Performance and deterioration mechanisms of PAC amended calcium bentonite in seawater with different compositions

There is a variation in the composition of seawater in different seas. Three major cations (Mg2+, Ca2+and Na+) constitute about 40.00% by weight of salinity of seawater in the Persian Gulf coast. The concentration of these cations has significant influence on the performance of calcium bentonite (CaB) slurries prepared from seawater. The high salinity of seawater results in the decrease of the physical stability of the slurry. Polyanionic cellulose (PAC) is commonly used to increase the viscosity and enhance the stability of the slurry. In this study, the performance and deterioration mechanism characteristics of polyanionic cellulose (PAC) modified CaB in seawater with different compositions were investigated through macroscopic performance tests and microscopic characterizations of seawater slurry. A series of slurry performance tests were conducted to determine various parameters, including the Marsh viscosity, filtrate loss and colloid fraction values of slurries. Then, the sensitivities of slurry performance to cation type and concentration were analyzed. An optical microscope (OM) test of fresh slurry was developed. Using this method, the porosity was extracted by image recognition technology to quantitatively analyze the deterioration degree of seawater slurry. The filter cake after filtration loss was examined by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), and the evolution processes of the microstructure and molecular structure of the filter cake subjected to seawater with different compositions were analyzed. The results showed that the slurry performance was most sensitive to Ca2+ concentration, followed by Mg2+ and Na+. In a low cationic concentration environment, bridging and hydrogen bonding occurred between the slurry modifiers and CaB particles, the fresh slurry exhibited a low porosity and homogeneous structure, and the filter cake microstructure was layered. As the cationic concentration increased, the slurry porosity increased, the structure tended to flocculate. SEM morphological analysis of the filter cake showed an agglomerate structure.