Obtaining a Bioadsorbent from Orange Peel suitable for Batch and Continuous Treatment
Abstract
One form of chemical contamination involves the contribution of heavy metals to the ecosystem mainly from industrial spills and mining operations. The most toxic heavy metals are cadmium, copper, chromium, mercury, nickel, lead and zinc. The importance of this type of toxic lies in the tendency to be accumulated and concentrated by the different species, being more dangerous as it ascends the evolutionary chain towards man.
Chemical precipitation is the most widely used technique for metal recovery. This process is conditioned by the pH, metal concentration and anions present in the water to be treated.
Bioadsorption is considered a viable alternative to the physico-chemical methods currently used for the recovery or removal of heavy metals dissolved in liquid effluents. Its main attractiveness, from an industrial point of view, is its low cost due to the great abundance, easy obtaining and low price of the bioadsorbent material. Bioadsorption is very effective in the treatment of metal concentrations below 100 mg/L, where the application of physical-chemical methods is not technically and economically feasible.
One of these materials of interest is orange peels because, due to their abundance as a waste product of the food industries, they have problems for their disposal and currently have little economic value. However, this residue has alow adsorption capacity, so both physical and chemical modifications are required to increase its adsorption properties. The objective of this work has been to optimize the treatment of orange peel intended to obtain a bioadsorbent that allows the removal of heavy metals both in a discontinuous process (Batch) and in an ongoing process. The verification of the characteristics of the bioadsorbent obtained has been carried outwith a series of synthetic solutions of Cu (II). The particle size and consistency of the final bioadsorbent has been optimized. In addition, to achieve a homogeneous elution in the continuous process, additional heat treatment has been necessary to prevent the development of microorganisms in a period of timeless than one week.
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Introduction
The described wastewater treatment techniques are divided into three sections: a) Separation or clarification techniques, which are mainly used in combination with other operations, or as a first step (to protect other facilities treatment against damage, obstruction or solids fouling) or a final clarification step (to remove solids or oil formed during a previous treatment operation). b) Physicochemical treatment techniques for non-biodegradable wastewater, mainly used for inorganic or non-biodegradable organic contaminants. c) Biological treatment techniques for biodegradable wastewater (Ministry of Environment, rural and marine environment of Spain 2009).
Traditionally, the elimination (but non-recovery) of heavy metals present in wastewater from industrial processes has been carried out in a majority way through precipitation processes of these metals as hydroxides in the middle basic. This technique has two drawbacks: on the one hand, the high solubility of some of the formed species, which entails alow elimination of the metal of interest of the dissolution to be treated and, therefore, requires a post-treatment that adjusts the concentration of heavy metal in the effluent to the environmental regulations of discharge, and on the other, the management of the sludge generated, which contain high concentrations of metal ions. The usual precipitation agents are: lime (calcium hydroxide), caustic soda, sulphides and sodium and calcium carbonates (Metcalf & Eddy, 2014). Inmost cases, calcium hydroxide is the most effective reagent because it results in the formation of very stable precipitates and has the ability to destabilize colloids.
Alternatively, among several methods, adsorption is one of the treatments most accepted for its versatility and simplicity. This technique, based on a physical process that is produced by weak long-range interactions (van der Waals forces), allows particles, molecules, or ions to be trapped or retained on the surface of a material, is highly effective for the removal of heavy metals has removal of a wide variety of contaminants, with rapid kinetics (Liu & Lee, 2014).
In the removal processes it is sought to prevent molecules from transforming or degrading, by breaking or by exchange of functional groups, avoiding the generation of compounds more reactive and toxic than the original compound (Sivaraj, et al; 2001).
Because of its high organic matter adsorption capacity, activated carbon is the most widely used adsorbent material, so it is considered a conventional adsorbent (McDougall 1991), having prepared different types of activated carbons from walnut shells (Yalcin & Sevinc 2000), eucalyptus bark (Bello, et al. 2002), corncobs (Abdel-Nasser, et al. 2001), and other residues. However, its high cost of production limits its application in wastewater treatment. Clays, biopolymers, zeolites, silica beads and plants or lignocellulosic wastes are some of the adsorbents, commonly used to remove ion dyes, heavy metals, radioactive materials among other organic pollutants and generated by different types of industries (Osei Boamah, et al; 2015).
Bioadsorption is the application of low-cost materials obtained from different biomasses from microbial flora, algae and agro-industrial waste to replace the use of conventional methods in the removal of contaminants (Hala, 2013). Bioadsorption has emerged as an ecological alternative to conventional technologies for the treatment of effluents containing concentrations of diluted metal (Suresh et al., 2015). It is usually applied in the removal of heavy metals by passive binding to non-living biomass from aqueous solutions (Feng et al., 2011).
Currently, the term bioadsorption, is used to describe the phenomenon of passive uptake of pollutants, based on the property that certain types of inactive or dead biomass possess to bind and accumulate different types of contaminants. This accumulation does not occur by metabolic mechanisms, which is what happens in bioaccumulation and that occurs with living cells (Adebayo et al., (2018).
The biomaterials used in these processes (rice, orange, lemon, etc.) actin short contact times and generate high quality effluents by different mechanisms (Sharma et al., 2007). In the presence of heavy metals, the most widely accepted mechanism is the ion exchange (Davis et al., 2003). Recently, new types of biomaterials have been introduced (Ouldmoumna et al., 2013).
Another area of bioadsorption use as an alternative process (economic and with an acceptable environmental impact) is that of wastewater in the textile industry (Sivaraj, et al 2001). Traditionally, these wastewaters have been treated with physical and chemical processes that are expensive to remove the dyes present. These processes incur operating and maintenance costs that most small industries cannot absorb (Lu et al., 2010). Bioadsorption mechanisms in this field are being investigated and it has been found preliminaryly to be by complexation, ion exchange, or by formation of hydrogen bonds (Zumriye 2005).
The bioadsorption process involves a solid phase (biomass) and a liquid phase (water) containing dissolved substance of interest that will be adsorbed (in this case, Cu2+ ions). In order for the bioadsorption process to be successful, there must be a strong affinity between the functional groups of biomass and the contaminant, since the latter must be attracted to the solid and bound by different mechanisms (Chojnacka 2010). In all cases, biosorption processes depend on the nature of the substance to be removed, the structure and characteristics of the bioadsorbent (specific area, diameter and volume of pores, surface loading, active sites, chemical composition) and experimental conditions (Gautam, et al.2014). One of these materials of interest is orange peels because, due to their abundance as a waste product of the food industries, they have problems for their disposal and currently have little economic value. However, this residue has alow adsorption capacity, so both physical and chemical modifications are required to increase its adsorption properties (Feng, et al 2010), (Left, et al 2013). However, many by-products can be obtained such as essential oils, carotenoids, flavourings and other derivatives applicable in the food, pharmaceutical and cosmetic industries (Pacheco, et al. 2019).
Conclusion
Checked: The importance of particle size in the chemical treatment of orange peel (500-1000 µm). The importance of a correct elimination of organic matter in acid attack (elimination of non-methylated pectin and carbohydrates) to prevent the formation of a bioadsorbent in the form of a gel that makes it difficult to exchange cationic both in batch and continuous. The development of microorganisms in the final bioadsorbent mass subjected to a period of three or more days of hydration.
It has been determined: Adjustment to the Langmuir model at concentrations below 250 mg/L of Cu(II). Adjustment to the Brunauer model when concentrations below 250 mg/L of Cu(II) have been taken into account together in conjunction with those above 250 mg/L of Cu(II). The temperature necessary to prevent the development of microorganisms has been to heat treat the final bioadsorbent for a period of 24 hours at 110oC. Optimal pH for Cu(II) bioadsorption at 4,4 pH units.
It has been established: The final bioadsorbent, dried at 110 oC, at pH 4,4 has a yield of 653 mL/gbioadsorbent to reduce an initial concentration of 40 mg/L of Cu(II) to a final concentration of 2 mg/L in a continuous process with a flow rate of 26,04 mL/min.