A remarkable number of laboratory investigations have illustrated the essential role of pH in the processes of chemical weathering (Schnoor, 1990; Guicharnaud and Paton, 2006). In this study, our results indicates that the concentration of H+ in the reaction environment is an important impact factor for weathering kinetics, consistent with those of previous studies (Huang et al., 2013; Wu et al., 1998; Zhang et al., 2007; Probst et al., 1992). The rate of the release of cations from the purple rocks obviously enhanced as the solution pH diminished, especially when the value of pH was less than 3.5 (Table 3). Similarly, the effect of acidification on chemical weathering is low (Sverdrup and Warfvinge, 1995), therefore, long-term evaluation is necessary to reveal the effects of acid deposition on chemical weathering (Probst et al., 1992). However, a pH less than 3.5 is infrequent in natural environments, the impacts of acidity on chemical weathering involve the relatively long procedures, and in simulation assessments, a lesser pH is commonly utilized to stimulate the long-lasting impacts of acidic media in nature to diminish the involvement of humidity, temperature, and other external parameters (Liu et al., 2010). In the current research, the performed analysis of cation release divulged substantial impacts of pH on chemical weathering among the two purple rocks, particularly when the amount of Mg2+ and Ca2+ released during the pH 2.5 processing was considered. The effects were significantly greater than those caused by the other treatments, which was attributed to the fact that compared to univalent cations, the divalent cations released were more affected by the pH of the environment (Liu et al., 1990; Hartikainen, 1996). Calcium in the rock mainly exists in inorganic form, and inorganic calcium can be further divided into mineral and exchange state calcium (Yuan, 1983). Of these, mineral calcium mainly exists in the crystal lattice of the rock in its solid phase and can seldom be released through hydrolysis. In contrast, the calcium-bearing minerals in purple rock are mainly silicate minerals and carbonate minerals. The acidities of silicic acid and carbonic acid are weaker than those of the simulated solutions due to the action of mixed acids of SO42− and NO3−. In this way, the calcium within silicate and carbonate minerals can be easily dissolved by acid. In contrast, the released quantity of potassium is evidently lower than that of calcium, which is mainly because most of the calcium-bearing primary minerals in the rock are easily weathered. In particular, the weathering rate of calcium-bearing plagioclase is much higher than that of potassium feldspar or sodic feldspar (Yuan, 1983). Meanwhile, the lattice energy of calcium-bearing minerals is much smaller than those of sodium- or potassium-containing minerals of the same type (Yuan, 1983). Additionally, the major clay minerals of the purple block (including illite, montmorillonite, and chlorite) are all phyllosilicate-based minerals with a ratio of 1:2 in illite and montmorillonite and in chlorite a ratio of 2:1:1 (Verburg and Baveye, 1994). The reactions of these minerals in acidic solutions may represented as follows:
Montmorillonite (Amram and Ganor, 2002):
Illite (Vieillard, 2000):
(Si3.55Al0.45)(Al1.27Fe(III)0.36Mg0.44)O10(OH)2(Ca0.01Na0.13K0.53)+7.80H+3.55SiO2+1.72Al3++ 0.36 Fe3++0.44 Mg2++0.01 Ca2++0.13 Na++0.53K++4.90H2O
Chlorite (Tang et al., 2007):
(Mg, Fe, Al)6[AlSi3O10](OH)8+16H+ [6(Mg, Fe,Al)]13++ Al3++ 3H4SiO4+6H2O
These reactions promoted the replacement of intercrystalline Ca2+, Mg2+, Fe3+ and Na+ by H+ in the solution. However, the crystal structure of illite, which accounts for the vast majority of the clay mineral composition of the rock, constitutes two tetrahedral sheets with face-to-face top oxygen molecules. Moreover, the layers are tightly stacked in a staggered arrangement, and a coordinate, co-edge, octahedral combination is produced by the superposition, forming an octahedral sheet (O). Thus, a basic structural layer is formed, the interlayer space of which is filled with K+ with an equilibrium electrovalence. The K+ exposed between the layers is directly absorbed onto the silica tetrahedra and is not easily replaced by H+ in the solution.
On the basis of transition state theory, chemical weathering processes may be influenced by the proton or aqueous activities of elements (Zhu et al., 2013). The distribution of cations (K+, Na+, Ca2+ and Mg2+) on the ternary diagrams presented in Fig. 4 can be used to indicate the relative contributions of major ions under various acidic environments (Huh, 2003). Silicate- and carbonate-rich minerals are major components of the purple rock lithologies; therefore, considering that they can be weathered easily (especially carbonates), they are expected to contribute significantly to the major cation budget of the chemical weathering process (Zhu et al., 2013). Therefore, Ca2+ was the dominant species in J3p under various acid treatments, while K+ and Na+ were remarkably high in J2s except for the pH 2.5 treatments. Notably, the relative contributions of major cations to chemical weathering varied with pH and time. The proportion of Ca2+ released by pH ≥ 3.5 treatments was substantially decreased when compared with the Ca2+ released by the treatment at pH 2.5. The proportions of Na+ and K+ behaved in the opposite manner, and the contribution rate of Mg2+ changed only slightly with changing pH. On average, the release of Mg2+ only accounted for a small fraction of the major cations, and its contribution in J3p approached 0 with an increase in time; it should also be noted that the contributions of Na+ and K+ increased.
The rate of chemical weathering is usually quantified by measuring solute concentrations and flux (Drever and Zobrist, 1992; Stonestrom et al., 1998). Past research has focused on the effects of changing temperature and reactive fluid composition on the chemical rates of individual minerals in the laboratory (Oelkers and Schott, 2001; Schott et al., 2009). However, natural rocks consist of many different minerals with distinct reactive and surface areas; thus, different rocks have distinct responses to pH and temperature. Concerning the constituents of its materials, purple rocks include a considerable percentage of clay minerals, namely, illite, montmorillonite, chlorite, and so forth. According to studies of influence factors on clay mineral weathering, the clay mineral dissolution rates increase with temperature strongly depends on pH (Huertas et al.,2001; Köhler et al., 2003), and previous investigations have exhibited that more quickly dissolution could be achieved by a departure from neutral pH circumstances. Furthermore, at greater temperatures, the rate of solution is greater (Oelkers et al., 2008; Bray et al., 2015). A thorough study on phyllosilicate weathering indicates that the rate of weathering is lesser at low temperatures and moderately acidic to neutral pH, and it is greater at higher temperatures and lower pH, which are divided through a transition zone (Lamarca-Irisarri, et al., 2019). Regression analysis of our results revealed that the relationships between cation release rate, H+ concentration and natural temperature were significant for the tw purple rock samples (p < 0.001). According to partial correlation coefficients (Table 4), both the H+ concentration and temperature significantly influenced the cation release rates (p < 0.001). Temperature affected not only the absorption of protons on the surface (Cama et al., 2002) but also the physical erosion of purple rock (Zhang et al., 2015). Previous researchers have indicated a positive relationship between chemical and physical weathering rates (Riebe et al., 2003; Singh et al., 2005). Therefore, any attempts to model cation release rates based on the independent effect of pH alone and without consideration of temperature effects will be unsatisfactory; our developed model (equation ) would be more accurate for the prediction of cation release rates under environments with variable acidity.