J. Chem. Chem. Eng. 10 (2016) 147-152 doi: 10.17265/1934-7375/2016.03.006 D DAVID PUBLISHING Chromate and the Environment: Removal and Utilization of Industrial Waste Fernando B. Mainier, Pedro Paulo B. Leite, Marcone F. Reis and Thiago Teobaldo Silva Engineering School, Federal Fluminense University(UFF), Niterói, Rio de Janeiro 24220-261, Brazil Abstract: Chromate and dichromate sodium as a function of oxidizer characteristics are used in several industrial areas; for example, in surface protection of coated parts of cadmium, zinc and aluminum (chromate coated treated), corrosion inhibitors, the treatment of leather, the manufacture of pigments, etc. However, the use of such products has been questioned due to the problems of toxicity and pollution that can be caused in the environmental. The Brazilian environmental agency has established that the concentrations of 2- chromate in water courses are less than 0.5 ppm. In order to reuse chromate (CrO4 ) from industrial effluent, laboratory experiments have been proposed based on chemical reduction or electrolytic processes, in order to transform these chromate ions in a final mix of oxides (in solid form), which can then be packed and sent to the production process of sodium chromate. The results of these experiments have become useful industrially (without regard to costs) considering the environmental reuse and the life cycle of the chemical compound. Key words: Chromate, dichromate, contamination, chemical reduction, electrolytic process. 1. Introduction waste, among others [1, 2]. In order to individualize the problem of Industrial production processes, especially chemical contamination of workers and environmental pollution, ones, continue to exert strong pressure on the chromate and dichromate are highlighted, usually environment, in view of the inherent risks of their called hexavalent chromium compounds, because, due manufacturing processes in e.g.: the security industry, to their oxidizing characteristics, such products are used contamination, tailings, effluent, packaging, reuse, in various industry segments. Certain studies have recycling, toxic metals and toxic waste, which often argued that such products are harmful to human health, get confused or are interlinked. because contact can promote dermatitis, and inhalation Large industrial complexes such as oil and gas and ingestion promote irritation and ulceration in the production, petrochemical metallurgical industries, for intestines, stomach and the risk of lung cancer [3-5]. the most part, become polluters and end up having an Fig. 1 presents chromite ore (FeO·Cr2O3) that is the impact upon the world with their waste, becoming main raw material for applications in different enemies of society, sustainability and man himself. industries, such as: 76% for production of chromium They give the impression of not taking precautions in and chromium alloys [6, 7]; 13% for refractory bricks relation to the present or the future, according to leaks, and 11% for chemicals [8-10]. accidents and disasters that occur in various parts of Currently, chromite ore is extracted and purified in the world. Some of these disasters can be associated South Africa (39%), India (16%), Kazakhstan (15%), with obsolete technologies and the direct or indirect and other countries as Brazil, Finland, Oman, Russia use of recycling or recovery of packaging and toxic and Turkey (representing 21%). In the 1970s to 1990s in Brazil, most of the chromite ore was intended for Corresponding author: Fernando B. Mainier, Ph.D., professor, research fields: corrosion, corrosion protection, the production of chromate [11]. Currently, chromate materials and environment. is imported from South Africa. 148 Chromate and the Environment: Removal and Utilization of Industrial Waste Fig. 1 Industrial routes using chromite ore. Globally, roughly 11% of chromite ore and chrome generated. It is believed that the treatment or waste are intended for the production of chemical processing of waste should be carried out by those compounds, mainly: chromic oxide (CrO3) for companies that generate the source of pollution chromium plating, sodium chromate (Na2CrO4), (segregated treatment), because usually a great effort sodium dichromate (Na2Cr2O7), lead chromate is accompanied by high cost of treatment to recover (PbCrO4), chrome alum (KCr(SO4)2·12H2O), etc. The the final system when, in fact, it is contaminated [1, 15]. quality of these products depends on the origin of the The reactions for obtaining sodium chromate and raw material and industrial processing; therefore, the industrial sodium dichromate from chromite ore are treatment of waste and the final disposal of this waste presented below: 4FeO.Cr O + 8Na CO + 7O → in landfill are both essential. 2 3 2 3 2 2Fe O + 8 Na CrO + 8CO (1) In this industrial scenario, application of the rules 2 3 2 4 2 2Na CrO + H SO → and regulations of Clean Technologies [1, 12] and the 2 4 2 4 Na Cr O + Na SO + H O (2) premises of LCA [13, 14] of materials (i.e. from the 2 2 7 2 4 2 cradle to the grave from raw materials to final landfill) To illustrate the use of segregated waste treatment, are important bases of environmental sustainability the use of chromate as a corrosion inhibitor in a and human health protection from the toxicity of such cooling system was chosen. Generally, the chromate 2- products. concentration (CrO4 ) can vary from 50-600 mg/L Clean technologies should be established in order to depending on the operating conditions and the water meet the demands for sustainability and to ensure that used, mainly in relation to the content of chloride the environment, public health, worker safety, and the (Cl-). safety of the inhabitants surrounding industrial To evaluate the removal and reuse of chromate, in complexes are protected using methods that reduce the form of “chromite”, double oxide of iron (II) and energy, and programs that reuse or recover raw (III) chromium (4FeO·Cr2O3) were selected two materials, in order to reduce the amount of waste processes. Chromate and the Environment: Removal and Utilization of Industrial Waste 149 The first is based on chemical reduction using 2. Materials and Methods reducing substances such as: sodium sulfite (Na SO ), 2 3 The test of chemical reduction consisted, essentially, sodium bisulfite (NaHSO ), ferrous sulfate (FeSO ), 3 4 of a glass container 500 mL in volume, equipped with sulfur dioxide (SO ), whose reactions are listed below: 2 a magnetic stirrer, in which were placed 200 mL of 2Na CrO +5H SO + 3Na SO → 2 4 2 4 2 3 sodium chromate solution referred to as the chromate 5Na SO + Cr (SO ) + 5H O (3) 2 4 2 4 3 2 2- ion (CrO4 ). The chromate solutions evaluated were 4Na CrO + 7H SO + 6NaHSO → 2 4 2 4 3 100, 200 and 600 mg/L, were used a high purity of 7Na SO + 2Cr (SO ) + 10H O (4) 2 4 2 4 3 2 sodium chromate. The pH for each solution was 2 Na CrO + 6FeSO + 8H SO → 2 4 4 2 4 adjusted in the range of 2.7-2.8 using a solution of 2 2Na SO + 3Fe (SO ) + Cr (SO ) + 8H O (5) 2 4 2 4 3 2 4 3 2 M H2SO4. The masses of Na2SO3, NaHSO3 and 2Na CrO +2H SO + 3SO → 2 4 2 4 2 FeSO4 used in the chemical reduction were 2Na SO + Cr (SO ) + 2H O (6) 2 4 2 4 3 2 determined, stoichiometrically and using, respectively, Neutralization of the acid solution and the Eqs. (3-5). To guarantee the reduction of chromate precipitation in their hydroxides and calcium sulfate 2- 3+ ions (CrO4 ) from the chromium ions (Cr ), the (CaSO4) can be made using calcium hydroxide or calculated masses were multiplied by 1.1. Then, sodium hydroxide, whose reactions are presented aiming at the removal of precipitates, were added to below: the resulting solution sodium hydroxide or calcium Cr2(SO4)3+ 3Ca(OH)2→ 2Cr(OH)3↓ + 3CaSO4↓ (7) hydroxide based on earlier Eqs. (7, 9-11), with an Na2SO4+ Ca(OH)2→ CaSO4↓ + 2NaOH (8) increase of 1.1 of the mass calculated. Finally, the Fe2(SO4)3 + 3Ca(OH)2→ 3CaSO4↓ + 2Fe(OH)3↓ (9) solution is filtered through a filter press to remove the Cr2(SO4)3+ 6NaOH → 2Cr(OH)3↓ + 3Na2SO4 (10) precipitate formed and, in the resulting solution, the Fe2(SO4)3 + 6NaOH → 3Na2SO4 +2Fe(OH)3↓ (11) chromate content was determined that had not been The second process is based on reduction by ferrous removed. 2+ ions (Fe ) obtained by the electrolytic dissolution of In the experiments based on the reduction by carbon steel electrode by applying a continuous ferrous ions (Fe2+), an electrolytic cell (Fig. 2) electric current, in acidic medium (H2SO4), as shown consisting of an acrylic container with a maximum in the following reactions: capacity of 500 mL was used, containing the anode 2+ Anodic reaction: Fe - 2e → Fe (12) and the cathode of carbon steel connected to a power + Cathodic reaction: 2H + 2e → H2 (13) supply of adjustable direct current (current rectifier) Ferrous sulfate (FeSO4) formed in solution, reacts with a maximum voltage of 30 V. with chromate as shown in the reaction: The current ranged from 0.2-0.6 A and the time 2Na CrO + 6FeSO + 8H SO → 2Na SO + 2 4 4 2 4 2 4 ranged from 10-30 minutes, were placed in the cell 3Fe (SO ) + Cr (SO ) + 8H O (14) 2 4 3 2 4 3 2 200 mL of high purity sodium chromate solution (100, 2- Later, the solution is neutralized with calcium 200, 600 mg/L), referred to as the chromate (CrO4 ) hydroxide or sodium hydroxide as shown in the ion. The pH for each solution was adjusted in the following reactions: range of 2.7-2.8 using a solution of 2 M H2SO4. Cr2(SO4)3+ 3Ca(OH)2→ 2Cr(OH)3↓ + 3CaSO4↓ (15) Some of the operational conditions, such as the time Na2SO4+ Ca(OH)2 → CaSO4↓ + 2NaOH (16) and current/voltage for this experiment, were based on Fe2(SO4)3 + 3Ca(OH)2→ 3CaSO4↓ + 2Fe(OH)3↓ (17) Faraday’s law, Cr2(SO4)3+ 6NaOH → 2Cr(OH)3↓ + 3Na2SO4 (18) m = Zit (20) Fe2(SO4)3 + 6NaOH → 3Na2SO4 + 2Fe(OH)3↓ (19) where, 150 Chromate and the Environment: Removal and Utilization of Industrial Waste platinum electrode.