Effect of pH on the surface charges of permanently charged soils and clay minerals

Abstract

Traditionally, the surface charge number (SCN) of permanently charged soils/clay minerals is believed to be unaffected by environmental pH. However, recent studies have revealed the occurrence of polarization-induced covalent bonding between H+ and the surface O atoms of permanently charged clay minerals. This discovery challenges the traditional notions of “permanently charged soils” and “permanently charged clay minerals”. The purpose of this study is to confirm that there is no true “permanently charged clay” or “permanently charged soil”. In this study, the SCNs of two permanently charged clay minerals, two variably charged clay minerals, five permanently charged soils, and four variably charged soils were measured at different pH values using the universal determination method of surface charge number. The results showed that: (1) The SCNs of both the permanently and variably charged soil/clay minerals decreased significantly with decreasing pH; (2) the SCN of montmorillonite decreased less with decreasing pH than the SCNs of the variably charged minerals, whereas the SCN of illite decreased to nearly the same extent, indicating strong polarization-induced covalent bonding between H+ and the surface O atoms of illite; (3) the SCNs of the permanently charged soils decreased to a similar extent as those of the variably charged soils with decreasing pH. This study demonstrated that there is no true “permanently charged clay mineral” or “permanently charged soil” because of the polarization-induced covalent interactions between H+ and the surface O atoms of clay minerals.