = 0.8–0.9) as well as strong association of Sc with clay minerals (r > 0.8) and not Fe. To summarize, the present investigation demonstrated that extraction and selective precipitation of calcium from karst soils is feasible and can be potentially explored to utilize such which are otherwise considered waste.