Reactivity of biogenic silica: Surface versus bulk charge density
Reactivity of biogenic silica: Surface versus bulk charge density
复制标题
DOI：
10.1016/j.gca.2009.10.038
复制
发表时间：
2010-01
影响因子：
5
通讯作者：
S. Loucaides;T. Behrends;P. Cappellen
中科院分区：
文献类型：
--
作者：
S. Loucaides;T. Behrends;P. Cappellen
酸碱滴定进行了在三个不同的离子强度（0.01，0.1和0.7M NaCl）的范围内的海洋和大陆生物硅质材料。各种材料的充电行为的大的可变性与两个可电离基团池的存在一致，一个在二氧化硅颗粒的外表面上，另一个在二氧化硅颗粒内。内部和外部硅烷醇的相对量估计通过拟合两个站点的络合模型，以获得在不同的离子强度的过量质子与pH值曲线。对于新鲜的硅藻硅藻壳和植硅体，以及最近沉积的生物硅质沉积物，丰富的内部硅烷醇的数量级相同，或超过，外表面上的硅烷醇。较老的生物硅质材料表现出较低的内部基团的比例，而减少内部硅烷醇的相对量也观察到人工老化的硅藻硅藻壳在海水中。内部可电离官能团的存在解释了生物二氧化硅的电荷密度，大大超过二氧化硅表面的理论密度。内部与表面硅烷醇相对丰度的变化进一步解释了电荷对离子强度的非均匀依赖性，总电荷密度和溶解动力学之间缺乏相关性，以及生物二氧化硅红外光谱中950 cm − 1峰强度的变化。溶解速率与外部电荷正相关，而不是总电荷积累，如预期的那样，如果溶解仅涉及从颗粒的外表面去除硅酸盐单元。随着时间的推移，内部与外部的硅烷醇浓度比的逐渐减少代表了改变影响地球表面环境中生物二氧化硅的回收和保存的材料性质的机制之一。
Acid–base titrations were carried out at three different ionic strengths (0.01, 0.1 and 0.7M NaCl) on a range of marine and continental biosiliceous materials. The large variability in electrical charging behavior of the various materials is consistent with the existence of two pools of ionizable groups, one on the outer surface of and the other within the silica particles. The relative amounts of internal and external silanols were estimated by fitting a two-site complexation model to excess proton versus pH curves obtained at the different ionic strengths. For fresh diatom frustules and phytoliths, as well as recently deposited biosiliceous sediments, the abundance of internal silanols was of the same order of magnitude as, or exceeded, that of silanols on the external surface. Older biosiliceous materials exhibited lower proportions of internal groups, while a decrease in the relative amount of internal silanols was also observed for diatom frustules artificially aged in seawater. The existence of internal ionizable functional groups explains measured charge densities of biogenic silicas that largely exceed the theoretical site density of silica surfaces. Variations in the relative abundance of internal versus surface silanols further explain the non-uniform dependence of electrical charging on ionic strength, the lack of correlation between total charge density and dissolution kinetics, and the variable 950cm−1peak intensity in the infrared spectra of biogenic silicas. Dissolution rates correlate positively with the external charge, rather than the total charge build-up, as expected if dissolution only involves the removal of silicate units from the external surfaces of the particles. The progressive reduction with time of the internal to external silanol concentration ratio represents one of the mechanisms altering the material properties that affect the recycling and preservation of biogenic silica in earth surface environments.