Cellulose nanocrystal-supported sodium alginate adsorbent

Abstract

The present disclosure discloses a cellulose nanocrystal-supported sodium alginate adsorbent and use thereof in enriching organic phosphorus in wastewater, belonging to the technical field of environmental engineering. Cellulose nanocrystals are prepared from papermaking deinking sludge through chemical conditioning, drying, crushing, chemical oxidation and microwave assisted separation, and then supported on alginate under weakly acidic conditions to provide the cellulose nanocrystals with the ability to enrich organic phosphorus in wastewater. By using this method, the content of organic phosphorus in the wastewater can be significantly reduced, the total phosphorus in the effluent can be reduced, and the wastewater treatment effluent can satisfy the discharge standard. The enriched organic phosphorus can be recycled as a phosphorus resource through incineration. In addition, the papermaking deinking sludge is made into the product with a high added value, so the applicability of the papermaking deinking sludge is broadened. The present disclosure has high feasibility.

Claims

1. A method for preparing a cellulose nanocrystal-supported sodium alginate adsorbent wherein the method is supporting cellulose nanocrystals on sodium alginate; wherein a process of preparing the cellulose nanocrystals comprises the following steps: (1) carrying out solid-liquid separation on a papermaking deinking sludge mixture, collecting the solid sludge, and then diluting the solid sludge with a buffer solution to obtain a sludge-water mixture; (2) adding a flocculant and slaked lime to the sludge-water mixture obtained in step (1), mixing the mixture uniformly, then carrying out membrane filtration, and collecting sludge trapped on the filter membrane; (3) treating the sludge obtained in step (2) by thermal drying until a water content of the sludge is not more than 30%, and crushing the sludge; (4) rinsing the crushed sludge with the buffer solution, then adding 2,2,6,6-tetramethylpiperidinooxy free radical and sodium bromide, and mixing the mixture uniformly to form a mixed system; and (5) adding a terminating oxidizer to the mixed system obtained in step (4) until a pH of the reaction system no longer changes, terminating the reaction, carrying out solid-liquid separation, and collecting the solid to obtain the cellulose nanocrystals.

2. The method according to claim 1, wherein the method for preparing the cellulose nanocrystal-supported sodium alginate adsorbent comprises the following steps: (a) dispersing sodium alginate in a slightly acidic medium, slowly adding a dewatering filtrate obtained after the membrane filtration in step (2), and mixing the mixture uniformly to obtain a corresponding sodium alginate dispersion; dispersing the cellulose nanocrystals in a slightly acidic medium, and mixing the mixture uniformly to obtain a cellulose nanocrystal dispersion; and (b) then, slowly and dropwise adding the cellulose nanocrystal dispersion to the sodium alginate dispersion to obtain a sodium alginate-cellulose nanocrystal mixed solution, and concentrating and drying the sodium alginate-cellulose nanocrystal mixed solution to obtain the cellulose nanocrystal-supported sodium alginate adsorbent.

3. The method according to claim 1, wherein a mass ratio of the sodium alginate to the cellulose nanocrystals is 2:0.2.

4. The method according to claim 2, wherein the slightly acidic medium is an acetic acid solution with a pH of 3.8-4.6.

5. The method according to claim 2, wherein a concentration of the sodium alginate dispersion is 10 g/L; and a concentration of the cellulose nanocrystals is 1 g/L.

6. The method according to claim 1, wherein in the process of preparing the cellulose nanocrystals, the terminating oxidizer in step (5) is NaClO.

7. The method according to claim 1, wherein in the process of preparing the cellulose nanocrystals, a mass fraction of the flocculant in step (2) relative to the dry weight of the solid sludge is 5%-10%.

8. The method according to claim 1, wherein in the process of preparing the cellulose nanocrystals, a mass fraction of the slaked lime in step (2) relative to the dry weight of the solid sludge is 10%-25%.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 shows equilibrium adsorption capacities and enrichment rates of different adsorbents for TCEP.

DETAILED DESCRIPTION

(2) The present disclosure will be described by examples according to the contents in the claims.

Example 1: Dewatering and Drying of Papermaking Deinking Sludge

(3) 10 L of papermaking deinking sludge mixture was acquired from a deinking wastewater treatment process section of a papermaking wastewater treatment site (a paper mill in Jiaxing). 500 mL of the sludge-water mixture that had been uniformly mixed was taken out (there were 6 duplicate samples), and centrifuged. Then, the solid sludge was diluted with a phosphate buffer with a pH of 7.0 for 3 times, and the final volume was adjusted to 500 mL. For 3 of the duplicate samples, the sludge-water mixture was subjected to vacuum filtration, the sludge sample trapped on the filter membrane was placed in an oven and dried at 105° C. for 4 h, and the dry weight of the papermaking deinking sludge was obtained. For the other 3 duplicate samples, AlCl.sub.3 (5% of the dry weight of the sludge) and slaked lime (20% of the dry weight of the sludge) were sequentially added to the 500 mL of sludge-water mixture. After 2 min of high speed stirring (at a speed of 300 r/min) and 10 min of low speed stirring (at a speed of 50 r/min), vacuum filtration was carried out to obtain the sludge trapped on the filter membrane, and the dewatering filtrate was collected for later use. Thermal drying was carried out at 200° C. until the water content in the sludge was reduced to 30%, and the sludge was crushed for later use.

Example 2: Oxidation of Dried Papermaking Deinking Sludge and Separation of Cellulose Nanocrystals

(4) The crushed dried papermaking deinking sludge in the duplicate samples was taken and rinsed with the phosphate buffer with a pH of 7.0 to make the volume of the mixture reach 100 ml. With stirring at a speed of 150 r/min, 80 mg of (0.2 mmol) 2,2,6,6-tetramethylpiperidinooxy free radical (TEPMO) and 80 mg of (5.0 mmol) NaBr were sequentially added. After the TEPMO and the NaBr were completely dissolved, a 10.0 mmol/L NaClO solution was slowly and dropwise added until a pH of the reaction system no longer changed. Then, the reaction was terminated with a slight excess of anhydrous ethanol, and finally microwave assisted separation and centrifugation were carried out to obtain the cellulose nanocrystals.

Example 3: Establishment of Alginate-Cellulose Nanocrystal System

(5) Under slightly acidic conditions provided by acetic acid, 2 g of sodium alginate was added to 100 mL of acetic acid solution (pH=4.5), 50 mL of the dewatering filtrate of the papermaking deinking sludge was slowly added, and stirring was carried out to make the sodium alginate thoroughly mixed with calcium ions in the dewatering filtrate and make the sodium alginate uniformly dispersed, thereby obtaining an alginate-acetic acid mixture. 0.2 g of the obtained cellulose nanocrystals was dissolved in 50 mL of acetic acid solution (pH=4.5). After the mixture was stirred uniformly, the alginate-acetic acid mixture was slowly added. Then, the obtained alginate-cellulose nanocrystal mixed solution was dried in an oven at a low temperature (75° C.) for 24 h. After reaching an equilibrium water content, the alginate-cellulose nanocrystal was collected for later use.

Example 4: Effect of Alginate-Cellulose Nanocrystal on Enriching Organic Phosphorus in Printing and Dyeing Wastewater

(6) The obtained alginate-cellulose nanocrystal was made into an adsorbent. The adsorbent was mixed with 50 mL of printing and dyeing wastewater (2.2 g of the adsorbent was used to treat 50 mL of printing and dyeing wastewater). It was found that the alginate-cellulose nanocrystal has a good effect on enriching organic phosphorus. Its equilibrium adsorption capacity for TCEP was up to 35.7 mg/g, and the enrichment rate was more than 85%. Therefore, the contribution of TCEP to the total phosphorus in the effluent was effectively reduced, and the effluent could satisfy the discharge standard. The alginate-cellulose nanocrystal could be regenerated through ozone oxidation and reused to adsorb TCEP. After the regeneration, the equilibrium adsorption capacity was still higher than 30.0 mg/g, which met the requirement of cyclic utilization. Therefore, it is determined that the cellulose nanocrystal prepared above has a good effect on enriching organic phosphorus and thus provides a new way for utilizing papermaking deinking sludge as a resource.

Comparative Example 1: Determination of Concentration of Terminating Oxidizer in Oxidation System

(7) Those that do not meet the conditions cannot become cellulose nanocrystals, nor can they be used to enrich organic phosphorus. Based on the average diameter D, the average length L and the L/D ratio, the concentration of the oxidizer was finally determined. As shown in Table 1, when NaClO was used as the terminating oxidizer, cellulose nanocrystals could not be obtained with relatively low concentrations of NaClO. When the concentration of NaClO was higher than 10 mmol/L, the oxidation system could successfully prepare the cellulose nanocrystals. Considering the preparation cost, 10 mmol/L was finally determined as the concentration of NaClO.

(8) TABLE-US-00001 TABLE 1 Effect of NaClO terminating oxidizer on preparation of cellulose nanocrystals Concentration 5.0 8.0 10 12 15 mmol/L mmol/L mmol/L mmol/L mmol/L Average 2.7 nm 2.9 nm 3.0 nm 3.0 nm 3.1 nm diameter D Average 295 nm 277 nm 270 nm 263 nm 255 nm length L L/D 109.3 69.3 90.0 87.7 82.3

Comparative Example 2: Separation of Cellulose Nanocrystals

(9) Both the ultrasound method and the microwave method could separate the substance from the matrix. After investigating the characteristics of the cellulose nanocrystals under different energy densities, the most suitable method for separating cellulose nanocrystals was determined. As can be seen from Table 2, when the ultrasound method was used to separate the cellulose nanocrystals, satisfactory cellulose nanocrystals could not be prepared under the energy density of 1100 J/mL. As the energy density increased, the cellulose nanocrystals could be produced successfully. When the microwave method was used to separate the cellulose nanocrystals, the obtained cellulose nanocrystals were satisfactory in average diameter, average length and L/D. Considering the energy consumption, the microwave method with the energy density of 1000 J/ml was selected to separate the cellulose nanocrystals.

(10) TABLE-US-00002 TABLE 2 Characteristics of cellulose nanocrystals obtained by different separation methods under different conditions Microwave method Microwave method Microwave 1000 1250 1500 1100 1350 1600 method J/mL Average  2.9 nm  2.8 nm  2.8 nm  2.6 nm  2.7 nm  2.8 nm diameter D Average  285 nm  265 nm  230 nm  272 nm  265 nm  260 nm length L L/D 98.3 94.6 82.1 104.6 98.1 92.9

Comparative Example 3: Effects of Cellulose Nanocrystals Supported on Different Matrices on Enriching Organic Phosphorus

(11) In order to investigate the effect of cellulose nanocrystal-based adsorbents on enriching organic phosphorus (TCEP) in printing and dyeing wastewater, alginate-cellulose nanocrystal (with additional calcium ions), alginate-cellulose nanocrystal (with the sludge dewatering filtrate), chitosan-cellulose nanocrystal and zeolite-cellulose nanocrystal were respectively tested for their effect on enriching organic phosphorus. By adding CaO to condition the papermaking deinking sludge, the sludge dewatering filtrate had a relatively higher concentration of calcium ions. As shown in Table 3, the chitosan-cellulose nanocrystal and the zeolite-cellulose nanocrystal were selected to enrich the TCEP, their equilibrium adsorption capacities and enrichment rates were respectively lower than those of the adsorbents using alginate as the matrix. When the alginate solution was prepared with the additional calcium ions, its initial equilibrium adsorption capacity and enrichment rate were higher than those of the alginate solution prepared with the sludge dewatering filtrate. However, after eight times of regeneration, their equilibrium adsorption capacities and enrichment rates for TCEP were almost the same. Therefore, considering the convenience in operation and the cost, the alginate prepared with the sludge dewatering filtrate was selected, and the alginate-cellulose nanocrystal formed therewith was the more effective and economical adsorbent for enriching organic phosphorus in printing and dyeing wastewater.

(12) TABLE-US-00003 TABLE 3 Effect of cellulose nanocrystal adsorbents on enriching organic phosphorus .sup.aAlginate- .sup.bAlginate- Chitosan- Zeolite- cellulose cellulose cellulose cellulose nanocrystal nanocrystal nanocrystal nanocrystal Equilibrium 38.5 35.7 26.5 28.9 adsorption capacity for TCEP (mg/g) Enrichment 88.7 85.3 81.0 80.5 rate for TCEP (%) Equilibrium 30.1 30.0 23.1 21.2 adsorption capacity.sup.c for TCEP (mg/g) Equilibrium 82.0 82.1 75.7 70.5 adsorption capacity.sup.c for TCEP (mg/g) .sup.aalginate matrix prepared with additional calcium ions; .sup.balginate matrix prepared with calcium ions in the sludge dewatering filtrate; .sup.ceffect on enriching TCEP after eight times of regeneration.