Bis-pyridinium dibromides modified organo-bentonite for the removal of aniline from wastewater: A positive role of π–π polar interaction

Highlights

• HEMBP-B was firstly prepared and applied in the treatment of aniline wastewater.

• π–π polar interaction played a significant role in aniline adsorption on HEMBP-B.

• It provided an idea for the removal of aromatic contaminants from aqueous solution.

Abstract

Aniline removal is of great importance for water treatment, and adsorption is an efficient treatment method. Hexamethylene bis-pyridinium dibromide modified bentonite (HEMBP-B) was firstly prepared and characterized by XRD, FT-IR, BET, SEM, TEM and TG–DTG analysis. Adsorptive experiments of aniline on the organo-bentonite were carried out using batch method as a function of the length of linking carbon chain (n), concentration of HEMBP (C), contact time (t), temperature (T) and pH. The optimal conditions for aniline removal on HEMBP-B were as follows: n of 5 or 6, C of 1.0 CEC, t of 120 min, T of 298 K, and natural pH of 6.6. The π–π polar interaction existed between the aromatic rings of aniline and HEMBP has successfully explained the main adsorption mechanism. The equilibrium data of aniline removal on HEMBP-B followed Freundlich and D-R isothermal models precisely, which indicated a less favorable and physical process of aniline adsorption. The kinetic data could be best described by the pseudo-second-order model. Besides, the thermodynamic study revealed that aniline adsorption on HEMBP-B was non-spontaneous, exothermic and physical.

Introduction

Aniline is an important raw material used extensively in the areas of dyestuff, rubber, pharmacy, plastic, paint and other chemical industry. However, its existence could cause various health issues and environmental problems. Aniline-containing wastewater even in very low concentration has been shown to be harmful to human and aquatic life. Once aniline intrudes the body through pneogaster, digestive tract and skin absorption, it will convert hemoglobin to methemoglobin and results in cyanosis by influencing oxygen supply of tissue cells. So the increasing attentions have been paid to the development of aniline removal technologies. Several processes to remove aniline from wastewater have been described, including adsorption [1], [2], biodegradation [3], [4], advanced oxidation [5], [6], photo-catalysis [7], [8] and some other processes [9], [10]. Among these technologies, adsorption has been proven to be an effective way in treating organic wastewater through decades of research [11], [12]. Activated carbon is the most well-known and high efficiency adsorption material because of its larger surface area and pore volume. But its high cost of regeneration limits the further application in wastewater treatment [13].

Bentonite is also recognized as an effective adsorption material for the treatment of organic contaminants because of its high specific surface areas, low cost and ubiquitous presence in most soils [14]. But the natural bentonite must be modified before practical application in order to overcome its intrinsic disadvantages, such as the stronger hydrophilicity at surface and lower organic carbon content [15], [16]. Organic surface modification is one of the most extensive researches among those modifying methods, and the modifiers are usually conventional quaternary ammonium salts with long chain or other kinds of surfactants, whose removal mechanism to organic contaminants is mainly due to the partition adsorption [17], [18]. Compared with those researches, there have been very few reports about the wastewater treatment by the use of organo-bentonite modified from the bivalent cationic compounds with short chain. Furthermore, it is well-known that most organic contaminants are aromatic compound (e.g. benzene, phenol, aniline and so on), and there might be other more effective interaction between aromatic contaminant and the modifier if the aromatic ring is introduced into the structure of modifier. In other words, the π–π polar interaction between the two kinds of aromatic rings might improve the removal efficiency. For this purpose, hexamethylene bis-pyridinium dibromide (HEMBP, Py⁺(CH₂)₆Py⁺·2Br⁻) was taken into account due to its short linking carbon chain and heteroaromatic ring in the structure.

In this paper, a novel organo-bentonite (HEMBP-B), prepared from the reaction of hexamethylene bis-pyridinium dibromide (HEMBP, Py⁺(CH₂)₆Py⁺·2Br ^–) and natural Na-bentonite (Na-B) through wet-process, was applied in the removal of aniline from aqueous solution. The effects of the length of linking carbon chain in bis-pyridinium dibromide, concentration of HEMBP, contact time, temperature and pH on aniline adsorption were investigated. The adsorption mechanism of aniline on HEMBP-B was obtained by the comparison of adsorption capacities of Na-B, HEMBP-B and hexamethonium bromide modified bentonite (HM-B). The adsorption isotherms, kinetics and thermodynamics modeling of aniline were also intensively studied. The results obtained from this work would not only enrich the species of adsorbents for the treatment of organic wastewater, but also provide the theoretical basis for its further research.