Purpose

Antibiotics often coexist with heavy metals in contaminated soils, making their behaviors more complicated than their individual presences. Sorption to soil components deeply affects the mobility, bioavailability, and fate of contaminants. However, limited studies have investigated the cosorption characteristics of tetracycline (TC) and heavy metals to typical soil minerals, particularly phyllosilicates.

Materials and methods

This study investigates the effect of Cd(II) on TC sorption by kaolinite (Kao, 1:1-type) and montmorillonite (Mont, 2:1-type) via isothermal adsorption, coupled with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).

Results and discussion

Isothermal results show that TC sorption on both kaolinite (Kao, 1:1-type) and montmorillonite (Mont, 2:1-type) is little affected by Cd(II) at pH 4.5 but increases significantly at pH 8.5. The overall effect of Cd(II) on TC sorption depends on the intensities of two positive processes, i.e., (1) the formation of soluble Cd-TC complex increases sorption compared to free TC molecules and (2) the formation of a ternary clay-Cd-TC complex at clay variable edge sites increases sorption, and one negative process, i.e., competition of Cd(II) with cationic and zwitterionic TC species at the permanently charged sites decreases sorption. XRD analyses demonstrate that TC intercalates into the interlayer (d₀₀₁ = 18.7 Å) of Mont as one monolayer parallel molecule both with and without Cd(II) but adsorbs predominately on external surface of Kao. FTIR results suggest a flat-lying horizontal configuration of adsorbed TC molecules on clays, accompanied by a strong complexation between the carbonyl and amino groups of TC molecules and the surfaces. Besides, the presence of Cd(II) more or less facilitates this complexation.

Conclusions

We propose that Cd(II) can significantly promote TC adsorption on clay minerals at alkaline pH rather than acidic pH. TC and Cd(II) interact intensely and form strong surface complexation on clay surfaces. Hence, coexistence with Cd(II) may lower the mobility and thus the bioavailability of TC in alkaline soils, which provides a new direction for soil remediation by regulating soil pH using alkaline materials.

Graphical abstract

中文翻译：

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pH和矿物质类型对四环素在土壤层状硅酸盐上与Cd（II）的结合

目的

抗生素通常与重金属共存于受污染的土壤中，因此其行为要比其单独存在更为复杂。对土壤成分的吸附会严重影响污染物的迁移率，生物利用度和命运。但是，有限的研究已经研究了四环素（TC）和重金属对典型土壤矿物（尤其是页硅酸盐）的吸附特性。

材料和方法

本研究通过等温吸附，X射线衍射（XRD）和傅里叶耦合研究Cd（II）对高岭石（Kao，1：1型）和蒙脱石（Mont，2：1型）TC吸附的影响变换红外光谱（FTIR）。

结果与讨论

等温结果表明，高岭石（Kao，1：1型）和蒙脱石（Mont，2：1型）上的TC吸附在pH 4.5时几乎不受Cd（II）的影响，但在pH 8.5时显着增加。Cd（II）对TC吸附的总体效果取决于两个积极过程的强度，即（1）可溶性Cd-TC络合物的形成与游离TC分子相比增加了吸附，以及（2）三元粘土的形成粘土可变边缘位点处的-Cd-TC复合物增加了吸附作用，而一个负过程，即Cd（II）与阳离子和两性离子TC物质在永久带电位点上的竞争降低了吸附作用。XRD分析表明TC插入中间层（d ₀₀₁Mont（18.7Å）是具有和不具有Cd（II）的一个单层平行分子，但主要吸附在Kao的外表面。FTIR结果表明，吸附的TC分子在粘土上呈水平分布，并伴有TC分子的羰基和氨基与表面之间的强络合。此外，Cd（II）的存在或多或少促进了这种络合。

结论

我们建议Cd（II）可以显着促进TC在碱性pH而非酸性pH下对粘土矿物的TC吸附。TC和Cd（II）相互作用强烈，并在粘土表面形成强表面络合。因此，与Cd（II）共存可能会降低TC在碱性土壤中的迁移率，从而降低其生物利用度，这通过使用碱性物质调节土壤pH值为土壤修复提供了新的方向。

图形概要