Comparison of Dissolution Rate-Determining Mechanisms between Limestone and Dolomite
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摘要: 碳酸盐岩溶解的速率控制过程包括: (1) 岩石表面上的非均相化学反应; (2) 离子从岩石表面通过扩散向溶液中的传输; (3) CO2向H+和HCO3-的转换.通常是这3个过程中的最慢过程决定着碳酸盐岩的溶解速率.然而, 实验和理论分析发现, 在条件相似的情况下, 白云岩的初始溶解速率不仅只有灰岩的1/3~1/60, 而且灰岩和白云岩的溶解呈现出不同的速率控制机理.如对灰岩而言, 在实验中加入能催化CO2转换反应的生物碳酸酐酶(CA) 后, 其溶解速率增加出现在CO2分压>100 Pa的区域, 最高可达10倍; 而对白云岩, 其溶解速率增加出现在CO2分压 < 10 000 Pa的区域, 且增加仅3倍左右.此外, 虽然2类岩石的溶解也均受水动力条件(旋速或流速) 的控制, 且主要出现在CO2分压 < 1 000 Pa的区域, 但灰岩的溶解对水动力条件的变化比白云岩溶解更敏感.这些发现在解释和揭示自然界白云岩和灰岩岩溶发育及其相关资源环境问题的差异方面具有重要意义.Abstract: The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surface; (2) mass transport of ions into solution from rock surface via diffusion; and (3) conversion reaction of CO2 into H+ and HCO3-. Generally, it is the slowest of these three processes that limits the dissolution rate of carbonate rock. However, it was found from experiments and theoretical analysis that under similar conditions, not only are the initial dissolution rates of dolomite lower by a factor of 3 to 60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, under conditions of CO2 partial pressures (pCO2) >100 Pa, limestone dissolution rates increase remarkably, by a factor of about 10 after the addition of carbonic anhydrase (CA) into the solution, which catalyzes the conversion reaction of CO 2. For dolomite, the increase of dissolution rate after the addition of CA appears at pCO2 < 10 000 Pa. The enhancement factor of CA on dolomite dissolution rates is much lower (a factor of only about 3). In addition, though dissolution of both limestone and dolomite is also determined by hydrodynamics (rotation speed or flow speed), especially under pCO2 < 1 000 Pa, the dissolution of limestone is more sensitive to hydrodynamic change than the dissolution of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resources and environments of dolomite and limestone areas.
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图 1 旋速3 000 r·min-1 (a)和100 r·min-1 (b) 时溶解速率与CO2分压的关系
对于灰岩, 加入CA对溶解速率的增加主要发生在CO2分压>100 Pa条件下, 而对白云岩, 则出现在CO2分压 < 10 000 Pa条件下
Fig. 1. Dissolution rate vs. pCO2 for rotating speeds at 3 000 r·min-1 (a) and 100 r·min-1 (b). Addition of CA increases the dissolution rates mainly at pCO2>100 Pa for limestone, andpCO2 < 10 000 Pa for dolomite
图 2 旋速3 000 r·min-1 (a)和100 r·min-1 (b) 时白云岩和灰岩溶解速率CA (碳酸酐酶) 增加倍数与CO2分压的关系
增加倍数等于1 (虚线) 表明CA对溶解速率的增加没有影响
Fig. 2. Factor of CA enhancement as a function of pCO2 at rotating speeds of 3 000 r·min-1 (a) and 100 r·min-1 (b). Factor of 1 (dashed line) means no influence of CA at all
图 3 加或不加CA条件下白云岩和灰岩溶解速率水动力增加倍数(R3 000 r·min-1/R100 r·min-1) 与CO2分压的关系
增加倍数等于1表明旋速对溶解速率的增加没有影响
Fig. 3. Factor of hydrodynamic enhancement (R3 000 r·min-1/R100 r·min-1) as a function of pCO2 under the condition with or without addition of carbonic anhydrase. Factor of 1 (dashed line) means no influence of hydrodynamics at all
图 4 白云岩(a)和灰岩(b)溶解时[CO2(aq)]/[H2CO3]比率在扩散边界层内的变化
为加入CA后CO2转换因子, 即CO2转换反应速率增加的倍数
Fig. 4. [CO2(aq)]/[H2CO3] in the diffusion boundary layer as a function of distance to the bulk solution when dolomite (a) and limestone (b) dissolution happens. Here, fCO2 is the factor of rate increase of the CO2 conversion due to the addition of CA fCO2
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