Synthesis and characterization of carboxylic cation exchange bio-resin for heavy metal remediation
Graphical abstract
Introduction
Biomaterials are composed of various surface functional groups and effective in removing heavy metals even if their use is limited due to leaching of lignin, pectin, and tannin [1], [2]. The chemical modification of biomaterials has gained the attention of researchers to overcome these problems [3], [4]. Metal removal capacities of biomaterials depend upon its density, swelling behavior as well as the type of cellulose, i.e., cellulose I or cellulose II [5], [6]. The crosslinking, grafting and/or functionalization in favorable experimental conditions can improve leaching resistance, swelling properties as well as thermal stability and, induce new functional groups on biomaterials [2]. It also increases the metal adsorptive capacity and selectivity for a specific application through the formation of different surface chelates [7]. Functionalization of lignocellulosic biomaterials usually involves a reaction of anhydride groups of the modifying agent with the primary alcoholic groups present on glucose units within the polymer. Cyclic anhydrides such as succinic, phthalic, EDTA, and maleic anhydride are often used to introduce the functional groups such as carboxylic, ester, and amine on lignocellulosic biomaterials [8]. However, the reactive groups of biomaterials are tightly packed within the lignocellulosic mass and are inaccessible in ordinary conditions. However, boiling water or treating with acid or alkali treatment could expose them [3]. Treatment of biomaterials with alkali is known as mercerization; a wherein, the crystalline arrangement of cellulose I is converted into cellulose II. It solubilizes a part of cellulose, degrades lignin, and hydrolyzes ether bond forming water soluble phenolic compounds [5]. It exposes the −OH groups, which could react with cyclic anhydrides to arouse functional sites, effective for heavy metal binding [3], [9]. It also helps to reduce leaching of lignin, pectin, and tannin.
Selection of biomaterial for modification is the foremost; it should be abundant, cheap and locally available, to minimize adsorbent production cost. India is one of the major producers of arecanut, and in India, Assam holds the 3rd rank [10]. Despite its abundance and cheap availability, utilization of arecanut husk is minimal, and most of it is thrown away as solid waste. Arecanut husk contains about 44% cellulose, 28% hemicellulose, and 11% lignin [11]. It has high ash content and is difficult for the enzymatic digestion, which limits its application as a biofuel feedstock [11]. Studies on arecanut utilization are also scanty.
Ethylenediaminetetraacetic dianhydride (EDTAD) is cyclic anhydride comprising of two anhydride groups in a molecule, which reacts with amide and/or alcoholic groups of biomaterials forming chelating groups such as ethylenediamine-N,Ń-diacetic acid (EDDA), carboxylic, amine, and/or iminidiacetic (IDA). These groups are the potential binding sites for the metal complexation in wastewater [12]. A study on Zn(II) removal reported that the carboxylic groups are involved in metal binding at higher pH, whereas, Zn(II) removal in acidic pH is facilitated by IDA groups [8]. Carboxylation of various agricultural materials using EDTAD for heavy metal removal is studied by several authors [6], [12], [13]. Modification of baker’s yeast biomass by acylation reaction was conducted, wherein 3 of 4 carboxylic groups were successfully attached to the ionized biomass. The modified biomass showed adsorption capacities up to 10 and 14 times for Pb(II) and Cu(II), respectively than that of the unmodified biomass [12]. Removal of cationic dyes by EDTAD modified sugarcane bagasse showed about 4.2 and 6.7 folds increase in adsorption capacity over the unmodified biomass [13], [14]. Mercerized sugarcane bagasse showed Cu(II) adsorptive capacity of 92.6 mg/g against 66.7 mg/g for raw biomass [6].
The fundamental insights such as the accessibility of the impregnated −COOH groups for metal binding from the theoretical considerations are not yet studied and, further, there is an imperative need for its validation from the experimental studies.
In this paper, a cation exchange bio-resin is synthesized from arecanut husk using EDTAD as a carboxylating reagent. Two-stage synthesis method for the conversion of arecanut husk into a cation exchange is proposed. The synthesized bio-resin is characterized using elemental analysis, thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), and Fourier transform infrared (FTIR) spectroscopy. Furthermore, the concentration of −COOH groups impregnated is determined by the potentiometric titration using a proton adsorption model (PAM) and, validated from the gravimetric analysis (based on N-content). The synthesized cation exchange bio-resin was successfully applied for the removal of Pb and Cd from a battery industry wastewater. The possible chemical regeneration of spent bio-resin is explored to investigate the practical implication to industrial wastewater.
Section snippets
Chemical and reagents
All solutions were prepared from salts of either analytical reagent or laboratory reagent grade chemical in Milli-Q water (conductivity <5.5 × 10−8 S/m) (Millipore S.A.S., France). Lead nitrate (minimum assay 99%) was obtained from S.D. Fine Chemicals, Mumbai, India. Sodium nitrate (99% purity), ammonium ferrous sulphate hexahydrate (purity 98.5%), nitric acid (minimum assay 69%), hydrochloric acid (minimum assay 35%), sulphuric acid (purity 90%), sodium hydroxide (minimum assay 98%), EDTA
mpg/mpl and CI of mercerized husks
The extent of mercerization was assessed in terms of mpl and CI. The X-ray diffractograms of MHs were used for the determination of CI at various test conditions and shown in Fig. S2 of Supplementary Material. It can be seen that the mpl was increased from 16 to 40% and CI (Eq. (3)) was decreased from 49 to 27% (Fig. 1) with the increase in NaOH concentration from 10 to 20%. The mpl was increased only 2% and, the CI was decreased by 1% with 30% NaOH concentration. So, NaOH concentration of 20%
Conclusions
The study develops a two-stage protocol for the synthesis of carboxylic cation exchange bio-resin. The carboxylation took place by acylation of anhydride group of EDTAD with −OH group of MH through an ester linkage. Concentration of carboxylic groups present on FMH was found to be 0.735 mM/g, determined by a gravimetric method using proximate & ultimate analyses at the optimal synthesis conditions. The carboxylating agent, EDTAD, was mostly ridged in the vesicles opened by mercerization as
Acknowledgements
The authors acknowledge the instrumental facilities provided by the Central Instrumental Facility (CIF) of IIT Guwahati for characterization of FMH in FESEM and BET surface area analyzer. Authors are thankful to Sophisticated Analytical Instrument Facility (SAIF) of Guwahati University for XRD analysis of FMH. The authors are also thankful to Exide industries, India, for allowing collection of battery wastewater to carry out this research. This research did not receive any specific grant from
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