Acidic condition accelerates cation release from purple rock in Southwestern China.

ABSTRACT

In spite of the fact that rock weathering performs an essential task in the evolution of the Earth's surface, the quantitative assessment between pH and rates of chemical weathering remain unclear. This study aims to characterize the chemical weathering rate of purple rocks and then develops a model to calculate the release rates of cations (K+, Na+, Ca2+ and Mg2+) under various pH conditions. Two types of purple rock were sampled from the Shaximiao Group (J2s) and Penglaizhen Group (J3p), and a series of laboratory experiments were performed by soaking the purple rocks in solutions with pHs from 2.5 to 7.0, over 24 treatment cycles. The results showed that the release rates of cations apparently increased as the pH decreased. The release of Ca2+ was the dominant process of chemical weathering in J3p under various pH treatments, while K+ and Na+ were remarkably high in J2s (with the exception of the pH 2.5 treatment). Quantitative analysis revealed that the rate of cation release was significantly related to the H+ concentration (p < 0.001) and the air temperature (p < 0.001). The relationship between cation release and acidity was found to be an exponential function. Our results suggested that solution acidity serves as an important driving force for cation release rates from purple rocks and that environmental acidification would enhance rock weathering.