Nickel (ii) Ion adsorption by native and treated pea pods

. The paper describes a study on static Ni 2+ ions adsorption by native and treated shells of pea pods ( Pisum sativum ). Pea pod shells were treated with 1-3% sulfuric acid or sodium hydroxide solutions. Adsorption isotherms were constructed and calculated according to the Langmuir, Freundlich, Tyomkin, and Dubinin-Radushkevich models. It was found that all constructed isotherms were most accurately described by the Freundlich model. It was determined that the highest sorption capacity for Ni 2+ ions was shown by 3% sulfuric acid solution-treated pea pods. The paper also describes a study of the kinetics of Ni 2+ ion adsorption by native and treated pea pod shells under static conditions. It was determined that the main mechanism of the process is mixed diffusion. The calculated thermodynamic parameters indicate physical adsorption.


Introduction
The World Ocean pollution by various pollutants is a pressing issue at the moment.Heavy metal ions (HMI) rank top in the list of priority pollutants.The latter enter water bodies with poorly treated wastewater (WW) from chemical, mechanical manufacturing, ore dressing, and other facilities.
Currently, WW treatment in the above applications is based on various methods: chemical, electrochemical, physico-chemical, biological, etc.However, the use of the abovementioned methods is deterred by their high cost, low efficiency, and high energy intensity.
One of the efficient processes to remove pollutants from aqueous media is adsorption.However, activated carbons (AC) used in industrial production are very expensive, have low efficiency for HMI.In addition, AC require regeneration, which also raises the process's cost.

Materials and methods
Currently, the world community is rapidly developing a new innovative environmental protection area -the use of industrial and agricultural waste as sorption materials (SM) to remove pollutants from aqueous media [1][2][3][4][5][6][7][8][9][10].Of particular interest are bulk lignocellulose SM resulting from the processing of agricultural raw materials.The latter have the benefit of a yearly renewable raw material base, efficient removal of various pollutants, cheapness, natural abundance, etc. Legume processing by-products are of particular interest for pollutants removal due to a large amount of proteins which have various functional groups in their composition and react with HMI.It was shown that bean (Phaseolus vulgaris) pod shells effectively remove ions of Pb 2+ [11,12], Cd 2+ and Hg 2+ [12], Ni 2+ and Zn 2+ [13] and other metals.Fava bean (Vicia faba) pod shells are reported are an effective sorption material for removal of such ions as Co 2+ [14], Cd 2+ and Pb 2+ from aqueous media [15] and soybeans husk helps remove Cr(VI) ions [16].
Plantations of seed peas (Pisum sativum) are the most prevalent in Russia.The latter's processing by-products are pea pod shells (PPS) which, as a rule, after the extraction of pea grains, rot in piles and deteriorate the environmental situation in the storage areas.One of PPS applications is using them as a sorption material for HMI removal from aqueous media.
[ [17][18][19] provided data on Cu 2+ and Zn 2+ ions adsorption from simulated solutions by native and treated PPS.This paper deals with a study of Ni 2+ ions adsorption by native and treated PPS biomass.
In connection with the above, PPS surface was initially treated with thinned solutions of sulfuric acid (C(H2 SO4) = 1%, 2%, 3%) and sodium hydroxide (C(NaOH) = 1%, 2%, 3%).To this end, 10 g of PPS was placed in 1,000 cm 3 round-bottomed flasks and 500 cm 3 sulfuric acid or sodium hydroxide solutions of a given concentration was added.The flasks contents was intensively stirred using an agitator for 5 hours at 22-23 ° C. At the end of the interaction time, the sorption material was filtered through a paper filter under vacuum, washed with distilled water, and dried at 95-100 ° C in a drying cabinet until a constant weight was reached.
At the next stage of the study, the sorption properties of native and treated PPS for Ni 2+ ions were studied under static conditions.To this end, 1 g sorption material sample and 100 ml of nickel salt solution with an initial of Ni 2+ ions concentration ranging from 0 to 2,000 mg/dm 3 were placed in a conical 250 ml flask and stirred with a magnetic agitator for 3 hours.At the end of the above time, the sorption material with sorbed nickel(II) ions was filtered through a paper filter under vacuum and the residual (equilibrium) content of Ni 2+ ions was determined in the filtrates.
The sorption capacity of the materials was determined using formula 1.
where Сs is the initial concentration of nickel (II) ions in solution, mg/dm 3 ; Ce is the equilibrium concentration of nickel (II) ions in solution, mg/dm 3 ; 100 is the volume of the solution in which the sorption interaction was carried out, cm 3 ; 1,000 is the conversion from cm 3 to dm 3 .Isotherms of Ni 2+ ions adsorption by native and treated PPS are shown in Figures 1 and  2. These adsorption isotherms show that both acid and alkaline surface treatment of the sorption material provides an increase in its sorption capacity for Ni 2+ ions.The maximum sorption capacities of native and treated PPS for nickel (II) ions, in accordance with the experimental data, are presented in Table 1.  1 shows that the largest sorption capacity for nickel (II) ions is found in PPS treated with 3% sulfuric acid solution.
The equations of adsorption processes were derived from linearization of the adsorption isotherms in coordinates 1/А = f(1/Ce), logА = f(logCe), lnА = f((ln(Cs/Ce))2) and А = f(lnCe) within the framework of four two-parameter sorption models: Langmuir, Freundlich, Dubinin-Radushkevich and Tyomkin, respectively, with approximation coefficients R2 (Table 2).The processes of nickel (II) ions adsorption by both native and treated PPS are best described by the Freundlich model, i.e., there are active centers with a larger sorption capacity on the surface of the sorption material [20].
Thermodynamic process parameters of nickel (II) ions adsorption by native and treated pea pods are calculated on the basis of the adsorption equations (Table 3).
Table 3 shows that according to Gibbs energies (-100 <∆G<0 kJ/mol) and sorption energies (0<E<8 kJ/mol) all the processes under study are physical adsorption processes.The n < 1 values indicate that the sorbent-sorbate binding energy increases as the surface is filled.The kinetics of Ni 2+ ions sorption by native and treatied pea pods was studied under static conditions.To this end, 100 cm3ml of nickel (II) salt solution with a concentration of 1,000 mg/cm3 and 1 g of sorption material were placed in conical 250 cm3 flasks and stirred with a magnetic agitator for a certain period of time: 0, 1, 2, 3, 4 and 5 hours.At the end of the above period, the sorption material was removed and Ni2+ ion concentration was determined in the filtrates.The initial and final concentrations of nickel (II) ions were used to calculate the sorption capacities of materials (A, mol/g) and kinetic dependences of Ni2+ ion adsorption by native and treated pea pods were constructed (Fig. 3, Fig. 4).
The kinetic dependences of the processes of Ni2+ ions adsorption by native and treated pea pods were processed based on the Boyd diffusion model to calculate the Biot's coefficients provided in Table 4.

Results and discussion
The values of Bio's coefficients ranging from 1 to 20 show that nickel (II) ions adsorption by native and treatied pea pods is mixed diffusion.
Therefore, it was found that Ni 2+ ion adsorption by native and treated PPS is a physical process, the adsorption isotherms in all cases are most accurately described by the Freundlich model.It was determined that the main adsorption mechanism is mixed diffusion.

Table 1 .
Experimental values of maximum sorption capacities of native and treated pea pods for nickel (II) ions

Table 2 .
Equations of nickel (II) ions sorption by native and treated pea pods and their approximation coefficients

Table 3 .
Thermodynamic process parameters of nickel (II) ions adsorption by native and treated pea pods

Table 4 .
Results of processing kinetic dependences of nickel (II) ions adsorption by native and treated pea pods