Highly effective sorbents from products after processing agriculture raw materials

Abstract

Background. In Ukraine, a deterioration of the ecological state of water objects in the environment as a result of their contamination with heavy metals and organic compounds and the poor subsequent treatment of industrial wastewater takes place. One way to reduce the amount of pollutants in wastewater is to use purification methods based on the application of sorbents of different nature for toxins removal. Modern sorption technology usually consists of a multistage wastewater treatment, which involves the use of several types of sorption materials with selective action towards specific pollutants.
In recent years, special attention of researchers is directed towards sorbents and entero­sorbents derived from available raw materials such as components of plants and living organisms. These absorbers include pectin, chitin, and alginates. However, the techno­logies for such polysaccharide production are quite energy-consuming and multistage, and the low mechanical strength and high cost limit their widespread use.
From an economic point of view, it is promising to use solid plant wastes from agriculture and food industry as the sorbents.
The aim of current study is to obtain sorption materials from the walnut shell by organosolv and oxide-organosolv modifications and to investigate their sorption capacity towards organic and inorganic ecotoxicants.
Material and methods.Walnut shells, crushed to the size of 1–2 mm, were used as the raw material. Sorbents were obtained by the treatment of the initial raw materials with acetic acid and a mixture of acetic acid and hydrogen peroxide at the temperature of 90 °C and the ration solid to liquid 5: 1.
Sorption of dyes on initial and modified lignocellulosic materials was studied at 25 °C. Dyes solutions with the concentration of 100 mg/l were prepared on 0.15 N phosphate buffer with pH 6.0. In each experiment, the amount of sorbent was 0.20 g and the volume of solution was 25 ml. The initial and equilibrium concentration were determined by spectrophotometric method. Transmission spectra were recorded by "Specord M-40"at 664 nm wavelength for the methylene blue, at 576 nm for methyl violet and at 515 nm for murexide.
The study of absorption kinetics of methylene blue was performed by withdrawing at regular intervals of 4 ml of sample, measuring quickly its concentration and returning the sample back to the investigated solution to continue a sorption experiment.
Sorption of Fe3+ and Cu2+ was carried out by using the model solutions with concentrations of 10–50 and 50–250 mg/l, respectively. The amount of sorbent in each experiment was 0.50 g, the volume of metal solution was 50 ml and the length of sorption was 30 min. Fe3+ ions concentrations in initial solutions and in solutions after sorption was determined by spectrophotometry and Cu2+ ions by iodometry.
Results. During the modification of nut shells at high temperature in a medium of acetic acid, the hydrolysis of low and high molecular weight polysaccharide components take place. Acid hydrolysis promotes also the removal of mineral components, which has a positive impact on the pore volume of the sorbent. It was demonstrated that absorption capacity of the modified sorbents towards basic (methylene blue, methyl violet) and acid (murexide) dyes, which are markers of low molecular weight toxicants of organic nature , increases more than twice in comparison to the initial raw material.
As a result of treatment of nut shell with a mixture of acetic acid and hydrogen peroxide (concentration 9 %), a significant decrease in the yield of lignocellulosic material was observed. This was due to the removal of a significant part of substances of aromatic nature, mostly lignin, from nut shells. As a result, the content of polysaccharides in modified sorbent had increased. In addition, the content of mineral components was reduced twice in comparison with initial material.
Investigation of kinetic characteristics of sorption of cationic dyes by modified sorbent showed that the maximum absorption rate of the dye corresponds to the first 60 min of contact, during which the concentration of methylene blue in the solution is reduced by more than two times. Full sorption equilibrium is reached within 180 min.
Obtained plant sorbents are characterized by high sorption capacity towards heavy metal ions (Fe3+ – 18–29 mg/g and Cu2+ – 33–44 mg/g).
Conclusion. New sorbents derived from plant waste of food industry, namely crushed walnut shell, were obtained and their high efficiency in the sorption of ecotoxicants of organic and inorganic origin from the model aqueous solutions was demonstrated. It was shown that the sorption capacity of plant sorbents depends on their structure, which is determined by the conditions of processing the initial plant wastes. Due to the number of advantages, such as high sorption capacity, low cost and availability of raw materials, simplicity of disposal, biocompatibility, and nontoxicity, the obtained natural materials can have a wide practical use as polifunctional soebents solving thus environmental problems related to pollution of the environment.