Preparation of nano-materials and their recyclable application in the removal of organic dyes from aqueous solution

Abstract

In this study, different starch-based natural polymeric materials especially wheat powder (WP) and turmeric powder (TP) were collected from local market. Then the glutinous substances of these powder samples were removed by continuous washing with water and filtering followed by fornication to prepare their functional forms for the efficient adsorption of dyes from aqueous solution. Starch nanoparticles (SN) was synthesized from wheat powder by nanoprecipitation method. Furthermore, nanocomposites of polyaniline (PANI) and starch were prepared in situ and ex-situ polymerization of aniline using starch as a filler material in order to improve the mechanical and physical properties of the starch. Moreover, dispersion of the starch into PANI matrix makes the prepared composites biodegradable. Both kinds of nanomaterials were characterized with infra-red spectroscopic (IR), thermal analysis (DTA), scanning electron microscopic (SEM) and x-ray diffraction techniques (XRD).

The average size of SN estimated from Debye-Scherrer formula is 9 nm. Besides, the average particle size of PANI/starch Composite is about 18 nm which is smaller than that of PANI (20 nm). Comparison of thermal properties of the composites with the pure starch and PANI ensured the presence of both the starch and PANI in the composites. A good match was found also in the IR spectrum of the materials.

Adsorption behavior of the raw biosorbents (WP, TP and Starch) and synthetic adsorbents i.e., PANI, PANI/starch composite-1 and PANI/starch composite-2 were compared for the two typical ionic dyes, specifically, cationic methylene blue (MB) and anionic orange green (OG) based on the removal of the dyes as a result of adsorption from their neutral aqueous solution at room temperature. In the case of MB dye adsorption, the adsorption efficiencies of TP, PANI, Composite-1 and Composite-2 were found to be maximum and comparable. The adsorption efficiencies of WP, pure starch and SN were reasonably low, but the efficiency of SN was higher than that of pure starch. High adsorption efficiency of SN was revealed by SEM and XRD analysis to be due to the smaller particle size. The OG dye was not adsorbed onto TP and other starch-based biosorbents i.e., WP, Starch and SN and showed very little adsorption capacities compared to the synthetic adsorbents PANI, Composite-1 and Composite-2.

Pure starch and starch-based WP and TP undergo biodegradation. Even though the SN has a higher efficacy than the pure starch, it has no experience of biodegradation. We estimated a rough calculation of the expenses of the biosorbents comparing with that of the synthetic conducting polymer adsorbents. Indeed WP and TP are too cheaper adsorbents. Moreover these adsorbents are biodegradable. Thus, we were interested in the detail adsorption study of these low-cost, biodegradable adsorbents which could also be recycled to produce alcohol and to recover the dyes for reuse.

In case of WP, the adsorption of MB is more favorable at low temperature. Conversely, for TP, the adsorption process of MB is more favorable at higher temperatures. Moreover, the adsorption of MB onto WP and TP follow the pseudo-second order adsorption mechanism and the adsorption is of Langmuir type.

With increasing the concentration of the electrolytes from 0 to 6 mM, the adsorption of MB dye onto TP was not decreased mostly but the adsorption was declined drastically onto WP. MB dye desorption from WP adsorbent was higher (80 %) in presence of divalent cation calcium ion rather in the presence of monovalent cation sodium (55 %). These ions might block the active sites of the adsorbents surface thus quite deactivating TP and WP towards MB dye.

It is very interesting to observe that OG dye didn’t undergo any adsorption onto TP and showed a small adsorption onto WP in aqueous solution of pH 6.86. But in presence of FeCl3.6H2O, the adsorption of OG both onto WP and TP rapidly increases in the same pH. It is because the [Fe (III)] coagulant forms complex with the starch content of WP and TP.

The recovery (desorption) of MB adsorbed onto WP was carried out by changing temperature from 29 ºC to 37 ºC. Percent recovery of MB was found 70 % at 29 ºC but 81 % at 37 ºC. The higher rate of desorption at higher temperature would be due to the increasing of thermal energy of the adsorbed species.