Flow Injection Analysis of Indigo Carmine Using Green Coconut (Cocos Nucifera L.) Fiber as a Bioadsorbent
Abstract
This paper deals with a procedure for preconcentration and determination of indigo carmine in textile industrial effluent. The product of Indigo blue sulphonation, 5,5´-disulfonic indigotin (5,5´-DI) aqueous solution was run through a mini-column packed with green coconut fiber (Cocos Nucifera L.) in a flow injection system. The 5,5´-DI was preconcentrated on the sorbent at pH 2 and sample flow rate 3.9 mL min-1 and eluted by HCl 0.1 mol L-1 to be detected by UV-vis spectrophotometry at 612 nm. An increase of 18.7% in the indigo recovery was observed with fiber washed with albumin and 11.7% in fiber washed with NaOH. A linear response was observed within the range 0.157 to 75.2 µmol L-1 , with detection limit, coefficient of variation and sampling rate estimated as 0.0376 µmol L-1 (95% confidence level), 4.5% (n = 21) and 101 determinations per hour, respectively. The enrichment factor achieved was 1.64. The proposed method was successfully applied to the determination of 5,5´-disulfonic indigotin in textile industrial effluents.
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Introduction
The textile industry has a large environmental impact because it generates a significant volume of wastewater, which sometimes is discharged without treatment[1]. Some dyes and organic compounds from these industrial effluents are highly toxic and chemically stable[2,3]. Disodium salt of Indigotin 5,5´-disulphonic acid, FD & C Blue 2 or Acid Blue W (IUPAC name 3,3-dioxo-2,2-bis-indolyden-5,5-disulfonic acid disodium salt) is popularly known as indigo carmine. It is considered as a highly toxic indigoid class of dye[4]. Apart from its use as a textile-coloring agent and additive in pharmaceutical tablets and capsules, indigo carmine is also used for medical diagnosis[5]. Several methods have been reported for quantitative determination of Indigo carmine, including chromatographic[6], voltammetric[7] and spectrophotometric[8,9] ones. New treatment technologies have been developed in order to meet the legal requirements regarding effluent disposal of this dye as well as to reduce the operating costs of such processes[10]. Coconut fiber (Cocos Nucifera L.) has been used as a low cost alternative bioadsorbent for the removal of pollutants (dyes and other natural and inorganic substances) from natural waters[11-13]. Also, the retention capacity of several metallic ions in aqueous solutions has been previously reported using coconut fiber as the adsorbent [14-17]. This is a sustainable alternative because of waste reuse. Approximately 60 million tons of coconut were produced worldwide in 2008: 85% in Asia, 8.5% in the Americas, 2.9% in Africa and 3.2% in Oceania[18-20]. Specifically in Brazil, about 1.3 billion coconuts were produced in 2013, producing about 1.5 kg of solid residue per fruit; in general, 80% of this biomass is discarded in the environment[21-24]. The decomposition of coconut generates methane, which is a major greenhouse gas [25-28]. Aiming reuse, the coconut husk can be processed to the condition of fiber, which is composed by cellulose, hemicellulose, lignin, pectin and various minerals[29-32]. This fiber can be used in the preparation of composites, such as bioplastics, natural adsorbents and raw materials for several processes[33- 35]. Flow-based systems are an alternative to develop a new analytical procedure that focuses on the reduction of reagent consumption and waste [36,40]. Moreover, flow systems minimize the analyst intervention in chemical analysis, increase the number of samples that can be processed at a time and improves measurements precision[41-45]. Flow injection spectrophotometry has been proposed for determination of indigo carmine dye[8] but the use of coconut fibers as biosorbent for this dye has not been found in the literature. This study aimed the use of coconut fiber as a biosorbent for retention of the organic compound 5,5'-disulfonic indigotin (5,5´-DI), a sulphonated species of Indigo blue dye (Fig. 1). The sorption process was carried out in a flow injection system aiming analyte separation and preconcentration before spectrophotometric determination. In addition, the appropriate conditions for the adsorption and the elution of the 5,5´-DI were determined, as well as the mass of dye retained per mass of green coconut fiber, which can be used for an upscale project to be implemented to recover 5,5’-DI from industries wastewater.
Conclusion
The FIA-SPE method[55] developed and described in this paper provides a simple and efficient procedure for the preconcentration and determination of indigo blue. The lowest 5,5´-DI concentration analyzed was 3.76 × 10-8 mol L-1. It was possible to achieve a sampling rate of 101 samples per hour with a relative standard deviation of 2.03%. In the wastewater analysis, from textile industry, it was possible to recover 75% of 5,5´-DI. There is also a potential to apply this kind of retrieval in textile industries that use dyes with similar physicochemical characteristics, such as Indigo Blue, in jeans industry. This kind of process could be an alternative to biological treatment, which is also often applied to the wastewater treatment[41,42].