KR101155091B1 - Method of adsorbing cadmium using waste calcium carbonate obtained from carbonation reaction of flue gas desulfurization (fgd) gypsum - Google Patents

Abstract

The present invention comprises the steps of drying the FGD (Flue Gas Desulfurization) gypsum; Filtering the dried FGD gypsum; Reacting the filtered FGD gypsum with CO ₂ gas under ammonium hydroxide solution; Filtering the ammonium sulfate supernatant and then centrifuging to obtain an adsorbent; Drying the adsorbent; And adsorbing cadmium from the cadmium dissolved solution using the dried adsorbent.
According to the present invention, there is provided a method for adsorbing cadmium from wastewater using a product obtained by reacting FGD gypsum with carbon dioxide, and acting as a buffer having a pH buffering effect, thereby effectively adsorbing cadmium, and inexpensive FGD The use of gypsum can provide an economical way to remove cadmium.

Description

Cadmium adsorption method using waste calcium carbonate obtained by FDF gypsum carbonation reaction

The present invention relates to a cadmium adsorption method using calcium carbonate waste obtained by the FGD gypsum carbonation reaction, and more particularly, to the buffering effect of pH and the excellent cadmium adsorption efficiency of the calcium carbonate waste adsorbed by the reaction of FGD gypsum with carbon dioxide. The present invention relates to a method of adsorbing cadmium at low cost.

Recently, inexpensive and effective adsorbents have been studied to remove heavy metals from wastewater. Calcium carbonate (calcite) plays an important role as adsorbent of divalent metal cations and is known to affect their concentration in environmental and ecological systems.

Mineral carbonation using minerals containing Ca / Mg is one of the applicable techniques for CO ₂ sequestration in the atmosphere. Cadmium is a metal that is harmful to humans and occurs frequently in wastewater.

FGD systems are widely applied to control acid gases (mainly SO _x ) in coal-fired power plants. Limestone and calcium-based adsorbents have conventionally been used to remove sulfur dioxide, and calcium sulfate (gypsum) is considered a major product.

The present invention explores the potential as a pH buffer and adsorbent for cadmium removal from contaminated water using by-products obtained from carbon dioxide sequestration of FGD-gypsum. Cadmium was chosen as the representative metal for testing the adsorption effect of the product, based on its toxicity and frequent presence in harmful water and insensitivity to redox reactions.

The present invention has been made to solve the above problems, and an object thereof is to provide a method for adsorbing cadmium from wastewater using a product obtained by collecting carbon dioxide with FGD gypsum.

Another object of the present invention is to provide an adsorption method in which the adsorbent can effectively adsorb cadmium by acting as a buffer having a pH buffering effect.

It is another object of the present invention to provide a method for removing cadmium at low cost by using a product obtained in the reaction of FGD gypsum with carbon dioxide.

The present invention is to achieve the above object, the step of drying a flue gas desulfurization (FGD) gypsum; Filtering the dried FGD gypsum; Reacting the filtered FGD gypsum with CO ₂ gas under ammonium hydroxide solution; Filtering the ammonium sulfate supernatant and then centrifuging to obtain an adsorbent; Drying the adsorbent; And adsorbing cadmium from the cadmium dissolved solution using the dried adsorbent.

In addition, to remove 100 mg / L cadmium, the adsorbent is characterized in that the amount is added more than 20 g / L.

In addition, the initial pH of the cadmium dissolved solution is characterized in that the range of 6 to 11.

In addition, the FGD gypsum is characterized by reacting with 15% CO ₂ gas at a flow rate of 10L / min.

According to the present invention, there is provided a method of adsorbing cadmium from wastewater at low cost by using a product obtained by capturing carbon dioxide with FGD gypsum, and acting as a buffer having a pH buffering effect, thereby providing an economical method of effectively removing cadmium. Can provide.

1 is a powder X-ray diffraction pattern (XRD pattern) of the adsorbent according to the present invention.
2 is a graph showing the pH of a suspension of 2.0 g / L adsorbent before or after shaking for 1 or 2 days at 150 rpm.
FIG. 3 is a graph showing percent removal of cadmium (II) (100 mg / L) after 24 hours without pH adjustment as a function of adsorbent dose (g / L).
4 is a graph showing cadmium (II) adsorbed after a 24 hour duration without pH adjustment at a concentration of 10 g / L of the adsorbent according to the invention as a function of the final dissolution concentration of cadmium (II).
Figure 5a is a Langmuir isotherm of cadmium (II) adsorption by the adsorbent (10 g / L) according to the present invention in the range of the initial cadmium (II) concentration 20 ~ 200 mg / L.
Figure 5b is a Freundlich isotherm of cadmium (II) adsorption by the adsorbent (10 g / L) according to the present invention in the range of the initial cadmium (II) concentration 20 ~ 200 mg / L.
6A and 6B show the cadmium (II) adsorption rate by an adsorbent without pH control for 2 days and an adsorbent at pH 7 for 12 hours, respectively.
7 is a graph showing cadmium (II) adsorption in cadmium (II) contaminated solution as a function of pH.

The present invention comprises the steps of drying the FGD (Flue Gas Desulfurization) gypsum; Filtering the dried FGD gypsum; Reacting the filtered FGD gypsum with CO ₂ gas under ammonium hydroxide solution; Filtering the ammonium sulfate supernatant and then centrifuging to obtain an adsorbent; Drying the adsorbent; And adsorbing cadmium from the cadmium dissolved solution using the dried adsorbent.

The present invention characterizes calsite-rich products formed from FGD gypsum and carbon dioxide and provides them as a low cost effective adsorbent for the removal of cadmium (II) from wastewater.

Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings.

<Examples>

Batch experiments were performed for cadmium adsorption in aqueous solution and variously controlled adsorbent capacity, initial cadmium (II) concentration, contact time and pH of cadmium (II) -polluted solution.

Materials and methods

Adsorbent and Cadmium (II) Preparation

FGD-gypsum was obtained from Yeongheung thermal power plant in Incheon, South Korea, dried overnight at 60 ° C., and then filtered to 200 mesh (74 μm) for carbonization. FGD-gypsum of ammonium hydroxide solution was reacted with carbon dioxide gas (15%) at a flow rate of 10 L / min at an initial reaction temperature of 50 ° C. (yield 90%). After filtering the ammonium sulfate supernatant, the product was centrifuged and dried at 60 ° C. overnight.

Characteristics of Adsorbent

The properties of the product used as the adsorbent are XRD (X'pert MPD, Philips) using Cu / Kα radiation, XRF (XRF-1700, Shimadzu) for the main components, and acid decomposition (for trace elements). ICP-OES (Optima 5300dV, Perkin Elmer) after acid digestion (HNO ₃ / HClO ₄ )). The detection limits in the XRF analysis and the ICP-OES method employed for the adsorbent properties are 0.01% and 4.0 mg / kg, respectively. Particle size distribution was measured using a laser scattering particle size analyzer (HELOS / RODOS & SUCELL, Sympatec GmbH). The surface area of the product was determined by surface area and porosity analyzer (ASAP 2010, Micromeritics) using the BET method of adsorption of N ₂ gas.

ΔpH (= pH _equilibrium The point of zero charge (pH _pzc ), defined as the point where pH _initial ) is zero, was measured in the following way: 45 mL of 10 mM NaCl (supported electrolyte) was transferred to a series of 50 ml conical tubes. The initial pH (pH _initial ) values of these solutions were adjusted to pH 2-12 by adding 0.01, 0.05, or 0.1 mM HCl or 0.01, 0.05, or 0.1 mM NaOH. After adding 10 mM NaCl until a total volume of 50 mL, 0.1 g of product was added to each flask. The suspension was mixed for 1 day or 2 days, centrifuged at 5000 rpm for 5 minutes and the pH was measured.

Batch adsorption experiment

Except for the adsorption rate experiments, all adsorption experiments were carried out three times in a temperature controlled shaker at 150 rpm. The reaction temperature was kept constant at 25 ± 0.2 ° C. and the reaction duration was 24 hours.

Except for one experiment for the adsorption rate maintained at pH 7, the pH of the suspension is measured without pH adjustment during the reaction in all experiments because of the pH buffering function of the adsorbent. The pH buffering effect of the adsorbent was predicted from the pH of the suspension before and after the reaction.

The experiment for adsorption is divided into four parts.

The first experiment is done with varying initial adsorbent dose. The adsorbent was charged in the range of 2.5-40 g / L in an aqueous solution containing 100 mg / L Cd (II) (100 mL).

A second experiment was conducted to obtain adsorption isotherms with varying initial cadmium (II) concentrations. The initial cadmium (II) concentration was varied in the range of 20 to 200 mg / L (100 mL) in 20 mg / L increments and the adsorbent dose was 10 g / L.

In the third experiment, adsorption kinetics using 100 mg / L of cadmium (II) at 10 mM NaCl at a high liquid ratio of 10 g / L without constant pH (pH 7) (12 hours) and without pH adjustment (48 hours) ) Was studied. As the pH of the solution increased over time, the experiment at pH 7 was due to an increase in the volume of 0.1 M HCl or 0.1 M NaOH added to keep the pH constant at 7.0 ± 0.1. It lasted only 12 hours. The suspension was mixed at about 150 rpm with a stir bar. To determine the dissolved cadmium (II) concentration, 15 mL was extracted at each different time.

The fourth experiment is to measure the ability of the adsorbent in cadmium (II) contaminated water as a function of pH. The pH value of 100 mL of 100 mg / L cadmium (II) solution with 10 mM NaCl background electrolyte was 0.01, 0.05, or 0.1 M HCl or 0.01, 0.05, 0.1, 0.5, or 1.0 M The NaOH solution was adjusted in the range of pH 4-11 to less than 1% of the total volume, after which the adsorbent was added to the pH-adjusting medium (medium). Control tests without adsorbents were performed to determine the effect of carbonate or hydroxide concentrations on the amount of cadmium (II) adsorption. The experimental groups were filtered through a 0.2 μm membrane filter (cellulose acetate, Sartorius) and acidified with instrumental grade HNO ₃ , and the dissolved cadmium (II) was analyzed by ICP-OES (Optima 5300dV, Perkin Elmer). The limit of detection of cadmium was 0.02 mg / L and the uncertainty of the assay was within 15%, including repeated experiments.

result

Characteristics of Adsorbent

The particle size of the product ranges from 1 to 100 μm and the volume average diameter (VMD) is 4.37 μm, which is much smaller than the VMD of waste gypsum (27.4 μm). It was not necessary to grind the waste gypsum or product for each carbonation or adsorption of cadmium. This is both economical and environmentally advantageous compared to raw materials. The N ₂ -BET specific surface area is 3.76 m ² / g and the XRD pattern consists mainly of calsite and additionally contains small amounts of basanite, dolomite, and muscobite. (FIG. 1). Table 1 summarizes the major and minor constituents of the Calsite-rich product (XRF and ICP-OES results, respectively).

Major elements % Trace elements mg / L CaO 51.64 CD <4.0 SO ₃ 4.63 Pd <4.0 SiO ₂ 1.91 Cu <4.0 Al ₂ O ₃ 0.85 Cr <4.0 MgO 0.57 As <4.0 Fe ₂ O ₃ 0.41 Co <4.0 K ₂ O 0.31 Hg <0.05 Na ₂ O 0.04 TiO ₂ 0.03 P ₂ O ₅ 0.02 MnO <0.01 Ignition loss 39.59

Calcium in oxide form is 51.64%, the highest of the total weight, and toxic heavy metals such as Cd, Pb, Cu, Cr, As, Co, and Hg each account for less than 4.0 mg / L.

FIG. 2 is a graph showing the pH of a suspension of 2.0 g / L product before or after shaking for 1 or 2 days at 150 rpm. The point where the initial pH and the final pH meet (pH = 8.8) indicates that the surface of the by-product reacts with the aqueous phase and is neutral at this pH value without hydrogenation or dehydrogenation.

After shaking the product in aqueous solution for 1 day (1d) or 2 days (2d) as shown in FIG. 2, the point of zero charge, ΔpH (pH _initial pH _final ) is 8.8. No significant change in product pH was seen between day 1 and day 2. This shows that the development of charge at the interface between the product and water is constant for 1-2 days.

absorbent Input  effect

FIG. 3 is a graph showing the percent removal of cadmium (II) (100 mg / L) after 24 hours without pH adjustment as a function of adsorbent dose (g / L), with the graph inserted as a function of adsorbent dose Shows the change. Square and circle symbols mean initial pH (empty rectangle) and last pH (empty circle), respectively.

3 shows that as the adsorbent dose increases in the range of 2.5-40 g / L, the percentage of cadmium (II) (100 mg / L) removed increases from 21.5% to 98.2%. If the input volume exceeds 20 g / L, the removal percentage is constant at 100%. This confirms that 100 mg / L of cadmium can be completely removed when the dose exceeds 20 g / L. The relationship between cadmium (II) removal and adsorbent loading is the result of increased usability of the adsorption site due to the increasing surface area of the adsorbent, with increasing adsorbent loading. The inset graph of FIG. 3 shows that the initial pH and final pH values are a function of adsorbent dosage. The pH evolution pattern resembles the percentage change in cadmium removal as a function of adsorbent dose. With increasing adsorbent dosage, the decomposition of calcite increases, as a result of which the concentration of carbonate species increases, and consequently the pH in aqueous solution. When the adsorbent input is insufficient to remove cadmium (II) (<70%), the pH difference between the initial pH (before 1d duration) and the last pH (after 2d duration) is low (<0.2 unit of pH). In other words, when the adsorbent dose changed from 12.5 to 40 g / L, the initial pH was near neutral (~ 7.0), but the final pH increased to 7.9. This is due to the more residual surface area remaining after cadmium (II) covers the surface of the product when the adsorbent is sufficient to adsorb cadmium (II).

Adsorption isotherm

FIG. 4 shows cadmium (II) adsorbed after 1 day duration at a fixed adsorbent dose (10 g / L) as a function of dissolved concentration of cadmium (II).

The pH at product suspension before and after 1 day duration as a function of initial cadmium (II) concentration is shown in the inset graph of FIG. 4. Initial pH decreases with an increase in initial cadmium (II) concentration (20-200 mg / L), and the pH difference from pH evolution after 1 day duration is greater at lower metal concentrations, but at the saturation range of FIG. Similar (ΔpH to 0.3).

Experimental data were analyzed by Langmuir and Friendly's isotherm models. The Langlanger model assumes uniform monolayer adsorption of the adsorbate on the adsorbent with a limited number of independent and equivalent adsorption sites, and the shape of the line can be represented by the following equation (2). The Frenchrich model has an exponential distribution of active sites, and the adsorbed molecules have unlimited surface coverage due to the interaction between the adsorbed molecules as well as the adsorbents. 3) can be displayed.

Equation (2):

Equation (3):

Where q and Ce each represent cadmium (II) concentrations per unit volume of adsorbent and solution after 1 day of duration, K is a friendly value constant associated with adsorption capacity, q _max is maximum adsorption capacity, and b and n are adsorption energy Is a constant.

The parameters of the linear plot and the Langlanger and Friendly's isotherm equations are shown in FIGS. 5 and 2. 5A and 5B are clang adsorbed isotherms (FIG. 5A) and friendly-rich isotherms (FIG. 5B) of cadmium adsorbed by products of 1.0 g / L concentration in the initial cadmium concentration of 20-200 mg / L, respectively.

Langmuir Value q _max (mg / g) 8.03 b (L / g) 0.79 R ² 0.9993 Freundlich Value K 5.13 n 10.0 R ² 0.9829

Friendly parameters can be obtained from a straight plot of logq as a function of logCe.

From the value of R2 (0.9993) and the shape of the isotherm in the Langeru model, it can be seen that uniform monolayer adsorption is associated with cadmium (II) removal by the calsite-rich product. As can be seen in Figure 4, in the Langeru form, it can be seen that the adsorption of the adsorbate rapidly increases as the initial metal concentration increases and then stagnates. From the stagnation, it can be seen that the active site on the adsorbent surface reached saturation with increasing metal ion concentration. The product adsorbent is recycled from FGD gypsum and is an economical and environmentally promising adsorbent because it does not need to be ground or ground.

Adsorption rate

Cadmium (II) adsorption rates were monitored, respectively, at pH 7 fixed conditions and without a pH control system. FIG. 6A shows cadmium (II) as a function of time without pH adjustment and FIG. 6B is shown at pH 7.

As can be seen in FIG. 6A, the pH change during the reaction is evident at the point where at least 80% of the total cadmium (II) is adsorbed (about 12 hours). The efficacy of cadmium (II) removal is best for the first 5 minutes (about 20% of the total cadmium (II)), followed by up to 1 hour (about 35%), and then gradually decreases to stagnation. Initial high adsorption rates can be attributed to abundant excess adsorption sites on the surface. The increase in pH after 12 hours may be due to the diffusion of cadmium (II) adsorbed onto the surface into a deeper structure and accelerated dissolution of calsite. Compared to the stabilization constants of calcite (CaCO ₃ ) (pKsp = 8.47) and otavite (CdCO ₃ ) (pKsp = 11.3) reported by Davis et al., The hydrated CaCO ₃ covered by cadmium (II) was lower than calsite It may be assumed to have solubility.

solution pH Effect

The pH has a significant effect on the metal adsorption behavior by affecting the protonation or deprotonation of the adsorbent as well as the adsorbent. However, conventional assessments of pH effects in metal adsorption by calcite have been generally unavailable because of the calsite-water equilibrium in open systems. Since cadmium (II) adsorption was very fast within 5 minutes as shown in FIG. 6, it was practically impossible to monitor the reaction by adjusting to a constant pH in a solution containing the calsite-rich product from the beginning of the reaction.

7 is a graph showing cadmium (II) adsorption in cadmium (II) contaminated solution as a function of pH.

Experiments were conducted on the pH effect in a deliberately contaminated solution of cadmium (II) with a specific pH (4-11) (100 mg / L). Although cadmium ions in aqueous solution appear to be gradually removed by increasing the initial pH of the solution, the effect of pH in the solution before adding the adsorbent is not significant (FIG. 7). Cadmium removal reaches 70% in the low pH range of 4.0-5.0, reaching about 80% at pH 6.0-8.0, 90% at pH 9, and up to 100% at pH> 9, with pH 6.0 or higher being preferred. . The fact that cadmium (II) removal efficiency is 70% even in the low pH range of 4.0 to 5.0 is due to the buffering effect of the adsorbent. The initial pH immediately after adding the adsorbent was compared to FIG. 7. The pH of the initial solution, pH 4.0-8.0, rises to near 7.0, and falls below pH 9.0. The pH of the solution after adsorbent addition is expected to be a major factor in the removal of cadmium (II) by the calcite-rich adsorbent.

In the control sample, without adsorbent, cadmium (II) adsorption was not detected at pH 4-8, but markedly increased from pH 9.

conclusion

This example was carried out to study the ability of the product obtained by capturing carbon dioxide using FGD gypsum as a pH buffer and adsorbent for cadmium removal of waste water. The product was characterized as a _calsite -rich powder by VMD, N ₂ -BET specific surface area, and pH _zpc , at _4.37 μm, 3.76 m ² / g, and pH 8.8, _respectively . Adsorption of cadmium on the product was investigated by varying the adsorbent dose, initial cadmium (II) concentration, reaction time and pH value. The cadmium (II) removal percentage is increased with increasing adsorbent input and an adsorbent input above 20 g / L is required to remove 100 mg / L of cadmium (II). The adsorption isotherm fits well with the Langeru adsorption model and the maximum adsorption capacity is 8.03 mg / g. Cadmium (II) is rapidly adsorbed in the initial stage (within 5 minutes) and continues slowly slowly up to about 12 hours. After 12 hours, the adsorption rate became nearly constant and the pH increased due to calsite dissolution. Because of the calsite-water equilibrium, the adsorbent is added to the solution contaminated with cadmium (II), which has been previously adjusted to pH, to investigate the pH effect. Immediately after the addition of the adsorbent, the pH of the solution of 4-8 shifts to circumneutral (~ 7.0). After adding the adsorbent, the pH of the solution can be assumed to be a major factor in controlling cadmium (II) adsorption. These results demonstrate that the calsite-rich product obtained by capturing carbon dioxide using FGD gypsum is an effective pH buffer and adsorbent for cadmium removal of waste water.

Claims (4)

Drying the Flue Gas Desulfurization (FGD) gypsum;
Filtering the dried FGD gypsum;
Reacting the filtered FGD gypsum with CO ₂ gas under ammonium hydroxide solution;
Filtering the ammonium sulfate supernatant and then centrifuging to obtain an adsorbent; And
Drying the adsorbent;
Adsorbing cadmium from the cadmium dissolved solution using the dried adsorbent
Cadmium adsorption method comprising a.
The method of claim 1,
Adsorption method of cadmium, characterized in that the amount of adsorbent is added more than 20g / L to remove the 100 mg / L cadmium.
The method of claim 1,
The initial pH of the cadmium dissolved solution is a cadmium adsorption method, characterized in that in the range of 6 to 11.
The method of claim 1,
And adsorbing the FGD gypsum with 15% CO ₂ gas at a flow rate in the range of 10 L / min.

Images