Article Cite This: J. Chem. Eng. Data 2018, 63, 4241−4247
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Mechanistic Understanding of the Adsorption Behavior of Metal Lead Ions by Attapulgite-Induced Porous Nanocomposite Hydrogels Xinyou Mao,† Yanyan Duan,† and Chuanyi Wang*,†,‡ †
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Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics and Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, People’s Republic of China ‡ School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China ABSTRACT: Hydrogels have many advantages as a kind of heavy-metal adsorbent, such as simple separation, high efficiency, and recyclability, but their applications are limited by their poor mechanical strengths. To this end, versatile natural phyllosilicate minerals are introduced into hydrogels to enhance the mechanical strengths of hydrogels. However, mechanistically understanding the clay-induced effect is still limited. In this work, attapulgite-induced porous network structures were observed, which are interpreted in terms of the nonsacrificial starch-like pore formation mechanism. Furthermore, the influence of the porous structure of the attapulgite nanocomposite hydrogels on the adsorption of Pb2+ from an aqueous solution was comprehensively studied. The adsorption kinetics fits a pseudo-second kinetic model well, and the syllogistic Weber−Morris model study reveals that the porous structures of nanocomposite hydrogels improves their ability to internally diffuse. The adsorption isotherm displays over 98.2% of goodness of fit for the Langmuir isothermal model, with a maximum adsorption capacity of 864.5 mg/g for the adsorption of Pb2+. Moreover, the adsorption thermodynamics study indicates that the enthalpy changes of chemical adsorption decrease from 62.31 to 51.01 kJ/mol for hydrogels after the addition of 10% attapulgite. On the basis of the above findings, a reasonable adsorption mechanism including surface adsorption and internal diffusion is proposed. adsorption.15 The introduction of macroporous structures into hydrogels is one of the ways to solve this problem.16 Normally, pore-forming agents or porogens are added into the hydrogels.17,18 Nevertheless, pore-forming agents may cause contamination, and porogens need to be eluted, thus resulting in complicated processes. In our previous work,19 natural attapulgite, a kind of versatile natural clay, was introduced into sodium alginate−poly(acrylic acid) hydrogel, which results in porous structure, good mechanical property, and reusability for the efficient adsorption of Cu2+ and Pb2+ from aqueous solution. To fully take advantage of such polymeric clay mineral composites as adsorbents for heavy-metal removal, a mechanistic understanding of the formation of polymeric composites and relevant adsorption processes is crucial. Continuing from our previous work,19 the purpose of this work is to explore the mechanism for making porous microstructures by attapulgite. In particular, the effect of attapulgite on the adsorption behavior of Pb2+ by nanocomposite hydrogels is thoroughly studied by revealing the involved adsorption kinetic, isotherm, and thermodynamic
1. INTRODUCTION Metallic lead is an essential industrial raw material1,2 but may cause pollution in mining, smelting, and consumption.3 Lead ion (Pb2+) is a kind of accumulated toxic pollutant, which will destroy the ecological environment and endanger human health,4 especially to impair the intellectual development of children.5 Thus lead pollutant must be banned from the source of industrial pollution, especially industrial wastewater. Adsorption is a simple and feasible method to remove heavy metals from wastewater.6 Hydrogels, as novel potential heavymetal adsorbents, have been extensively studied for treating wastewater in recent years due to their advantages in simple separation, high efficiency, and recyclability.7,8 However, the applications of hydrogel-based heavy -metal adsorbents are limited by their poor mechanical strengths.9,10 Many reports show that the mechanical strengths of hydrogels can be enhanced by introducing an optimum amount of natural clay into the semi-interpenetrating polymer network (semi-IPN) hydrogels.11,12 In recent years, such ternary composite hydrogels, which consist of natural linear polymers, synthetic polymer monomers, and inorganic clay, are becoming a popular system.13,14 On the contrary, the larger the hydrogel size, the slower the rate of internal diffusion and the lower the rate of water © 2018 American Chemical Society
Received: August 22, 2018 Accepted: October 22, 2018 Published: October 30, 2018 4241
DOI: 10.1021/acs.jced.8b00744 J. Chem. Eng. Data 2018, 63, 4241−4247
Journal of Chemical & Engineering Data
Article
Figure 1. Schematic expression of the synthesis of nanocomposite hydrogels. (i) Free radical production, (ii) chain initiation, (iii) chain growth, and (iv) chain cessation.
Figure 2. Scanning electron microscopy (a), transmission electron microscope (b), and schematic networks (c) of hydrogels. Scanning electron microscopy (d), transmission electron microscope (e,f), and schematic networks (g) of hydrogel with 30% attapulgite. Note that loose surfaces and porous 3D structures are produced after introducing attapulgite.
Scanning electron microscopy (SEM) images were obtained at 20.0 kV on a field-emission scanning electron microscope (ZEISS SUPRA55VP) after gold plating. A transmission electron microscope (TEM, FEI Tecnai G20, US) was used to observe the morphologies and structures of the hydrogels. X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI) was conducted at 150 W using Al Kα.
processes. By comparing the difference in the adsorption behavior between hydrogel alone and hydrogels with attapulgite, a plausible mechanism was proposed.
2. EXPERIMENTAL SECTION The nanocomposite hydrogels were synthesized by following the protocol developed in our previous work,19 as schematically described in Figure 1. The test of adsorption was conducted via a batch adsorption experiments. The concentration of Pb2+ in lead chloride solution was measured by an inductively coupled plasma spectrometer (ICP−OES). It must be pointed out that all of the values of the adsorption capacity were measured three times, and the error ranges were all
