Removal of Aromatic Amines and Heavy Metals from Wastewater Using Nanoadsorbent

Abstract

Existence of water has led to evolution of life on our magnificent planet earth. But regrettably, quality of our water resources is declining continuously due to population explosion, industrialization, civilization, domestic activities and environmental changes. 5– 10 billion tons of industrial effluents is generated annually worldwide, out of which maximum amount is dumped untreated into rivers, lakes and oceans leading to water pollution. More than seven hundred organic (azo-dyes, benzene, phenols, DDT, aromatic amines etc.) and inorganic pollutants (heavy metals like arsenic, cadmium, chromium, cyanide, fluoride, lead, mercury, selenium etc.) have been reported in water. Some of these pollutants like aromatic amines and heavy metals are alarming because of their highly toxic, mutagenic and carcinogenic nature and urgently need to be removed from wastewater. Chemical compounds with one or more aromatic rings along with amino substituents (-NH2, -NH- or nitrogen group(s)) are classified as aromatic amines. They are well known components in wide variety of wastewater including those from textile and dye processing industries, synthetic polymer industries, tanneries, oil refining processes and several other chemical process industries. Carcinogenic and mutagenic nature of aromatic amines are known since 1895 due to which twenty-two types of aromatic amines including 4-chloro-otoluidine (4-COT) and 4-Amino-bi-phenyl (4-ABP) above 30 mg/L in effluents have been restricted by European Union Standards (1907/2006), therefore need to be treated before discharge. On the other hand, heavy metals are the high atomic weight and high density (≈5 times higher than water) elements present naturally in our ecosystem. Zinc is present in the earth’s crust in abundance. However, its increasing concentration in wastewater as Zinc ion [(Zn (II)] owe to various commercial manufacturing units results to certain adverse diseases including skin irritations, vomiting, stomach cramps and anaemia. Chromium, contrarily is present in our surroundings in two forms: trivalent [Cr (III)] and hexavalent [Cr (VI)]. Cr (VI) is about five hundred times more noxious than [Cr (III)] due to its ability to oxidize additional species. Innumerable industries are responsible for introduction of excess Cr with wastewater causing eyes infection, allergy, asthma, dermatitis, cancer and skin irritation including burns. Therefore, various agencies like World health organization (WHO) and Environmental protection agency (EPA) have set a bearable limit for these heavy metals in industrial effluents. Numerous physico-chemical techniques (precipitation, coagulation, flocculation, adsorption, ion-exchange); electrochemical methods and biological techniques are available in literature for removal of both aromatic amines and heavy metals from wastewater. However, these techniques have been demonstrated to be unfavourable because of high cost involvement, high energy consumption and production of large volume of toxic by-product as sludge. Adsorption is reported to be very efficacious for wastewater treatment with simple operational conditions. Many authors used activated carbon for organics removal from wastewater including aromatic amines like diphenylamine (DPAM), napthylamine (NAM), and aniline. Similarly, various researches had efficiently used activated carbon prepared from agricultural waste for heavy metals (Zn (II), Cr (VI) and Pb (II)) removal. However, its use is confined due to their limited adsorption capacity and high quantity requirement. Therefore, to overcome these demerits, latterly investigators have focused on the development of adsorbents possessing larger surface area and pore size such as zeolites and mesoporous silica nanoparticles. Discovery of ordered mesoporous materials has led a new path towards the research on synthesis and application of these materials. Mobil Composition of Matter No. 41 (MCM-41) is a synthetic-ordered mesoporous silica characterised by high surface area, high pore volume with uniform pore size distribution exhibiting excellent thermal, hydrothermal and hydrolytic stability. A number of researchers have reported removal of azo dyes from textile wastewater using MCM-41 as adsorbent. Adsorptive removal of aromatic amines such as p-chloroaniline, benzidine, aniline, 4-methylaniline and naphthalamine on polymeric nano-adsorbents has been reported by various authors but only single study for the adsorption of aromatic amine aniline onto MCM-41 has been reported so far. No study was available for the adsorptive removal of aromatic amines 4-chloro-o-toluidine (4-COT) and 4-amino-bi-phenyl (4-ABP) using MCM-41. If we consider heavy metal ions adsorption onto MCM-41, numerous work has been reported. However, no research has been reported on zinc removal using MCM-41. Also literature lacks study on simultaneous removal of two or more metal ions. No work has been reported concerning the multicomponent system for heavy metals removal using MCM-41 including Zn (II) and Cr (VI). Considering these gaps in literature, present study has been undertaken in which MCM-41 was synthesized and characterize for enhanced adsorption of aromatic amines (4- chloro-o-toluidine, 4- aminobiphenyl) and heavy metals (Cr 6 +, Zn2+). Effect of various adsorption parameters such as contact time, pH, adsorbent dose, adsorbate concentration and temperature on removal of both aromatic amines and heavy metals has been studied followed by kinetic and thermodynamic studies. MCM-41 nanoparticles were synthesized via hydrothermal method. Synthesized material was then characterized by various characterization techniques including XRD, BET, FTIR, FE-SEM and TEM. BET surface area of synthesized MCM-41 was found to be 502.77 m2 g–1. BJH model exhibited 0.85 cm3 g–1 pore volume with 3.21 nm pore size of MCM-41. Nitrogen adsorption/desorption isotherm plot obtained from BET analysis, was found to be of type IV with H1 hysteresis loop which confirms the mesoporous nature of synthesized MCM-41. XRD spectra of MCM-41 exhibited a single peak at 0.98, which is associated with the hexagonal mesophase structure. Since the material is not crystalline at the atomic level, no reflections are observed at higher angles. FE-SEM images clearly revealed the spherical uniform structure of MCM-41 within the size of 70-100 nm. TEM micrographs showed that the MCM-41 is hexagonal, and possesses evenly distributed pore system with a uniform mesoporous channels array. Batch adsorption experiments were performed for aromatic amines and heavy metals removal. Optimum conditions for 4-COT and 4-ABP treatment were obtained as: pHi =2, m=0.2 g/L and t=2h. Pseudo-first-order and pseudo-second-order kinetic models were applied to study the adsorption kinetics. Further, adsorption rate controlling mechanism was investigated using intra-particle diffusion model. Interactions of 4-COT and 4-ABP molecule with the surface of MCM-41 were investigated and adsorption process controlling mechanism was explored. Langmuir, Freundlich, Temkin and D-R isotherm (Dubinin- Radushkevich) models were used to represent the adsorption equilibrium data. Effect of temperature for adsorption of 4-COT and 4-ABP onto MCM-41was studied at C0 values ranging from 10-300 mg/L, pHi= 2, mopt=0.2 g/L and t= 2h by varying temperature (T) in the range of 283-318 K. It was observed that the adsorption of both 4- COT and 4-ABP onto MCM-41 increases with rise in temperature due enhanced movement of the amine molecules to the MCM-41 surface. Thermodynamic parameters were obtained from the linear plot of ln KD versus 1/T. Value of KD was found to decreases with raised T The positive ΔH◦ value illustrates the occurrence of energy barrier in amines adsorption on MCM-41, which reveals the endothermic nature of adsorption. Positive value of ΔS◦ indicates high degree of freedom of the adsorbed adsorbate. Negative ΔG◦ values represent the amines adsorption onto MCM-41 is feasible and spontaneous. Similarly, adsorptive interaction of toxic Zn (II) heavy metal in aqueous solution with synthesized mesoporous silica based MCM-41 was investigated. Further, the effects of presence of Cr (VI) heavy metal on the adsorption of Zn (II) and vice versa were investigated by simultaneous competitive binary adsorption from Cr (VI) and Zn (II) aqueous binary solution. Optimum conditions for Zn (II) treatment were obtained as: pHi =7, m=0.1 g/L and t=2h. Competitive adsorption of both Cr (VI) and Zn (II) from their binary solution was studied at 288 ̶ 308K temperatures. Obtained equilibrium data were then fitted to different competitive multicomponent isotherm models like Langmuir (extended), Freundlich isotherm (extended), Sheindrof-Rebuhn-Sheintuch (SRS) isotherm and Redlich-Peterson isotherm (modified) models, and various isotherms and interaction parameters were calculated. The adsorption capacity (qe) upsurges with the increase in C0 and T values, showing adsorption process as adsorbate limiting. It was observed that at any temperature, for persistent Cr (VI) concentration, the equilibrium adsorption uptake of Zn (II) upsurges with increase in initial Zn (II) concentration, whereas, the equilibrium adsorption uptake of Zn (II) declines uninterruptedly with increasing the Cr (VI) concentration. Similar trend was seen for Cr (VI) adsorption with increasing concentration of Zn (II). Zn (II) and Cr (IV) adsorption from their binary aqueous mixture was found antagonistic type. The modified R-P isotherm model suits best to the experimental data obtained from binary adsorption for Zn (II) and Cr (VI) onto MCM-41 at 308 K with lowest MPSD value of 74.47, followed by extended-Freundlich isotherm model and SRS model.