Ultrasonic Radiation Influence on the Bioadsorbent Characteristics of Citrus (Citrus x Lemon) & (Citrus x sinensis)

Conventional methods for the treatment of wastewater with low concentrations of heavy metals in the ion state are extremely costly. For this reason, adsorption techniques have gained acceptance due to their effectiveness in removing pollutants that are too stable for conventional methods, resulting in high-quality effluents (Basso et al, 2002). Beyond research with living plants (Seki et al, 1988), the use of plant waste material for the recovery of heavy metals has been addressed. An adsorbent can be considered inexpensive if it requires little processing, is abundant in nature, or is a by-product or waste from another industry. (Bailey et al, 1999).

Generally, the bioadsorbents studied come from waste that has problems for reuse and which do not represent any economic value, a clear example of this is waste from industry and agricultural area (Crini, 2006). They have adequate adsorption capacity: pine bark (Al-Asheh et al, 2000), conifers (Aoyoma et al, 2000), rice husk (Feng et al, 2004), etc. In this sense, the easiest and most accessible bioproduct is plant biomass (Mani & Kumar, 2014).

The cell walls of bioadsorbent materials contain polysaccharides, proteins and lipids. Therefore, they contain numerous functional groups capable of linking heavy metals on the surface of these (Wei et al, 2010). Among the functional groups present are the amino, carboxylic, hydroxyl, phosphate and thiol groups that differ in their affinity and specificity with respect to susceptibility to bind to the different metal ions (Ghimire et al, 2003). However, it should be noted that the content in functional groups in the bioadsorbent material may be different depending on the species, the season, the geographical area, etc. The most commonly consumed fruit peels are apple (Mallampati & Valiyaveettil 2013), banana (Memon et al, 2009) orange (Feng et al, 2011), lemon (Tejada et al, 2015), and mango (Iqbal et al,2009). All of them should be included among widely used bioadsorbents. Citrus fruits are a diverse group of species native to tropical and subtropical regions of Asia that are cultivated in the world. The fruits of these species, in particular oranges, mandarins, lemons, limes and grapefruits, play an important role in the feeding of millions of people, whether as fresh fruit, concentrate, drink or in culinary preparations. The two largest producers are Brazil and the United States, with 21.4% and 14.5% of world production respectively. China, Mexico, Spain and India follow in importance, representing together 27.6% of the global total (FAO,2017).

Citrus essential oils are obtained from fruit bark (flavedo) (Tranchida et al., 2011). Different extraction techniques are employed, including direct methods such as shell compression and indirect methods such as steam stripping distillation and microwave radiation-assisted water (HDMO) distillation. The best method to extract themis steam distillation due to the variety of volatile molecules extracted, such as terpenes and terpenoids, phenol-derived aromatic components and aliphatic components (Palazzolo et al., 2013). However, this process requires several hours, high energy consumption, conventional heating and hot water agitation (Durán & Villa, 2014). HDMO is avery fast and relatively inexpensive process and the essential oils obtained are free of thermal decomposition products and contaminants. Recently, the extraction of microwaves without the addition of solvent has been introduced and using only water that is removed in situ from the tissues (Ferhat et al., 2006) obtaining higher yields and better quality of the oil in a shorter time.

Citrus pectin, a thickener commonly used in the food industry, is obtained from the extraction of albedo (white layer between the shell and pulp). However, inmost processes, the production of pectins is linked with the obtaining of essential oil (Cerón-Salazar & Cardona-Alzate, 2011). Alternatively, high methoxyl pectins have been obtained from orange peels by performing an acid extraction to the orange albedo by a low-pressure steam injection (15 psi) (Fishman et al, 2003). The industrial production of citrus pectin has the following stages: in the first, the peel must be washed to remove the greatest amount of soluble solids and impurities, as these components hinder the purification process. The peels are then subjected to a drying process, which inactivates the pectinaesterase enzyme and decreases the moisture content, increasing the stabilization of the peel for storage and reducing the cost of transport (Martí et al, 2014). Subsequently, the dry matter suspended in hot water with the necessary amount of a strong acid, starting the hydrolysis process.

During this process, starting from the macromolecular structure formed by cellulose, hemicellulose and pectins, hemicellulose begins its degradation to glucose, galactose and fructose; cellulose to glucose and pectin to pectin monomer through a depolymerization process (Chen et al, 2015). After a while, by filtration, the liquid phase is separated from the solid phase. Then the liquid phase is mixed with alcohol. As a consequence, the polymer that precipitates rapidly in the form of pectin is recovered. The precipitate is extracted and purified by washing it with more alcohol. Finally, it is dried and ground (Claus, 2002).

Depending on the source, pectins may vary in molecular size, degrees of acetylation and methylation, galacturonic acid content and neutral sugar residues. Therefore, pectins exhibit versatile gelling properties and are able to form complexes with other natural compounds and, as a result, are useful for designing food products (Gawkowska et al., 2018). The resulting material from the extraction of pectin is a poor dietary supplement for animals due to its low protein content and high in sugars (Siles et al, 2016). However, it presents the optimal conditions for further treatment as a bioadsorbent (Masmoudi et al, 2008).

Acid-soluble pectin extraction is necessary for its ability to absorb large amounts of water and the formation of colloids. If not extracted, the final product would not have a sufficient degree of consistency to be used as a bioadsorbent. Along with pectin, acidic hydrolysis involves the solubilization and degradation of carbohydrates, especially xylan and hemicellulose, since glucomanane is relatively stable in acid medium (Van Buren, 1991).

Accepted the acid attack as the first phase to increase the adsorption capacity of the citrus shell, the second phase consists of an attack in alkaline medium, intended for the saponification of ester groups that have not been extracted during the acid attack and subsequent crosslinking with Ca(II) (Cardona Gutiérrez et al, 2013). Usually this process is carried outwith NaOH, which is followed by crosslinking with CaCl to increase the activation sites. However, it is proven (Arjona et al., 2 2018) that both processes can be carried out in a single stage with the use of a Ca(OH) 0.2 M solution. 2 Process intensification is currently one of the areas with the greatest development potential in the food industry. Through more sustainable technologies, the aim is to increase the productive performance, quality and safety of the processed product, as well as reduce the size of equipment, waste and energy needs (Benali & Kudra, 2010). Among the different techniques that can be used for the intensification of processes (microwave, infrared, electrical pulses) it is worth highlighting the application of acoustic energy (Knorr et al, 2004). One of the main characteristics of power ultrasound is its ability to improve matter transfer processes (Cárcel et al., 2011).

In recent years there has been increased interest in the use of ultrasound to intensify the pectin extraction process as they not only increase the performance of the operation, but also reduce the extraction time compared to the conventional process (Bagherian et al., 2011; Minjares-Fuentes et al., 2014; Maran & Priya, 2015; Wang et al., 2015; Sundararaman et al., 2016; Freitas de Oliveira et al, 2016; Grassino et al., 2016).