Study on adsorption mechanism of silicate adsorbents with different morphologies and pore structures towards formaldehyde in water

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Abstract

Silicate with various pore structures and morphologies showed different adsorption effects on removing organic pollutants, but the corresponding adsorption mechanisms are ambiguous. In this work, the adsorption mechanisms of formaldehyde in water by silicate adsorbents with different morphologies and pore structures were investigated. Diatomite with disc-like morphology and calcium silicate with short rod-like morphology were selected for comparison. Results show that the adsorption capacity of calcium silicate with superior specific surface area and pore volume is higher than that of Diatomite. The pseudo-first-order and pseudo-second-order models were employed to fit the dynamics behaviors of adsorption process, and the pseudo-second-order model was more suitable one. Furthermore, the adsorption data was also modeled by the internal particle diffusion model and it was found that the adsorption process could be divided into surface fast adsorption and internal particle diffusion. The adsorption mechanism of diatomite on formaldehyde includes surface adsorption and internal particle diffusion, while surface adsorption is the main mechanism of removing formaldehyde by calcium silicate. Four adsorption isotherm models are used to describe the isotherm adsorption process, while Freundlich isotherm shows the best fit results, which indicates that both adsorbents are multilayer adsorption. It can be classified as physical adsorption.

Introduction

Silicate based materials are the most abundant minerals in the earth’s crust, which are widely distributed and easy to collect. Due to rich porous structures and high specific surface area, silicate based materials have been used as adsorbents for the removal of environmental pollutants and have been intensively studied [1,2]. For instance, Zhang et al. [3] investigated the adsorption performance of modified kaolinite on Congo red; it is found that the specific surface area and pore volume of kaolinite increased and the corresponding Congo red removal efficiency increased by 70 % after chemical modification. Khalid et al. [4] researched the adsorption capacity of zeolites with various Si/Al ratios on phenol in water; the results show that a higher Si/Al ratio indicates higher removal efficiency and as the Si/Al ratio increases from 5 to 100, the corresponding removal efficiency increases from 15 % to 65 %.

It is known that specific surface area and pore structure of adsorbents are the key factors for adsorption capacity [5]. Different appearances and internal structures of adsorbents result in different specific surface areas and pore structures, which inevitably influence on the adsorption effect. Montmorillonite (MMT) is a typical silicate with nano-lamellar structure and can be used as adsorbents [6]. Elsherbiny et al. [7] mixed MMT with polyaspartate and prepared polyaspartate/MMT composite, which achieved a removal efficiency of 87.91 % and 29.84 % for Pb2+ and Cd2+ from water, respectively. This adsorbent shows adsorption selectivity for metal ions and high removal efficiency for Pb2+.

Different from MMT, diatomite with a disc-like shape is another commonly used adsorbent, and often be served as an adsorbing material in decorative coating to adsorb toxic gases (e.g. formaldehyde, ammonia and benzene). Liu et al. [8] investigated the adsorption capacity of diatomite and amine-modified diatomite for formaldehyde; the result indicated that adsorption capacity increased by approximately 3.5 mg g−1 after the amino modification.

Calcium silicate is a special adsorbent because that its morphology and pore structure depend on the preparation methods, and it shows significant differences in specific surface area and pore volume. Calcium silicate prepared by hydration has tremella-like surface structure and bring large specific surface area and pore volume. Wang et al. [9] prepared a tremella-like mesoporous calcium silicate hydrate with a hydrothermal method; this calcium silicate shows the specific surface area of 122.83  m2 g−1 and a pore size distribution range from 4 to 36 nm, which can remove formaldehyde effectively with the maximum removal efficiency of 98.94 %. Zhang et al. [10] prepared layered calcium silicate nanosheets with ultrasound-assisted sol-gel method, whose specific surface area ranges from 17.31 and 37.20 m2 g−1 and pore volume ranges from 0.0651 and 0.1779  cm3 g−1; the adsorption capacity could reach 109.4 mg g−1 during the phosphate adsorption process. Another shape of calcium silicate can be obtained by traditional calcination method; most of the calcined products are rod-like and this shape limits the specific surface area and porosity [11].

It can be seen from the above literatures, the adsorption mechanisms vary with the shapes of adsorbents. In this work, a comparative study about adsorption mechanism was carried out by selecting two kinds of silicate adsorbents with different morphologies and pore structures; this research aims to investigate the effects of morphology and pore structure on adsorption mechanism. Diatomite and calcium silicate are categorized as the typical silicate minerals with similar compositions, which are usually employed as effective adsorbents for wastewater treatment. However, the adsorption mechanism for diatomite and calcium silicate toward contaminants in aqueous solutions are meaningful to be differentiated due to the obviously different morphology and pore structures. On that account, disc-like diatomite (DL-D) and rod-like calcium silicate (RL-CS) were selected as the samples. Formaldehyde was chosen as target adsorbate, which is considered to be a pollutant with a large diffusion range and a great harm. The adsorption capacity and adsorption mechanism were studied systematically.

Section snippets

Adsorbents and reagents

Powder DL-D, was purchased from Lingshou Yanhui Mineral Products Trading Co., Ltd, Shijiazhuang, China. Analytical pure calcium silicate (RL-CS) powder was obtained commercially from Macklin Biochemical Technology Co., Ltd, Shanghai, China. Formaldehyde solution (37 %) was obtained from Damao Chemical Reagent Works (Tianjin, China). Analytical grade phenol reagent (C8H9N3S·HCl·H2O) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Hydrochloric acid solution

Specific surface area and pore structure

The specific surface area and pore structure of adsorbents are determined as the important factors to evaluate the adsorption capacity. The nitrogen adsorption-desorption isotherms and pore size distribution for DL-D and RL-CS are shown in Fig. 1.

As displayed in Fig. 1(a), the nitrogen adsorbed capacity of DL-D is very small, which indicates the weak interaction between adsorbent and adsorbate. The nitrogen adsorption-desorption isotherm of DL-D can be classified into type II according to BDDT

Conclusions

A comparative study has been performed between DL-D and RL-CS for formaldehyde adsorption, and the significant remarks are listed as follows:

  • (1)

    RL-CS with rod-like shape shows higher adsorption capacity than DL-D with disc-like shape due to the higher specific surface area and pore volume; the removal efficiency reached 81 % and 85 % by DL-D and RL-CS, respectively. The micro morphology of DL-D has changed a lot and surface pore size becomes smaller; while RL-CS has no obvious morphological

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Manman Wang: Writing - original draft. Bianying Wen: Conceptualization. Baomin Fan: Investigation. Huijuan Zhang: Methodology.

Acknowledgements

Financial supports from National Natural Science Foundation of China (No. 21606005), Beijing Municipal Natural Science Foundation (No. 2192016), Support Project of High-level Teachers in Beijing Municipal Universities in the Period of 13th Five-year Plan (No. CIT&TCD201904042), and the Innovative Research Team of New Functional Materials of Beijing Technology and Business University, are gratefully acknowledged.

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