Process development for the batch and bulk removal and recovery of a hazardous, water-soluble azo dye (Metanil Yellow) by adsorption over waste materials (Bottom Ash and De-Oiled Soya)

Abstract

Bottom Ash and De-Oiled Soya have been used as adsorbents for the removal of a hazardous azo dye—Metanil Yellow from its aqueous solutions. Adsorption of Metanil Yellow on these adsorbents has been studied as function of time, temperature, concentration and pH. Batch adsorption studies, kinetic studies and column operations enabled extraction of lethal dye from wastewaters. Adsorption equilibrium data confirms both Langmuir and Freundlich isotherm models and monolayer coverage of dye over adsorbents. Kinetic data have been employed to calculate specific rate constants, indicating thereby involvement of first order kinetics in the on-going adsorption and activation energy was determined as 0.813 and 1.060 kJ mol⁻¹ for Bottom Ash and De-Oiled Soya, respectively. For both adsorbents, the adsorption process has been found governing by film diffusion, over the entire concentration range. Column operations have also been performed for the bulk removal of the dye and also to examine the practical utilization of fixed bed adsorption technique in elimination of dangerous effluent. Saturation factors for Bottom Ash and De-Oiled Soya columns have been calculated as 99.15 and 99.38%, respectively. Attempts have also been made to regenerate the dye from the exhausted columns using aqueous sodium hydroxide as eluent.

Introduction

Removal of toxic industrial, water-soluble non-biodegradable waste, particularly organic dyes, is of great concern. Removal of colored waste from aqueous effluents is of significant environmental, technical and commercial importance and that is why various physico-chemical techniques like, coagulation, adsorption, chemical oxidation and froth floatation, etc., have been used for the removal of organics as well as inorganics from wastewaters. Using sophisticated instruments, electrochemical processes like electro coagulation [1], electrochemical degradation [2], electrochemical oxidation [3] and photoelectrocatalytic methods [4], [5] the task to eradicate toxic pollutants from water have also been accomplished. Amongst all these techniques adsorption is considered one of the most efficient one, due to its easy methodology and operations. The most commonly used adsorbent is activated carbon, which is considered as an expensive sorbent, making wastewater treatment a cost-challenging step. In past one decade, low cost adsorbents like, Chaff [6], Soymeal hull [7], Palm kernel fibre [8], Pinus pinaster bark [9], Sugar beet pulp [10], etc., have emerged as potential alternative resources for removing water-soluble toxic materials from contaminated water. In recent past, our laboratories have also developed some waste materials like Bottom Ash, De-Oiled Soya, Hen Feathers, etc., [11], [12], [13], [14], [15] as effective and efficient adsorbents.

Present studies explore the feasibility of using Bottom Ash and De-Oiled Soya as adsorbents for the removal of a hazardous coloring agent, Metanil Yellow from wastewaters. Metanil Yellow is highly water-soluble and belongs to azo group of the dyes. Although Metanil Yellow is a non-permitted color, but still it is widely used as a colorant in sweet meat, ice creams, soft drinks and beverages. Due to its orange-yellow color, the dye is also extensively used for coating turmeric. It is widely used in leather, paper and textile industries [16] and also as a stain and colorant for the wool [17], [18]. It is also used as coloring material for lacquers and cosmetic products. The dye is highly suitable for the preparation of colored water-fast inks [19] and can also be employed for determining trace amounts of Mo(VI) [20]. Apart from all these, Metanil Yellow can act as an indicator for the determination of H⁺ ion concentration in the pH range of 1.2–2.3 [21].

Toxicity data reveals that oral feeding or intraperitoneal and intratesticular administration of Metanil Yellow in animals produces testicular lesions due to which seminiferous tubules suffer damage and rate of spermatogenesis is decreased. On oral consumption, it causes toxic methaemoglobinaemia [22] and cyanosis [23] in humans, while skin contact results into allergic dermatitis [24]. Metanil Yellow also has tumour-producing effects and can also create intestinal [25] and enzymic [26] disorders in human body. Though it is not mutagenic but can alter the expression of genes [27]. Thus, keeping the hazardous effects of the dye in view, attempts have been made by various workers to remove Metanil Yellow from wastewater [28], [29], [30], [31], [32]. Present work is also an attempt to formulate an easy, reliable and feasible method for its removal from the wastewaters.

Two adsorbent materials—Bottom Ash and De-Oiled Soya chosen for the present studies are purely waste materials. Bottom Ash is obtained from thermal coal-fired power generation plants and appears as granules of dark gray black color. Its disposal in the surrounding lands is always a problem to the concerned authorities because it makes the agricultural land barren and unfit for cultivation [33], [34]. The adsorbent, De-Oiled Soya is obtained from Soyabean oil extracting mills as a by-product after extracting all possible nutrients of Soyabean. Captivatingly, our state Madhya Pradesh is one of the leading producers of the Soyabean crop and a large number of Soyabean oil extracting plants are surrounding our city. It is a porous and dry flaky material with brownish white color. About a decade back it was used as animal and fish feed but nowadays it is a banned edible substance due to toxicity as well as bitter taste. The toxicity of De-Oiled Soya is due to use of more than 170 ppm of hexane as solvent during oil extraction [35], [36], [37], while bitterness is due to the formation of anti-metabolites, like lipoxygenase and trypsin, during the oil extraction [38], [39].