RU2091318C1 - Method of chromium(vi) adsorption on activated carbon - Google Patents

Abstract

FIELD: waste water treatment. SUBSTANCE: method includes treatment of adsorbent and solution; bringing solution in contact with adsorbent; adsorption on activated carbon in low-acid or acid medium with adjusting pH via continuously neutralizing solution to optimal pH values. Deviation of adsorption pH value form optimal value in the course of adsorption process is maintained according to following relationship: ΔpH = ΔpH_о·exp(-kt^m) where ΔpH is deviation of pH from its optimal value; ΔpH_о maximum deviation of pH from its optimal value at zero time; k and m coefficients determined from experimental data. Along with purifying waste waters, invention may be applied in analytical chemistry and in production of chromium and its compounds. EFFECT: optimized process parameters. 3 dwg, 1 tbl

Description

 The invention relates to a method for adsorption of chromium (VI) from solutions on activated carbon (AC), relates to the field of extraction of substances by ion-exchange materials and can be used in non-ferrous and ferrous metallurgy, as well as for the treatment of industrial and domestic wastes.

Known methods for the adsorption of chromium (III) cation exchangers KU-1, KU-2, KRF-5p, KF-1, KFP-8 [1]
The disadvantage of this method is the need to convert chromium from oxidized to reduced form.

 Closest to the invention is a study on the removal of chromium (VI) compounds from water by activated carbon [2] in which it was found that during the adsorption of chromium (VI) from drinking water containing 50-500 μg Cr (VI) / l on AC with By decreasing the pH from 6 to 3, the adsorption capacity of the AC increases by almost 10 times.

The disadvantage of this method is that data are obtained on adsorption on AC of chromium (VI) from solutions with a very low content of chromium (VI), of the order of 0.05
0.5 mg / l, while in the effluents of galvanic and other industries, the chromium (VI) content can be two or more orders of magnitude higher. In addition, the adsorption values are given only depending on the pH of the initial solution and the pH change during the adsorption process was not taken into account, which does not affect the adsorption results.

 The objective of the invention is to find optimal conditions for fast and efficient adsorption of chromium (VI) from industrial and domestic wastewater at AC.

 The technical result of the invention is to improve the kinetic characteristics of the process while at the same time a high degree of adsorbate recovery from the solution, increase the sorption exchange capacity (SOE) of the adsorbent, reduce the consumption of reagents, use relatively cheap adsorbents, suppress redox processes developing in the system with prolonged contact of the adsorbent and adsorbate .

The technical result is achieved by the fact that in the known method of adsorption of chromium (VI) from a solution, comprising contacting the AC and the solution at pH 3 6, during the adsorption process, the deviation of the pH from the optimal value is maintained according to the following relationship:
ΔpH = ΔpH _о • exp (-kt ^m ), (1)
where ΔpH is the deviation of the pH from the optimal value;
t time;
ΔpH _about - the maximum deviation of the pH from the optimal value at the initial time;
k and m are constants determined from experimental data.

 The pH value of the medium is an integrating indicator of the complex physicochemical adsorption process, which, to the optimum value, improved the process’s qualitative characteristics.

 Continuous neutralization of the solution to optimal pH values was carried out in small portions of the neutralizer, since with the addition of significant amounts of the latter, it is possible to go to the pH region in which adsorption is slow.

As AU, bone charcoal with particle sizes by fractions was used
Particle size, mm:
+2.5 +2.0 +0.8 +0.315 +0.125 +0.08 -0.08
Weight, g:
0.7 28.8 14.4 5.5 0.5 0.1 -
Weight,
1.4 57.6 28.8 11.0 1.0 0.2 -
The essence of the method is illustrated by drawings, where in FIG. 1 3 shows indicators of the adsorption of chromium (VI) from an aqueous solution at AC in SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ (Fig. 1) -, H ₂ O (Fig. 2) - and OH ^- (Fig. 3) forms when neutralizing the solution to a given pH _ads and maintaining its constant adsorption time in accordance with equation (1), which for the processes shown in FIG. 1 3, had the form
ΔpH = ΔpH _о • exp (-0.01t ^3.5 ), (2)
where t time, min
Examples of specific performance of the method.

Adsorption was carried out from 200 ml of a stock solution of K ₂ Cr ₂ O ₇ with a concentration of 100 mg / L, calculated on CrO ₃ .

The initial solution was prepared by dissolving in water a salt of K ₂ Cr ₂ O ₇ grade H.H. the metal concentration was determined on a KFK-3 photocalorimeter; the acid-base characteristics of the solution were controlled by a pH meter.

Adsorption values are presented in the form of C, in mg / l per CrO ₃ -concentration of chromium at a given time from the start of adsorption, pH _{ads of a} constant pH of the solution during adsorption, СОЕ, in mg / g per CrO ₃ sorption exchange the ion exchanger capacity, in mg of CrO ₃ adsorbate per 1 g of adsorbent.

The concentration of adsorbate in the solution corresponded to the pH of the solution at a given time of adsorption. In FIG. 1 3 the dependence of the concentration of total chromium on pH _ads is represented by a solid line, and of reduced chromium by a dashed line.

The adsorbent (dry weight AC 2 g) was charged with counterions by saturation from 0.1 n solutions of H ₂ SO ₄ or NaOH (SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ OH ^- forms) or kept in distilled water (H ₂ O form).

Example 1 (Fig. 1). AC was processed by charging in SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ -form. A constant pH of _ads during the adsorption was maintained by neutralizing the solution with NaOH alkali in accordance with equation (2), in which I <ΔpH _o <2 at an optimum pH of _ads , which is in the range of 3–5.

 From the graph (Fig. 1) it follows that the adsorption of chromium (VI) by the proposed method was completed in less than a day, adsorption was carried out in the range of 2 <pH <7.

Example 2 (Fig. 2). ACs were processed by charging in the H ₂ O form. A constant pH of _ads during the adsorption was maintained by neutralizing the solution with NaOH alkali or H ₂ SO ₄ acid in accordance with equation (2), in which 0.5 <ΔpH _о <1.0 with an optimum pH of _ads within the range of 3–4.

 From the graph in FIG. 2 it follows that the adsorption of chromium (VI) by the proposed method was completed in less than a day, adsorption was carried out in the range of 1≅pH≅5.

Example 3 (Fig. 3). ACs were treated by charging the OH ^- form. A constant pH of _ads during the adsorption process was maintained by neutralizing the solution with H ₂ SO ₄ acid in accordance with equation (2), in which 6 <ΔpH _о <7 with an optimal pH of _ads within the range of 3-5.

 From the graph in FIG. 3 it follows that the adsorption of chromium (VI) by the proposed method was completed in less than a day, the adsorption was carried out in the range of 1 <pH <6.

From the graphs in FIG. 1 3 it follows that at pH <3, redox processes develop between the adsorbate and the adsorbent, leading to the appearance of reducing forms of chromium in the solution. These processes are all the more effective the smaller the pH value of _ads and the longer the contact time of the adsorbent and adsorbate.

 The table shows the SOE AC value for various adsorption conditions. From the table it follows that the proposed method dramatically increases the adsorption rate and the value of SOE.

 The proposed method in comparison with the prototype allows you to adsorb chromium (VI) on a cheap adsorbent activated carbon with a high degree of extraction and with high speed.

 The process is environmentally friendly, efficient; the solution is not contaminated with extraneous cations. Due to the high adsorption rate, redox processes leading to the destruction of the adsorbent do not have time to develop.

The optimum range of pH _ads is in the range of 2 to 6.

Claims (1)

A method for adsorption of chromium (VI) from a solution, comprising contacting activated carbon and a solution at 2 <pH <6, characterized in that during the adsorption process, the deviation of the pH value of the adsorption from the optimal value is maintained according to the following relationship:
ΔpH = ΔpH _o • exp (-k • t ^m ),
where ΔpH is the deviation of the pH from the optimal value;
t adsorption time;
ΔpH _o - the maximum deviation of the pH from the optimal value at the initial time;
k, m are constants determined from experimental data.

Images