Surface modified layered double hydroxide/polyaniline nanocomposites: Synthesis, characterization and Pb²⁺ removal

Highlights

• Calcium and iron based LDH intercalated citrate anions was modified by silane coupling agent.

• Nanocomposites of modified LDH and aniline was prepared by in situ polymerization.

• There is no accumulation between LDH in the polymer and good dispersion was observed.

• Nanocomposites were used for the adsorption of Pb (II) from aqueous solution

• The kinetic and adsorption isotherm were identified.

Abstract

In this study, Ca/Fe layered double hydroxide (LDH) with good accessible chemically active sites and with intercalate anions of citrate was prepared, and then 3-aminopropyl triethoxysilane as a silane coupling agent was used to modify the LDH surface. Then, in order to improve the thermal stability and removal efficiency toward bivalent Pb²⁺, polymer matrix nanocomposites (PMNCs) were fabricated by in situ polymerization of aniline in the presence of modified LDH. The as-synthesized PMNCs were characterized by XRD, FT-IR, TGA, TEM, and FE-SEM. Morphological studies revealed that modified LDH were dispersed homogeneously in polymer matrix without any aggregation. Finally, polyaniline (PANI) and synthesized PMNCs were used as adsorbents to removal of Pb²⁺ from aqueous solution. The effective parameters affecting metal adsorption including contact time, pH and initial concentration were studied and optimized. Also, the sorption kinetics and isotherm of these sorbents were investigated. The results revealed that the kinetic adsorption could be designated by a pseudo- first order model for the removal of Pb²⁺. Comparing the results of Langmuir and Freundlich equations showed that NC10 % with more adsorption capacity (q_max) and adsorption constant (K_l) were more suitable adsorbent than NC5 % and PANI for removal Pb²⁺. The results of R² values show that the Langmuir model showed the best fit on the lead absorption data by PANI and the synthesized PMNCs.

Introduction

The increasing worldwide contamination and heavy metals originated from rapid industrialization due to release of thousands of natural chemical and industrial compounds are becoming one of the great concerns facing humanity [1,2]. Unlike organic pollutants, heavy metals have a tendency to agglomerate in living organisms because they are not biodegradable and they tend to accumulate in living organisms [3]. For humans, prolonged exposure to heavy metals can seriously destruction to mental and central nervous function, lungs, blood components, liver and other vital organs [4]. Research has indicated that lead ions (Pb²⁺) as one of the high-risk and high toxicity heavy metal that is abundant in the environment [5,6]. The probable symptoms (Signs) and risks of lead includes anaemia, renal kidney disease, cancer, mental retardation, impaired intellectual aptitude and behavioral harms [7,8]. Infants, and the fetus are the most susceptible to lead than adults, exposure to lead in childhood has been cause the mental decline, slow bone growth, kidney damage, hyperirritability, stupor, intelligence quotient (IQ) decrease, aggression and delinquency [9,10]. The main Sources of contaminations of Pb²⁺ include paint production, battery manufacturing, pesticides, smoking, automobile emissions, pigment industry, burning of coal, and mining. As a result of these risks and concerns, it has created a potential impetus for the development of methods for correcting and removing toxic heavy metals in water [[11], [12], [13]]. Conventional techniques that have been used to removal of heavy metal ions include membrane filtration, sorption, electrochemical treatment, precipitation, and ion exchange [14,15]. Among them, sorption system has been widely used as it is economical, easy operating conditions, operates simply, high metal binding abilities, and cost-effective [16,17]. Zeolites, activated carbon and agricultural residues are the group of adsorbents that have been used for elimination of toxic metal ions [4,18]. Despite the benefits of these adsorbents, their low adsorption abilities, their weak interactions with metallic ions and difficulties of regeneration of them from water has limited their usage [7,19]. To overcome these constraints, recently, polymer based nanocomposites (NCs) with inorganic filler have been used for the removal of toxic species from wastewater because of their relatively high external surface areas, high adsorption kinetics, their intensive adsorption, and low toxicities [20].

Layered double hydroxides (LDHs) are in the category of the nano-ordered layered compounds which can be well prepared in the laboratory. The generic formula of LDH can be represented as [M^II_1-xM^III_x(OH)₂]^x+Aⁿ⁻_x/n. mH₂O that M²⁺, M³⁺ symbolizes divalent and trivalent metal and A^n- is an inorganic or organic interlayer anions. LDHs due to their chemical and physical properties are surprisingly similar to clay minerals [21]. Recently, LDHs have shown promising potential in the field of catalyst precursors, catalyst supports [22], electrocatalytic activity [23,24], bioactive moieties, biomedical applications, delivery of drug [24], superb adsorbents due to their layered structure [25], ion exchange properties, large interlayer space, and variety of chemical compositions [22,26].

Polyaniline (PANI) is currently recognized as a unique conjugated polymers that it has unique properties, such as low prices, ease of synthesis, adjustable electrical conductivity, high capacitive characteristics, and good environmental stability. According to this capabilities of PANI, it is a good candidate in many attractive applications include supercapacitors, photocatalytic activities, electronic devices, electronic displays, batteries, activators, and biological sensors [27,28]. PANI has adsorption sites for grafting organic and inorganic molecules owing to the presence of imine and amine functional groups on its backbone. Zhu et al. found that the polyaniline modified Mg/Al layered double hydroxides (LDHs) composites had high adsorption capacity to Cr(VI) removal [29]. Gao et al. synthesized organic-inorganic hybrid of polyaniline/titanium phosphate with an excellent removal capability for Re(VII) [30].

In the present study, NCs of polyaniline and 5 %, 10 % of silane modified LDH were investigated for the adsorption of Pb (II) from aqueous solution. For this purpose, at first, LDH based on calcium and iron with intercalate bio-safe anions of citrate have been synthesized via one-step co-precipitation and hydrothermal technique. Then, 3-aminopropyltriethoxy silane coupling agents was used to modify the surfaces of LDH and to stronger interaction between polymer matrix and LDH. PMNCs were prepared by in situ polymerization of aniline in the presence of modified LDH. The grafted intercalated LDH in PANI matrix on the thermal stability behavior, interaction, morphology, and adsorbents properties of the corresponding NCs were investigated and these findings were compared with pure PANI. For studying the optimum adsorption conditions, the effects of pH, contact time, heavy metal ion concentration on the adsorption of Pb(II) on the PANI/LDHs were carried out using batch adsorption experiments.