Starch-based carbon composite and use thereof in selective and efficient adsorption of mercury ion and methylene blue

Abstract

The present invention relates to a method for preparing a simply prepared, safe, nontoxic, and biodegradable modified starch; the present invention further relates to use thereof as an adsorbent in adsorbing heavy metal ions and organic compounds, and belongs to the technical field of modified starch material. Starch is used as raw material and modified with thiourea and tungsten chloride; the starch binds to heavy metal ions through a series of coordination or chelation, and thus produces a heavy metal ion adsorption effect; a starch-based carbon composite is prepared, i.e., a modified starch composite. The simply prepared, environmentally friendly, and recyclable modified starch adsorbent with excellent performance and higher practical value effectively removes He ions and such organic compounds as methylene blue from the wastewater; the modified starch adsorbent is expected to develop into a novel water treatment agent due to low loss rate, biodegradability, and recyclability.

Claims

1. A method for preparing a carbon composite from a starch, the method comprising the following steps: (1) adding glycerol and tungsten chloride to a reactor; after dissolving, adding a starch and thiourea thereto, and stirring, wherein a ratio of the tungsten chloride, the glycerol, the starch, and the thiourea is 1 g: 40 ml: 1 g: 0.5 g, respectively; (2) reacting at 150-250° C.; (3) after cooling, centrifuging and washing to collect precipitates; and (4) drying to obtain the carbon composite.

2. The method according to claim 1, wherein the reaction in step (2) is conducted for 5-15 h.

3. The method according to claim 1, wherein the centrifuging in step (3) is conducted for 20-40 min at 10,000-15,000 rpm.

4. The method according to claim 1, wherein the drying in step (4) is performed in a drying oven at a temperature of 50-80° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts scanning electron microphotographs;

(2) FIG. 2 shows a graph of adsorption capacity of mercury ion versus time;

(3) FIG. 3 shows a graph of adsorption capacity of methylene blue versus time.

DETAILED DESCRIPTION

(4) The present invention will be further described below in conjunction with a specific application example.

EXAMPLE 1

(5) A method for preparing a starch-based carbon composite was provided, including the following steps: (1) adding 40 L of glycerol and 1 kg of tungsten chloride to a reactor, and stirring until fully dissolved; then adding 1 kg of sorghum starch and 0.5 kg of thiourea, and thoroughly stirring until mixed well; (2) placing in a drying oven, setting a temperature at 150° C., and reacting for 15 h; (3) after cooling, centrifuging for 40 min at 10,000 rpm, washing with tap water, centrifuging for 40 min at 10,000 rpm again, and repeating three times, to collect precipitates; and (4) drying in the drying oven at 50° C., to obtain modified starch.

EXAMPLE 2

(6) A method for preparing a starch-based carbon composite was provided, including the following steps: (1) adding 40 L of glycerol and 1 kg of tungsten chloride to a reactor, and stirring until fully dissolved; then adding 1 kg of corn starch and 0.5 kg of thiourea, and thoroughly stirring until mixed well; (2) placing in a drying oven, setting a temperature at 250° C., and reacting for 5 h; (3) after cooling, centrifuging for 20 min at 15,000 rpm, washing with tap water, centrifuging for 20 min at 15,000 rpm again, and repeating three times, to collect precipitates; and (4) drying in the drying oven at 80° C., to obtain modified starch.

EXAMPLE 3

(7) A method for preparing a starch-based carbon composite was provided, including the following steps: (1) adding 40 L of glycerol and 1 kg of tungsten chloride to a reactor, and stirring until fully dissolved; then adding 1 kg of potato starch and 0.5 kg of thiourea, and thoroughly stirring until mixed well; (2) placing in a drying oven, setting a temperature at 200° C., and reacting for 10 h; (3) after cooling, centrifuging for 30 min at 12,000 rpm, washing with tap water, centrifuging for 30 min at 12,000 rpm again, and repeating three times, to collect precipitates; and (4) drying in the drying oven at 60° C., to obtain modified starch. The morphology of the starch-based carbon composite prepared is shown in FIG. 1.

APPLICATION EXAMPLE

(8) The starch-based carbon composite provided by the present invention can be used as an adsorbent in adsorbing heavy metal mercury ions and organic pollutants in wastewater. The adsorption step includes: adding the adsorbent modified starch into the wastewater, adjusting pH, controlling adsorption time, and adsorbing heavy metal mercury ions and organic pollutants in wastewater.

(9) To verify the adsorption effect of the adsorbent provided by the present invention, the following experiments were carried out:

(10) 1. Preparation of Solutions

(11) 2.7218 g of potassium dihydrogen phosphate (KH.sub.2PO.sub.4.Math.H.sub.2O, solution A) was dissolved in 1,000 ml of distilled water, and 4.5644 g of dipotassium hydrogen phosphate (K.sub.2HPO.sub.4.Math.2H.sub.2O, solution B) was dissolved in 1,000 ml of distilled water. Buffer solutions at different pH were prepared according to the following expression.

(12) Acetate buffer (0.02 M, pH=4): 2.8617 ml of glacial acetic acid was added to 500 ml of distilled water; pH was adjusted with certain concentrations of NaOH solution and acetic acid; then, the resulting solution was poured into a 500 ml volumetric flask to be filled to the pre-marked line.

(13) Acetate buffer (0.02 M, pH=5): 2.8617 ml of glacial acetic acid was added to 500 ml of distilled water; pH was adjusted with certain concentrations of NaOH solution and acetic acid; then, the resulting solution was poured into a 500 ml volumetric flask to be filled to the pre-marked line.

(14) Acetate buffer (0.02 M, pH=6): 2.8617 ml of glacial acetic acid was added to 500 ml of distilled water; pH was adjusted with certain concentrations of NaOH solution and acetic acid; then, the resulting solution was poured into a 500 ml volumetric flask to be filled to the pre-marked line.

(15) Acetate buffer (0.02 M, pH=7): 2.8617 ml of glacial acetic acid was added to 500 ml of distilled water; pH was adjusted with certain concentrations of NaOH solution and acetic acid; then, the resulting solution was poured into a 500 ml volumetric flask to be filled to the pre-marked line.

(16) Preparation of mercury ion solution: 50 mg of mercuric chloride was added to 500 ml of phosphate buffer solution (PBS) (pH=6).

(17) 2. Determination of pH and Time of Adsorption of the Starch-Based Carbon Composite Prepared in Example 3 to Mercury Ion

(18) To a 250 ml beaker, 100 ml of the well-prepared mercury ion solution and 0.02 g of modified starch were added, and then stirred in a heat collection type magnetic stirrer; adsorption capacities of modified starch to mercury ions were determined electrochemically at 15 min, 30 min, 1 h, 1.5 h, 2 h, 3h, and 4 h, respectively. As shown in FIG. 2, the adsorption capacity of mercury ions rises sharply to adsorption saturation within 30 min.

(19) After 500 ml each of PBS at different pH (=4, 5, 6, and 7) was prepared, 0.05 g each of mercuric chloride was added thereto; once particles were completely dissolved, 100 ml of the resulting solution was transferred to a 250 ml beaker; 0.02 g of modified starch was added thereto and stirred in the heat collection type magnetic stirrer for 30 min until adsorption thereof was saturated; after standing and centrifugation, adsorption capacities of modified starch to heavy metal mercury ions were determined at different pH by the electrochemical method or atomic absorption spectroscopy.

(20) TABLE-US-00001 TABLE 1 Effect of different pH on mercury ion adsorption pH = 4 pH = 5 pH = 6 pH = 7 Adsorption capacity 275 325 495 425 at 30 min (mg/g) Removal efficiency 55 65 99 85 at 30 min/%

(21) The experimental data indicated that the modified starch had the optimal adsorption effect on adsorption of heavy metal mercury ions when mercury ions were present in the PBS with pH=6 (Table 1).

(22) 3. Determination of pH and Time of Adsorption of the Starch-Based Carbon Composite Prepared in Example 3 to Methylene Blue

(23) To a 250 ml beaker, 100 ml each of the well-prepared methylene blue solution and 0.03 g of modified starch were added, and then stirred in the heat collection type magnetic stirrer; adsorption capacities of modified starch to methylene blue were determined electrochemically at 15 min, 30 min, 1 h, 1.5 h, 2 h, 3h, and 4 h, respectively. Results are shown in FIG. 3.

(24) After 500 ml each of PBS at different pH (=4, 5, 6, 7, and 8) was prepared, 0.025 g each of methylene blue was added thereto; once methylene blue was completely dissolved, 100 ml of methylene blue solution was transferred to a 250 ml beaker; 0.03 g of modified starch was added thereto and stirred in the heat collection type magnetic stirrer for 1.5 h until adsorption thereof was saturated; after the solution was removed for standing and centrifugation, an approximate amount of supernatant was pipetted, and adsorption capacities of modified starch to methylene blue were determined at different pH by the electrochemical method or atomic absorption spectroscopy.

(25) TABLE-US-00002 TABLE 2 Effect of different pH on adsorption of methylene blue pH = 4 pH = 5 pH = 6 pH = 7 pH = 8 Adsorption capacity 85 122 131 160 90 at 1.5 h (mg/g) Removal efficiency 34 49 52 64 36 at 1.5 h/%

(26) The experimental data indicated that the modified starch had the optimal adsorption effect on adsorption of methylene blue when methylene blue was present in the PBS with pH=7 (Table 2).

(27) 4. The starch-based carbon composites prepared in Examples 1 to 3 were investigated under optimal conditions explored in steps 2 and 3 to compare adsorption capacities of these composites to mercury ions versus methylene blue, as shown in Table 3.

(28) TABLE-US-00003 TABLE 3 Comparison of adsorption capacities of starch-based carbon composites prepared in Examples 1 to 3 Mercury ion Methylene blue Example 1 486 (mg/g) 158 (mg/g) Example 2 287 (mg/g) 95 (mg/g) Example 3 495 (mg/g) 160 (mg/g)