Wang Weixiao, Ha Jing, Li Zhaoyang, Liu Kui, Gao Leihong
(College of Science£¬Hebei University of Science & Technology, Shijiazhuang 050018, China)
Abstract Chromium(III) salts are the most popularly chemical agent used in the tanning process. Chrome tanned leathers are characterized by top handling quality, high hydro-thermal stability and excellent user properties. But in the tanning process with chromium(III) salts only 60%¡«70% of the total chromium(III) are up-taken by the hides. So the recovery of chromium from tanning wastewater is essential for environmental protection & economic reason. To this purpose, three precipitation and anion polyacrylamide (PAM) as flocculent are used in this research. The effects of pH and dosage on settling rate, Cr concentration in the handled wastewater£¬sludge size and volume were studied in this paper. The results show that the optimum method for recovery chromium is adjust pH of tanning wastewater about 7 by sodium hydroxide, then a small amount of PAM is put into the wastewater at stirring. At the optimum condition, not only high settling¡¡velocity, low volume of sludge and above 99.9% recovery rate of chrome can be obtained, but also the handled wastewater can be discharged to environment without alkalization to the soil because the pH of disposed wastewater is at neutral. Furthermore, the cost discharge is low with flocculation-sedimentation.
One of the most significant problems of the leather industry is waste generation. One tone of wet salted hide yields only 200kg of leather, but over 50m3 of waste water[1,2]. Chrome tanning is the most common type of tanning in the world. In the tanning process using chromium salt, only about 60%-70% of the total chromium are up-taken by the hides, in the other word, about 30%¡«40% of the chromium are discharged with the wastewater. So wastewater from tanning processing poses a significant chrome pollutant to environment and waste lots of chromium. In most countries, regulations governing chrome discharge from tanneries are stringent. All tanneries must thoroughly check their waste streams. Studies show that the concentration of chrome in the tanning wastewater varies from 2500 to 8000 mg.L-1 as the count of Cr2O3.The environmental impact of chrome waste from tanneries has been a subject of extensive scientific and technical dispute.
Several methods to remove chromium from tanning wastewater have been used. There are chemical precipitation, ion exchange, membrane filtration, extraction, reverse osmosis and absorption[4,5]. Chemical precipitation is the most popular method for recovery chrome and sodium hydroxide is the common precipitating regents. However, Cr(OH)3 precipitation forming from NaOH is very gelatinous. The settling rate for NaOH is very slow and the dewatering sludge is very difficult. In order to get a grainy, dense, easily settable precipitation, MgO and Ca(OH) 2 is used as precipitating regents, but MgO is more expensive than NaOH and precipitation forming from Ca(OH) 2 is not desirable.
In order to find a method to remove the chrome from wastewater with low cost and the precipitation easily settable, PAM has been used in this research. By comparing the size of precipitation’ grain, the velocity of precipitation, the settling rate and sludge volume, we confirmed that the optimum method to recover chrome from wastewater is to adjust pH of tanning wastewater 7-8 by sodium hydroxide and then to use a small amount of PAM as flocculent.
- MATERIAL AND METHODS
2.1 chemical reagents
The chemicals used in this research are all of analytical reagent grade. In this research, saturated lime, 1.0 mol.L-1 sodium hydroxide and saturated MgO were used to adjust pH of sample. PAM solution was prepared to yield concentration of 0.5g.mL-1 with deioned water.
2.2 Sample collecting and determination of the most important parameters
Wastewater sample from tanning process were collected in polythethylene bottles after tannery was finished (within 12h), then transported to laboratory and stored in a refrigerator at approximately 4¡ãC. The most important parameters including total chromium concentration, pH, total dissolved solids (TDS), color and COD were determined as soon as possible.
2.3 Treatment of tannery wastewater
Jar tests had been used to determine the effect of chromium removal and property of precipitation or flocculent object. 400mL of wastewater was added to each baker. Precipitant or flocculent were added to each sample separately to adjust pH from 6 to 11. Sample were mixed by magnetic stirrer at 100 rpm for one minute and then with the speed of 30 rpm for 15 min. In order to determine settling¡¡velocity¡¡and sludge volumes, discharged samples were poured into 500mL scaled cylinders and supernatant or sludge volume was read during the settling period. Then precipitation supernatant solution was filtered, chromium concentration was determined by atomic absorption spectroscopy method.
- RESULTS AND DISCUSSION
3.1 Characteristic of the tannery wastewater
The properties of the tannery wastewater are shown in Table 1.
Table 1 Chemical analysis of tannery wastewater
The average of chromium concentration was 2660 mg.L-1 in the raw tanning wastewater. The results showed that the wastewater from tannery process is one of the most important sources which pollutes environment as the concentration of chromium and other harmful material in the wastewater is extremely high. This is also confirmed by many other studies. That is why tanning wastewater should be disposed.
3.2 The optimum pH for removing chromium with different precipitant agents and PAM
Different alkali precipitants were added to each sample separately to adjust pH from 6 to 11. When using PAM as flocculants, the pH is adjust by NaOH and the volume of PAM solution is 2mL. According to the experimental method, precipitation supernatant solution was filtered, chromium concentration was determined after settling 3h. The results were shown in Fig.1.
Fig. 1 Chromium concentration in filter solution vs. pH using different method
Fig.1 shows the minimum solution of chromium is presented at pH 8-9.5 when using NaOH as the precipitant. In case of using Ca(OH)2 and MgO as the precipitant, the minimum chromium concentration of settled wastewater are present at pH8-10. Ether using NaOH as precipitant or using Ca(OH)2 and MgO, increasing pH above 10 will result in increasing the chromium concentration in filter solution. That is because the precipitation of Cr(OH)3 is amphoteric colloid, it will redissolve in acidic or basic solution. When pH is above 10, parts of Cr(OH)3 will be transferred to CrO2–, and CrO2– can be dissolved in water. So the pH should be control carefully when using precipitants to remove chrome from wastewater. Figure 1 also shows the removal rate of chromium from wastewater has little relationship with the kinds of precipitants, it depends mainly on pH of solution.
When using PAM as flocculation, the minimum chromium concentration of settled wastewater are present at pH7-10 and the Cr concentration in filter solution is below 0.2mg.L-1. This means chromium recovery from tanning wastewater can be handled at pH7 and the handled wastewater can be discharged to environment without alkalization to the soil.
3.2 The settling rate of the precipitation at different conditions
Removal chromium from tanning wastewater with different agents will get precipitations with different size and density. The different size and the density will have different settling rate.
The dosage of PAM will affect the size of the precipitation’s grainy, the settling velocity and the sludge volume. Settling rate with different dosage of PAM at pH7 and with different precipitants at pH8.5 is studied. The results are shown in Fig. 2.
Fig.2 Precipitation settling rate at different condition
The results show that precipitation forming from NaOH is very gelatinous and settling rate of precipitation is very slow. Using Ca (OH) 2 as precipitant can improve velocity of settling, but the improvement is a little and the precipitation is also gelatinous and difficult to filter. However, grainy precipitation with higher settling rate and lower volume of sludge was obtained by MgO, but the price of MgO is more expensive than NaOH and lime. This is the agreement with results obtained by Panswad.
Settling rate of the precipitation is fastest when 2mL PAM is used at pH7. Furthermore, a grainy, dense, easily settable precipitate can also be formed. Fig.2 also shows that the optimum dosage of PAM is 2mL/400mL wastewater. If the dosage is more than 4mL, flocculation-sedimentation will be large and light, some of sedimentation will float at the surface. So the optimum dosage of PAM is 2-3mL/400mL wastewater in the following researches.
3.3 Comparison of different methods
The Cr concentration in the filtrate, the size of precipitation and the settling rate are studied in this paper when using different precipitant and flocculent at optimum pH and dosage, the result is described in the Table 2.
Table 2 Comparison of different methods
|Size||Settling rate||Sludge volume
|NaOH||8.5||0.20||very small||Very slow||265|
|PAM||7.0||0.19||very large||Very fast||40|
Sludge volume of the precipitation in Table 2 is determined after 3 h.
Table 2 shows the sludge volume produced by MgO and PAM is much less than sludge volume by Ca(OH)2 and NaOH. The ratio of sludge is:
VPAM: V MgO: V Ca(OH)2: VNaOH=1: 1.4:4: 6.6
Cr concentration in the filtrate solution is very low and meet the demand of discharge standard of water pollutants for leather industry whatever method is used. However, a grainy, dense, easily settable and filtrate precipitate can be formed only when MgO or PAM is used as the precipitator or flocculent. Because using MgO to remove chromium is more expensive than using PAM at pH7, PAM flocculation-sedimentation is a suitable method for removal and recovery chromium from tanning wastewater.
Flocculation-sedimentation is an appropriate technology for recovery chromium from tanning wastewater. The results show that the optimum method for recovery chromium is adjust pH about 7 by sodium hydroxide, then a small amount of PAM solution is put into the wastewater at stirring. At the optimum condition, Not only a grainy, dense, easily settable and filtrate precipitation can be formed, but also the cost discharge is low and the handled wastewater can be discharged to environment without alkalization to the soil.
Acknowledgement This work was supported by the Natural Science Foundation of Hebei Province(No. 2006000305)