Wet reclamation of sodium silicate used sand and biological treatment of its wastewater by Nitzschia palea

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1 Wet reclamation of sodium silicate used sand and biological treatment of its wastewater by Nitzschia palea Wang Huafang, *Fan Zitian, Yu Shaoqiang, Liu Fuchu, and Li Xuejie (State Key Laboratory of Material Process and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan , China) Abstract: The massive amount of sodium silicate in the used sand was a pollution source, especially in the waste water from the wet reclamation of used sand. A new process of wet reclamation by biologically treating the waste water produced during the wet reclamation process of used sand was studied in the paper. In the work, the precultivation of N. palea was performed firstly, and three different scrubbing solutions: (1) tap water, (2) modified medium for N. palea, and (3) filtrate of the broth treated by N. palea for 15 days, were used. The results of the primary investigation show that a de-skinning ratio of 90% is obtained when using the scrubbing solution containing modified medium for N. palea at the ratio 1:2 of sand and scrubbing solution, and the maximal concentrations of Na + and SiO 3 ² - are 1.49 g L -1 and 0.51 g L -1, respectively. The results of the optimal biomass, ph value decrease and Na + and SiO 3 consumption indicate the optimal incubation conditions are at the irradiance of 5,000 lux and 25 ºC. Using the filtrate of the broth treated by N. palea for 15 days as the scrubbing solution directly, a de-skinning ratio of 93% is the highest compared to the results of the tap water and the modified medium for N. palea. In the biological process using N. palea, less water is used and little wastewater is produced, which is advantageous to the purpose of green manufacturing and environmental protection. Key words: sodium silicate; used sand; biological reclamation method; N. palea CLC numbers: TG Document code: A Article ID: (2012) Employing sodium silicate as an inorganic binder encouraged many foundries to switch over to it from organic binders, due to the fume-free characteristic sand and low concentration of volatile thermal decomposable products associated with the inorganic composition [1]. In the foundry industry, sodium silicate is widely used for molding sand due to such advantages as high productivity in molding practice, good size accuracy of castings, rich material source, and simple process equipment [2]. But the reclamation of used sand with sodium silicate was a terrible problem because spoiled used sand could cause great environmental problem. Some reclamation methods including wet reclamation, dry reclamation and chemical reclamation have been reported [3]. In the wet reclamation process, a de-skinning ratio of 90% was obtained because the residual binder membrane of sodium silicate could dissolve in the scrubbing solution. But a lot of scrubbing solution was used during the wet reclamation process *Fan Zitian Male, born in 1962, Professor. He is currently the advisor of doctoral candidates in the Huazhong University of Science and Technology, and his research interests are mainly focused on precision forming technologies of Mg/Al alloys, green sand and sodium silicate bonded sand casting technologies. The first author, Wang Huafang, is one of his doctoral students. fanzt@mail.hust.edu.cn, wanghfhust@163.com Received: ; Accepted: and the alkaline wastewater produced in the wet reclamation process needed to be treated by flocculants such as PAC or PAM before being discharged [4]. Especially in China, large amounts of used sand has been discarded in valleys, rivers and lakes; which caused the water and soil, even the groundwater to be polluted by the dissoluble sodium silicate and ester in the used sand [5,6]. Generally, the used sand is treated using tap water directly during the wet reclamation process, so a great amount of tap water is needed which leads to the production of a large mass of wastewater. Nitzschia palea (for short N. palea below), as an industrial diatom material, can breed rapidly in the environment of the alkaline solution through the consumption of Na + and SiO 3. The present work, therefore, just utilized this characteristic of N. palea; and an economical and recyclable scrubbing solution of this diatom for the wet reclamation of sodium silicate used sand was studied. 1 Materials and methods 1.1 Pre-cultivation of N. palea and modified medium for N. palea N. palea, FACHB-203 was obtained from the freshwater algae culture collection of the Institute of Hydrobiology, Chinese Academy of Sciences, China. This alga was a euryhaline 34

2 February 2012 organism, and was adaptable to a certain salinity variation. Especially, this alga was prone to breakout of water bloom when the conditions were favorable. Due to a small amount of N. palea obtained only, the precultivation of N. palea must be conducted firstly to get a large amount of medium for N. palea for the experiments. And the medium for N. palea should contain such ingredients as Na 2 SiO 3, NaNO 3, K 2 HPO 4, KH 2 PO 4, etc, which is very beneficial to the quick growth of this alga. However, because the nitrogen source of NaNO 3 in the medium have Na + that has interfere for the reclaiming results of Na + in the experiments, we changed NaNO 3 into CO(NH 2 ) 2. The medium for N. palea pre-cultivation is listed in Table 1 and Table 2. N. palea was trained to adapt to the high SiO 3 concentration until it could endure a SiO 3 concentration of 0.5 g L -1. The modified medium for N. palea was prepared according to Tables 3. In order to reduce the usage of water and make the best use of the wastewater, an additional experiment was conducted. The broth treated by N. palea after 15 days was filtered to discard the diatom and the filtrate was employed as scrubbing solution directly. Table 1: Medium for N. palea pre-cultivation Na 2 SiO g L -1 NaNO g L -1 K 2 HPO g L -1 MgSO g L -1 CaCl g L -1 KH 2 PO g L -1 MnSO g L -1 C 6 H 5 O 7 Fe g L -1 A5 (See Table 2) 1 ml L -1 Soil extraction solution 20 ml L -1 Table 2: and concentration of A5 H 3 BO 3 MnCl 2 4H 2 O Na 2 MoO 4 2H 2 O CuSO 4 5H 2 O ZnSO 4 7H 2 O Co(NO 3 ) 2 6H 2 O Table 3: Modified medium for N. palea 2.86 g L -1 dh 2 O 1.86 g L -1 dh 2 O 0.39 g L -1 dh 2 O 0.08 g L -1 dh 2 O 0.22 g L -1 dh 2 O 0.05 g L -1 dh 2 O *Note: A5 is a reagent available in market Na 2 SiO 3 (Na + and SiO 3 ) Na + : 1.49 g L -1, SiO 3 : 0.51 g L -1 CO(NH 2 ) g L -1 K 2 HPO g L -1 MgSO g L -1 CaCl g L -1 KH 2 PO g L -1 MnSO g L -1 C 6 H 5 O 7 Fe g L -1 A5 (See Table 2) 1 ml L -1 Soil extraction solution 20 ml L -1 Research & Development In the study, three different scrubbing solutions: (1) tap water, (2) modified medium for N. palea, and (3) filtrate of the broth treated by N. palea for 15 days, were used, and the results were compared. 1.2 Treatment of sodium silicate bonded used sand In the study, sodium silicate bonded used sand with CO 2 -cured process was obtained from the Steel Foundry of Chang Jiang Energy Corporation, Wuhan, China. In the used sand, the adding amount of sodium silicate is 8% of sand weight. In order to obtain optimal scrubbing efficiency, five different rates of used sand and scrubbing solution: 1:0.5, 1:1, 1:2, 1:3, and 1:4, were adopted, respectively, in the experiments. The used sand was scrubbed according to the above rates of used sand and scrubbing solution, respectively, in a sand-washing machine at a stirring speed of 2,000 rpm. 1.3 Effects of temperature and irradiance on growth of N. palea, Na + and SiO 3 ² - consumption by N. palea The temperature and irradiance are two very important cultivation parameters to N. palea. In order to find basic conditions to adapt to the cultivation of N. palea, we used an intelligent illumination box with the capacity of self-regulating temperature and irradiance to incubate. The incubations were carried out at different temperatures of 20, 23, 25, 27 and 30 and different irradiances of 1,000, 3,000, 5,000, 7,000 and 9,000 lux, respectively. After 10 days of cultivation in the illumination box, 5 ml of pre-cultivation medium with N. palea were directly added to 150 ml filtrate of scrubbing solution with the rates 1:2 of used sand and scrubbing solution of modified medium for N. palea. 1.4 Analytical methods At different times in the incubation period, 5 ml of culture broth was taken from the flask, and the cell growth (optical density) was measured by a spectrophotometer (V-1100, Shanghai MAPADA Instruments Co., Ltd) at 680 nm according to the method of Becker [7]. Na + and SiO 3 were measured referring to GB/T , China. The de-skinning ratio was measured as described by Fan et al. in Reference [8]. 2 Results and discussion 2.1 Effects of different scrubbing solutions on de-skinning ratio As shown in Fig. 1, the de-skinning results of sodium silicate bonded used sand by the modified medium for N. palea and tap water indicate that the former was advantageous to reclamation. The optimal de-skinning ratio using the modified medium for N. palea reaches 90% and is higher than 85% using tap water; which means the modified medium for N. palea is more suitable for wet reclamation and propitious to the biological treatment of wastewater by N. palea. The ph 6.30 of the modified medium for N. palea was a little lower 35

3 Fig. 1: Effects of ratio of sand and scrubbing solutions on de-skinning ratio than that of tap water (ph 6.80) and because the soil extraction solution in the modified medium for N. palea was acidic and can react with the residual sodium silicate to a certain extent, so the modified medium for N. palea is advantageous to the de-skinning process of the used sand [9]. 2.2 Effects of ratio of sand and scrubbing solution on concentration of Na + and SiO 3 The amount of water used in the wet reclamation process remarkably influences the concentration of Na 2 SiO 3, which could affect the growth of N. palea, Na + and SiO 3 consumption by N. palea. As shown in Fig. 2, the maximal concentrations of Na + and SiO 3 were 1.49 g L -1 and 0.51 g L -1 respectively at the ratio 1:2 of sand and scrubbing solution. During the process of wet reclamation, only part of the Na 2 O msio 2 could be dissolved in the scrubbing solution by being turned into SiO 3, but nearly all of the Na + could be dissolved in the scrubbing solution [10]. From Fig. 1, the optimal de-skinning ratio was also obtained at ratio 1:2 of sand and scrubbing solution. It indicates that the used sand was treated Tap the best at this ratio. During the process of wet reclamation, there are two processes (dissolving and scrubbing) which influence the de-skinning ratio and are impacted by some reasons such as the amount of scrubbing solution and ph value. In this study, employing the modified medium for N. palea as scrubbing solution at the ratio 1:2 of used sand and scrubbing solution was propitious to the wet reclamation. 2.3 Effects of temperature on growth of N. palea, ph value, Na + and SiO 3 ² - consumption by N. palea The SiO 3 concentration of 0.51 g L -1 was a very high concentration to the growth of the normal Nitzschia sp., so we used a trained N. palea to treat the filtrate obtained during the wet reclamation process of the used sand. Figure 3 shows the effects of temperature on the growth of N. palea, ph value and Na + and SiO 3 consumption by N. palea. As shown in Fig. 3(a), temperature of 25 was optimal for the growth and the highest biomass of 1.2 g L -1 was obtained at the cultivation time of day 14. Under the optimal temperature, the ph value of the broth was decreased very fast to 8.12 after 15 days cultivation (see Fig. 3b). As shown in Figs. 3(c) and 3(d), Na + and SiO 3 were consumed quickly at 25 and their residual concentrations were 0.16 g L -1 and 0.87 g L -1 respectively, at day 15. Temperatures higher or lower than 25 were not propitious to the growth of N. palea, ph value decline and Na + and SiO 3 consumption by N. palea. The results indicated that the growth was closely correlated to the ph value decrease, Na + and SiO 3 consumption by N. palea. 2.4 Effects of irradiance on growth of N. palea, ph value, Na + and SiO 3 consumption by N. palea The different irradiances from 1000 to 9000 lux were investigated in this study (see Fig. 4). As shown in Fig. 4(a), though the biomass was increased with the rising of the irradiance (up to 5,000 lux), we could conclude that an irradiance of more than 3,000 lux was adequate for the growth of N. palea. The diatom grew the fastest at the best irradiance of 5000 lux. The results of ph value, Na + and SiO 3 consumption by N. palea were similar to those of the temperature. Under the optimal irradiance, the maximal biomass, lowest ph value, and minimal residual Na + and Fig. 2: Effects of ratio of sand and medium for N. palea on concentration of Na + and SiO 3 36

4 February 2012 Research & Development SiO Fig. 3: Effects of temperature on growth of N. palea, ph value and Na + and SiO 3 consumption by N. palea SiO 3 Fig. 4: Effects of irradiance on growth of N. palea, ph value and Na + and SiO 3 consumption by N. palea SiO 3 by N. palea were 1.49 g L -1, 8.11, 0.83 g L -1 and 0.16 g L -1, respectively. It proved further that the faster the N. palea grew, the better the efficiency of biological treatment of the wastewater was. Many other factors influencing the reclaiming ratio of this biological treatment would be discovered in the further investigation, such as nutritional factors (N, P, Mg) and other environmental factors (ph, DO, flow velocity), etc. 2.5 Effect of filtrate of broth treated by N. palea on de-skinning ratio Figure 5 show the effects of the filtrate of broth treated by N. palea for 15 days as scrubbing solution directly on the deskinning ratio. In Fig. 5, a better result with the de-skinning ratio of 93% than those of the tap water and the modified medium for N. palea was obtained, which means a very good 37

5 De-skining ratio (%) Fig. 5: Effects of filtrate of broth cultivated by N. palea used as scrubbing solution directly on de-skinning ratio: 1-Tap water; Modified medium for N. palea; 3-Filtrate of the broth treated by N. palea for 15 days result for the wet reclamation. After N. palea was cultivated for 15 days at the optimal conditions (25 and irradiance of 5,000 lux), the ph value, Na + and SiO 3 concentration of the broth filtrate were 8.15, 0.87 g L -1 and 0.16 g L -1, respectively. In this alkaline solution with high Na + concentration and low SiO 3 concentration, the sodium silicate with high modulus value in the used sand was turned into dissolved sodium silicate [10]. Some kinds of proteins and surfactants produced by N. palea also could accelerate the dissolution of the sodium silicate [11]. 3 Conclusions (1) A new recyclable scrubbing solution and biological reclaiming process of waste water from wet reclaiming of sodium silicate used sand was introduced in this paper. (2) Using the modified medium for N. palea as the scrubbing solution and at the ratio 1:2 of sand and scrubbing solution, the optimal de-skinning ratio of 90% is obtained and the maximal concentrations of Na + and SiO 3 ² - are 1.49 g L -1 and 0.51 g L -1, respectively. When employing the filtrate of the broth treated water by N. palea for 15 days as the scrubbing solution directly, the de-skinning ratio of 93% is the highest compared to the results of the tap water and the modified medium for N. palea. It indicates that the modified medium is an economical and recyclable scrubbing solution for wet reclamation of sodium silicate in the used sand; and could be recycled many times by being topped up with tap water. This scrubbing solution meets the purpose of green manufacturing and environmental protection. (3) The biological reclaiming results of the optimal biomass, ph value decrease and Na + and SiO 3 ² - consumption indicate the optimal incubation conditions are at the irradiance of 5,000 lux and 25 ºC. References [1] Parappagoudar M B, Pratihar D K, and Datta G L. Neural network-based approaches for forward and reverse mappings of sodium silicate-bonded, carbon dioxide gas hardened moulding sand system. Materials and Manufacturing Processes, 2009, 24 (1): [2] Rabbii A. Sodium silicate glass as an inorganic binder in foundry industry. Iranian Polymer Journal. 2001, 10 (4): [3] Wang Jina and Fan Zitian. "Freezing-mechanical reclamation of used sodium silicate sand. International Journal of Cast Metals Research, 2010, 23 (5): [4] He Fuqiang, Fan Zitian and Wang Jina. Study of sewage treatment in wet reclamation process by used sodium silicate sand. Foundry Technology, 2008, 29(11): (in Chinese) [5] Ji S, Wan L, and Fan Z. The toxic compounds and leaching characteristics of used foundry sand. Water Air and Soil Pollution, 2001, 132 (3-4): [6] Dungan R S, Kukier U, and Lee B. Blending foundry sand with soil: Effect on dehydrogenase activity. Science of the Total Environment, 2006, 357 (1-3): [7] Becker E W. Microalgae Biotechnology & Microbiology. Cambridge: Cambridge University Press, 1994: [8] Fan Zitian and Lu Jianyuan. Measurement of the Na2O content and ratio of the remained binder film shredded by reclaimer in sodium silicate used sand reclamation. Research Studies on Foundry Equipment, 1999 (4): (in Chinese) [9] Zeng Hui, Li Huaying, Gao Zhijun, et al. Study on acidconsuming and waste water treatment technique of sodiumsilicate-bonded sand. Journal of Wuhan Institute of Technology. 1992, 14(1): (in Chinese) [10] Zhu Chunxi, Lu Chen, Ji Minsheng, et al. The chemical reclamation of waterglass sand. Foundry, 1998 (7): (in Chinese) [11] Морозов И В. Современное направление улуцшене выбиваемости жидкостк -ольных смесей.лит. Произ., 1984(2): (in Russian) This work was financially supported by the National Natural Science Foundation of China (No ) and the Research Fund for the Doctoral Program of Higher Education of China (No ). 38