论文期刊

Scale dependence of surface complexation capacity and rates in heterogeneous media




作者: Li Wang, Hang Wen, Li Li*
发表/完成日期: 2018-05-06
期刊名称: Science of the Total Environment
期卷: 635
相关文章:
论文简介
Reactive solutes flow, transport, and transform in natural subsurface that is ubiquitously heterogeneous. How
much are the heterogeneity effects on solute reactive transport? At what spatial scale heterogeneity effects di-
minish? Answers to these questions are important to understand controls of and to predict solute reactive trans-
port in natural environments. Although these questions have been explored for non-reactive tracers and solutes
that go through relatively simple single-component sorption, heterogeneity effects remain poorly understood for
solutes that go through surface complexation. These reactions typically includemulti-component and are subject
to influences of local geochemical conditions. Thiswork aims to address these questions and to understand how
and how much effective capacity (Csc,m), early (kc1) and late (kc2) rates of Cr(VI) surface complexation are influ-
enced by heterogeneity characteristics and length scales. A two-dimensionalmodelwith Cr(VI) surface complex-
ation were calibrated using column data. Numerical experiments were carried out in heterogeneous media
characterized by different permeability variance σ2
lnκ (0.2, 4.5), correlation length λL (0–6.0 cm), and domain
length (0.1–1.0m). Results showthat long λL and high σ2
lnκ facilitate the formation of preferential flowand elon-
gate the diffusion length, leading to high connectivity that minimizes Csc,m and kc1. In highly connected media
(N2.0), Csc,m and kc1 can be N2 orders ofmagnitude lower than those in corresponding homogeneousmedia. Con-
nectivity however is not a good indicator for kc2 that is predominantly controlled by diffusion. The heterogeneity
effects are scale dependent. As the domain length increases, Csc,m and kc1 values approach those of homogeneous
media due to longer mixing lengths and continued pH increase during Cr(VI) surface complexion. The “critical
length” at which heterogeneity effects disappear occurs at about 20λL and differs only slightly at different σ2
lnκ
values. The heterogeneity effects however linger atmuch longer lengths for kc2, the values ofwhich are N2 orders
of magnitude lower in heterogeneous media even at 20λ because of the longer characteristic times of diffusive
transport. These results underscore the strong regulation of solute reactive transport bymineral spatial patterns,
suggesting the importance of subsurface characteristics and spatial scales atwhich they are important in estimat-
ing natural attenuation, residence times, and natural system capacity to sorb chemicals.
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