Clay minerals of Pliocene deposits and their potential use for the purification of polluted wastewater in the Sohag area, Egypt

In our study we investigated the clay fraction composition of Pliocene clay deposits in the Sohag area, Egypt. Our goal was to obtain insights into the origin of the deposits, and to assess their potential for use in inexpensive wastewater purification. The rationale for the latter was that in Egypt both industrial wastewater and irrigation water are often polluted with heavy metals (HMs), the load of which can be significantly reduced using the Pliocene clay. We combined physico-chemical analyses with detailed X-ray diffraction (XRD) mineralogical investigations and Specific Surface Area (SSA) measurements of samples from 16 locations in four areas containing Pliocene clay deposits. The grain size distribution of the studied samples was dominated by silt (75-89%) with lower quantities of clay (6-20%) and sand (2-15%). Neither grain size distribution nor the distribution of individual clay minerals varied between the tested samples, suggesting that they all originate from a single source area. The effect of differential cation saturation (K+ and Mg2 +) and differential heat treatments (25°C, 300°C, and 550°C) on the XRD patterns of oriented aggregates of the clay-sized fractions revealed 4 different clay mineral groups in the tested samples. The relative abundances of the clay minerals were semi-quantified and revealed a dominance of smectite (69-91% on average) with relatively low contents of kaolinite (9-29% on average) and minor amounts of illite (1-7% on average) and chlorite (0 ≤ 1%). This mineral assemblage suggests chemical weathering and indicates warm climatic conditions of the source area during the period of deposition. The higher CEC values of the Pliocene clay deposits (32.3-65.4 cmolc/kg) also pointed to the occurrence of smectite in the soils. The SSA of the Pliocene clay fractions (26.25-128.97 m2/g) correlated well with their exchangeable cation contents (K+ and Ca2 +, R2 = 0.96 and 1.0 respectively) and micropore volumes (R2 = 1.0). Micropore volumes and SSA of the studied samples increased with the size of the exchanged cation: K+ > Ca2 + > Na+. The mineralogical composition suggests that Pliocene smectite-rich deposits in the studied area have great potential to be used as raw material for inexpensive, local purification of wastewater polluted with HMs.