Studies of removing copper ions from water with the seed coats of grain crops

. This paper presents the studies of sorbing copper ions in simulated water with the seed coats of barley, wheat, and oat grains. Isotherms were built for the Cu 2+ sorption at the initial concentrations of 0.315-62.947 mmol/dm 3 . It is shown that the sorption isotherms of all grain samples correspond with IUPAC 1b and are described by Langmuir equation with the confidence degree of 0.999. It is also shown that sorption runs intensively at the initial contaminant concentration ranging from 0.315 mmol/dm 3 through 15.737 mmol/dm 3 , while the sorption rate observed decreases significantly at the concentration exceeding 15.737 mmol/dm 3 , which works out to 1,000 mg/dm 3 . The sorption isotherms built is indicative of monomolecular adsorption running.


Introduction
Water contamination with metal ions is a severe ecological challenge, since it affects adversely the environment and human health. Metal ions can get into water both naturally and as a result of industrial activities, agriculture, mining operations, and using various chemicals. Moreover, water contamination with metal ions affects adversely vegetal and animal life, which leads to lowering biodiversity, rapidly decreasing the population of some species of animals and plants, and distinction of rare and unique ecosystems [1]. Thus, contaminating waters with metal ions is an urgent problem and requires being immediately managed and taking relevant measures aiming to purify and protect the environment.
Copper is one of the most common metals used in industry and agriculture, and its ions may get into the environment from various sources, including industrial effluents, domestic sewage, agricultural fertilizers, and pesticides. Contaminating wastewater with Cu 2+ is a grave issue related to the environment and human health. Wastewater concentration of copper ions over 1 mg/dm 3 leads to severe environmental issues, such as poisoning aquatic life and contaminating potable water. Moreover, copper ions may accumulated in the tissues of animals and plants, which provides adverse effects upon human health and causes food contamination. Hence, process-based measures are taken to reduce the sewage contamination with copper ions, including the use of physical, chemical, physicochemical, biological, and other techniques of sewage treatment in industry. Sorption-based techniques for removing Cu 2+ from sewage are the most efficient, and there are many various sorption materials (SM), among which there are ion-exchange resins, activated carbons, and magnetic and polymeric sorbents. Notwithstanding that sprawling the raw materials base to manufacture SMs for wastewater treatment is a topical problem, not all SMs comply with the resource-saving and closed-loop economy principles at the sufficient efficiency [5]. For this reason, SMs based on vegetable raw materials have been spread widely, they are effective regarding copper ions, environmentally friendly, and mostly processing waste, which may solve the problems of industrial and agricultural waste utilization.

Main part
Literature analysis has shown the removability of Cu 2+ ions using SMs based on rice husks [6,7], buckwheat straws and husks [8][9], and other gramineous waste [10]. Russia is traditionally considered the leader of grain exports, waste amounting to 10% and grain to 30% in the complete grain crop processing cycle according to ITS 44-2017 (Information and Technical Reference Book on the Best Available Techniques for Food Manufacturing). Thus, over 20 million tonnes of barley grain seed coats (BGSC), wheat grain seed coats (WGSC), and oat grain seed coats (OGSC) are formed in Russia annually.
Further to works [11,12], we studied the sorption of copper ions from simulated waters with SMs based on BGSC, WGSC, and OGSC and identified the sorption mechanism.
To simulate contaminated wastewater, we prepared a solution of salt CuSO4·6H2O with the Cu 2+ ion concentration of 0.315-62.947 mmol/dm 3 . Quantities of salts were taken considering crystallization water.
Wastewater were purified in flat-bottom flasks in the volume of 250 cm 3 , 200 cm 3 of solutions containing Cu 2+ ions and 1 g of the samples under research in each. Experiment was performed with constant stirring for three hours. Then SM was removed, while the residual contents of Cu 2+ ions were detected in filtrates photometrically [13].
Quantity of pollutant ions sorbed by 1 g of SM in mg/g was calculated by formula: Where Сs is the initial concentration of metal in the solution, mmol/dm 3 ; Сe is the end concentration of metal in the solution, mmol/dm 3 ; V is the solution volume, cm 3 ; and m is the mass of SM, g According to the methods described above, we obtained the following results of removing copper ions from simulated solutions at different initial concentrations of the pollutant. Data of the Cu 2+ sorption capacity with native BGSC, WGSC, and OGSC are shown in Fig. 1.  Fig. 1 can be classified as type 1b (by IUPAC), which indicates the presence of micropores in the materials. Such sorbents have a higher potential ability to interact with the adsorbed molecules, due to the proximity of walls between pores [14].
All isotherms are identical. Therefore, to study the nature of the process and compute the adsorption parameters, we used Langmuir and Freundlich equations that correspond with the localized monomolecular adsorption on a uniform surface. Within the above models, we processed the experimental sorption isotherms for the samples of BGSC, WGSC, and OGSC using the least square method and data approximation. Theoretically, a 1b isotherm must be described by Langmuir equation, which is confirmed by the data presented in Table 1. From data shown in Table 1, the correlation coefficient (R 2 ) of the samples within each model is approximately equal (within the measurement accuracy). The highest approximation coefficient is observed in Langmuir, which indicates that the monomolecular adsorption runs on the sample surfaces. In this case, adsorption runs un active centers that always exist on the surface of adsorbent, each of the centers being able to adsorb only one molecule [15][16][17]. As a rule, for this type of adsorption, adsorbate is held on the surface of adsorbent for some are also observed in Freundlich model, which can also be indicative of an insignificant multilayer sorption and the linear intermolecular interaction between the adsorbate molecules. The studies above indicate that the copper ion sorption isotherm with the samples of grain crop seed coats (GCSCs) does not provide any insights into the adsorption process. It is fair to assume that it is a mixed adsorption that is described by different models. From Fig. 1, the samples are saturated actively at the Cu 2+ ion concentration of 0-17 mmol/dm 3 , while with 17-68 mmol/dm 3 the curve passes into horizontal position, which may suggest the saturation of the samples and slight desorption. Thus, the on-site sorption process is described by Langmuir equation at the initial copper concentration in simulated water (SW) ranging from 0.315 mmol/dm 3 through 15.737 mmol/dm 3 , while it is described by Freundlich equation at the concentration of 15.737-62.947 mmol/dm 3 , that is, 1,000-4,000 mg/dm 3 . Table 2 shows the parameters of the equations depending on the concentration.  Table 2 shows that, at the initial concentrations of copper ions in SW ranging within 0.315-15.737 mmol/dm 3 , the process runs by Langmuir model, as indicated by the high approximation degrees of the equations (0.9993-0.9999). Verification of the hypothesis of how different sorption processes run depending on the initial concentration is also observed at the concentration ranging within 15.737-62.947 mmol/dm 3 , which are described by Freundlich equation with the results having a high confidence level. Hence, at the initial stage, the adsorbent surface is covered by the substance molecules, adsorption takes place on the vacancies only, and the adsorption rate decreases with the increasing adsorbate concentration in the solution. As soon as all the surface vacancies are filled, the balance is achieved (15.737 mmol/dm 3 ), at which the surface concentration of the adsorbate does not change despite the high correlation indices of Freundlich equation, which indicate a nonuniform surface and are validated by coefficient n. Coefficient K that determines the largest amount of the adsorbed substance per the adsorbent mass unit at the infinitely low concentrations in the solution is quite small, that is, the adsorption capacity of BGSC, OGSC, and WGSC remains low at the high Cu 2+ concentrations. initial pollutant concentrations of 0.315-15.737 mmol/dm 3 and at the concentration of above 1,000 mg/dm 3 , sorption is observed, but at a lower rate. Sorption isotherms constructed indicate the running of monomolecular adsorption. However, the constantly increasing quantity of adsorption models keeps the option open for more precisely describing the model of sorbing metal ions with vegetable SMs at a wide range of initial concentrations. Yet, the sorption of copper ions with the samples of sorption materials based on BGSC, WGSC, and OGSC allows minimizing the losses of metal ions and achieving the enhanced quality of wastewater.