Study on the pore surface fractal dimension and surface acid–base properties of natural particles around Guanting reservoir

Abstract

Nitrogen absorption–desorption isotherms were obtained on different natural particles around Guanting reservoir, and their data points were used to analyze the pore structure and fractal properties of the obtained solids. Other microstructure parameters of these particles are the following: 9.5669–4.5605 m 2 g −1 of BET specific surface area, 0.01784–0.06656 cm 3 g −1 of BJH cumulative absorbed volume and 7.20–7.74 nm of BJH desorption average pore diameter. The peak values of pore size distribution (PSD) curves were found at near 3.74 nm for samples 2, 3 and 4, and near 3.25 nm for sample 1, and near 5.17 nm for sample 5. The average pore diameter was larger than that corresponding to the peak value of pore size distribution (PSD). The micropore areas are very small in these particles, but their micropore volume of them can occupy over 15% of total pore volume. The self-similar and rough surface was observed on these particles, and the pore surface fractal dimensions D s of them were between 2.69 and 2.80, calculated by fractal FHH equation with the data of the adsorption branch of the isotherm, are a little difference from ones with the isotherm data of desorption branch, and also their corresponding fractal scales are different. It may be attributed to the different p/ p 0 range for D s values calculation and pore structure property. Otherwise, the computing D s values by thermodynamic model do not give meaningful indication for particle surface irregularity. There are about 1–2 specific surface sites ( D OH) in each square nanometers on these particles surface, and at most pH range, the average number of protons reacted per surface site ( Z) have negative values, and only when pH values are lower than about 4, Z can have positive values. Potentiometric titration experiment and two surface complexation model simulation results indicate that CCM simulation results can better interpret the experiment data points than DLM at 0.005 mol L −1 ion strength. Based on the CCM, the protonation and deprotonation reactions model can successfully depicted the acid–base behavior of natural particles surface, and the surface intrinsic acidic constants and the total concentration of surface sites for these natural particles, could be obtained.