Article pubs.acs.org/EF
Flocculation of Coal Waste Slurry Using Bioflocculant Produced by Azotobacter chroococcum Zhichao Yang,* Wei Wang, and Shengyu Liu College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China ABSTRACT: The flocculation technique is the most common sedimentation method for the dewatering of coal waste slurry, which is fine grained and has high ash content. Bioflocculants are flocculants which are vital to accelerating the settling of coal waste slurry via the flocculation technique. In this work, the bioflocculation of coal waste slurry was studied using a biofloccuant produced by Azotobacter chroococcum. Quartz and clay minerals were used for comparison. Flocculation tests showed that the bioflocculant effectively flocculated coal waste slurry. Approximately 83% of the flocculation rate was achieved at a bioflocculant dosage of 80 mg/L when the coal waste slurry concentration was 30 g/L. The maximum flocculation of coal waste slurry occurred around pH 2. The flocculation of clay minerals and quartz under the same conditions indicated that the bioflocculant flocculated clay minerals effectively and efficiently, but quartz was scarcely flocculated. ζ-Potential measurements showed that the surface charges of demineralized coal, clay minerals, and quartz became more negative after interaction with the bioflocculant. Adsorption tests and FTIR analysis results suggested that the bioflocculant shows a higher affinity to the demineralized coal, kaolinite, and illite than to quartz. Quartz had a negative impact on coal waste slurry flocculation with the bioflocculant.
1. INTRODUCTION It is difficult to achieve dewatering and solid−liquid separation in coal waste slurry, which is the inevitable byproduct of preparation plants. For the dewatering of coal waste slurry containing a high percentage of fine coal particles and high ash, the flocculation technique using various flocculants is the most common sedimentation method to accelerate the settling of particles. The flocculant type and dosage are closely related to the desired sedimentation rate and the water clarity values of coal waste slurry. The problems of residual aluminum and unpleasant metallic taste are usually caused by inorganic flocculants, while organic flocculants lead to environmental problems. Therefore, bioflocculants are more appropriate for coal waste slurry flocculation because of their biodegradability. Bacteria and their products can be excellent flocculants, and they function as selective flocculants in some cases.1 The flocculation of coal using bacteria, sometimes selectively, has been reported in the literature. For instance, Rhodopseudomonas sphaeroides cells have been shown to function as flocculants for coal slurry.2 The bioflocculant consisting of Paenibacillus polymyxa cells removed approximately 55−60% ash of the ash from high-ash Indian coals in a single-stage flocculation experiment.3 The bacterium Mycobacterium phlei shows selective flocculation in fine coal reducing ash or pyritic sulfur, and the differential adsorption ability of coal and clay minerals on the bacterium is responsible for the selectivity.4−6 Additionally, secretions from bacteria, fungi, and yeast, such as proteins, polysaccharides, glycoproteins, and glycolipids, can be used for flocculation.7 For example, Zheng and coauthors proposed that the extracellular biopolymer derived from Bacillus sp. F19 is effective for flocculation. The extracellular biopolymer is found to be a sugar−protein derivative.8 The bioflocculant produced by white-rot fungi demonstrated good performance in the flocculation of high-concentration microparticle slime water.9 © XXXX American Chemical Society
Although these bioflocculant responses produced remarkable effects in flocculating experiments, metal cations also play important roles. The flocculating abilities of bioflocculants are strongly dependent on the concentrations and valences of these cations in the destabilization of colloidal systems. All of the metal ions such as Ca2+, Mg2+, and Mn2+ can enhance the flocculating ability of the bioflocculants produced by the consortium of Cobetia sp. and Bacillus sp., while Li+ and K+ completely inhibit flocculation.9 Ca2+ is more effective in stimulating the flocculating activities of bioflocculants.7 The high cost and secondary pollution are the disadvantages of adding cations during flocculation. Therefore, cation-independent bioflocculants are preferred. The price of conventional media components and the flocculant yields seriously affect the application of bioflocculant in different industrial fields.9,10 Inexpensive substrates have been widely applied to reduce the production costs of bioflocculants. Studies have reported using agricultural and industrial wastewaters, sludge, and rice straw fermentation liquor for bioflocculant production.11−17 Additionally, using semicontinuous fermentation processes and developing better strains are feasible methods for improving bioflocculant yield.18−20 There are very few studies investigating the flocculation behavior of coal waste slurry using extracellular biopolymer flocculants with cation-independent flocculating capabilities. The objective of this study is to report an extracellular biopolymer flocculant with cation-independent flocculating capability in the flocculation of coal waste slurry. Sedimentation tests have been used to determine the effects of bioflocculant treatment on the flocculation rate of coal waste slurry. Received: November 16, 2016 Revised: January 12, 2017 Published: January 17, 2017 A
DOI: 10.1021/acs.energyfuels.6b03052 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Hebei province of China. The results of X-ray diffraction (XRD) spectra and chemical analyses of the mineral samples indicated that the purities of the kaolinite, illite, and quartz were 99, 90, and 99%, respectively, which could meet the desired requirements of this study. The mineral samples were crushed in an agate mortar, and the −5 μm size fraction was used for ζ-potential measurements. Approximately 0.1 g of sample was added to 100 cm3 of 80 mg/L bioflocculant solution. The dilute suspension was stirred, and the pH was adjusted to the desired value using NaOH or HCl. The suspension was further conditioned and allowed to stand for 5 min until the pH stabilized. Then, the ζ potential was measured and the average of five separate readings and its standard deviation were reported. 2.2.3. Adsorption Tests. Adsorption experiments were carried out on demineralized coal and minerals. In each case, 5.0 g of sample was added to 200 mL of bioflocculant suspension (800 mg/L) at the required pH. The suspension was then shaken for some time using a vortex shaker. The effect of the mixing time on the adsorption was investigated at pH 7.8. Similarly, the effect of the pH was studied after 12 h of mixing on the vortex shaker. The optical density (OD550) of the supernatant was then measured to determine the adsorption rate. The adsorption rate was defined by the following formula:
2. MATERIALS AND METHODS 2.1. Materials. 2.1.1. Coal Waste Slurry. The coal waste slurry was supplied from Wangping Coal Preparation Plant, China. The solid percentage of the slurry sample is approximately 6.85%, and the natural pH value is 7.8. The sedimentation tests were carried out using coal waste slurry samples that were concentrated or diluted to the appropriate concentration. 2.1.2. Bioflocculant. Strains isolated from soil were used for bioflocculant production. Soil samples were collected from the Taiyuan University of Technology in Taiyuan, China. The selective medium consisted of the following (in g·L−1): sucrose, 5; kaolin, 0.5; Na2HPO4, 2; MgSO4·7H2O, 0.5; CaCO3, 0.1; and FeCl3, 0.005 (pH 7.0). The isolated strains with different colony characteristics were inoculated in 250 mL Erlenmeyer flasks containing 100 mL of medium and incubated with shaking at 30 °C and 150 rpm for 3 days. Among the 12 isolated bioflocculant-producing strains, the mucous colonies were almost all convex, round edged, and russet as indicated by morphological tests. According to the assessment of flocculating activities, strain G7 has the highest flocculating ability for coal waste slurry in the absence of CaCl2. Therefore, strain G7 was selected and stored at 4 °C for further studies. Strain G7 was observed to form smooth, circular colonies and aerobic bacteria. The bacterium was Gram-positive and rod-shaped under microscopic observation. Subsequently, the 16S rDNA of strain G7 was sequenced following PCR amplification. The 16S rDNA of strain G7 was deposited in databases (Accession No. JQ692178). A high percentage of the 16S rDNA sequence was similar to Azotobacter chroococcum, and the similarity was over 99%. Therefore, the strain G7 was identified as A. chroococcum. The bioflocculant produced by the strain Azotobacter sp. G7 was named BFG7. To produce the bioflocculant, strain G7 was inoculated in 250 mL flasks containing 100 mL of culture medium and grown at 30 °C on a shaker at 150 rpm. After 3 days of cultivation, the cells were removed by centrifugation at 6000 rpm for 20 min. The supernatant was subsequently added to cold ethanol with stirring at the ratio of 2:1 (v/v). Then, the resulting insoluble portion was obtained by centrifugation at 10 000 rpm for 3 min and purified with distilled water. After three such treatments according to the steps above, the bioflocculant was dialyzed against deionized water overnight and then lyophilized to obtain purified bioflocculant. The bioflocculant mainly consisted of protein (36.7%) and sugar (61.8%), including 39.5% neutral sugar, 3.6% uronic acid, and 2.1% amino sugar. The total protein content of the bioflocculant was measured using the method of Bradford with bovine serum albumin as the standard.21 The total sugar content of the bioflocculant was determined using the phenol−sulfuric acid method with glucose as the standard solution.22 The analysis of the sugar composition was performed via the following methods after the bioflocculant was hydrolyzed with trifluoroacetic acid at 121 °C for 1 h. The amino sugars and uronic acid were examined using the Elson−Morgan and carbazole−sulfuric acid methods, respectively.22 The neutral sugar content was determined via the carbazole−sulfuric method.23 The purified bioflocculant was measured for its flocculating ability using the method described in section 2.2.1. 2.2. Methods. 2.2.1. Flocculation Tests. Known volumes of the bioflocculant were added to several suspensions in beakers and stirred using magnetic mixers. For each of the flocculation tests, the mixture was stirred at 50 rpm for 2 min and then transferred to a 250 mL graduated cylinder. After settling for 72 h, the supernatant at the halfheight was withdrawn for optical density determination at the wavelength of 550 nm. The flocculating rate, representing the flocculating activity of the bioflocculant, was defined and calculated by the following formula:
adsorption rate = (C − D)/C × 100% where C is the OD550 (optical density at 550 nm) of the control, while D is the OD550 of the sample supernatant. 2.2.4. Fourier Transform Infrared (FTIR) Spectroscopy. The purified bioflocculant was characterized using a Fourier transform infrared (FTIR) spectroscopy instrument (BRUKER ALPHA-T, Germany) using the disk technique with KBr as the matrix, over the frequency range 4000−400 cm−1. The spectra of the demineralized coal and minerals were evaluated before and after interaction with the bioflocculant. 2.2.5. Analysis of the Coal Waste Slurry Characterization. The method of proximate analysis was used to analyze the fixed carbon, volatile matter, and total ash of the solid in the coal waste slurry. The sulfur content and bomb calorific value were analyzed using an autosulfur-fixation meter (WDL-9A, China) and a reaction calorimeter (ZDHW, China). The mineral components of the solid in the coal waste slurry were determined via X-ray diffraction. The pH was measured with a pH meter (S20K, Mettler). The average of five separate ζ-potential measurements was obtained using a Zeta-Meter 3.0 system. The pulp density was measured using a slurry density meter (MOHO, China). Briefly, 100 g of coal waste slurry was filtered, and the filter cake was dried to a constant weight. Solids by weight of coal waste slurry were obtained via multimeasurement average of the weight of filter cake.
3. RESULTS AND DISCUSSION 3.1. Characterization of Coal Waste Slurry. The composition of the solid particles in the coal waste slurry is the key influencing factor in the flocculation of colloidal suspensions.24−26 With the exception of coal, ash is the most important component of coal waste slurry. The ash content and constitution analysis are shown in Table 1. The X-ray diffraction and proximate analysis of the solid in coal waste slurry indicate that the total ash content is 31.70%, and the main minerals present are kaolinite, illite, and quartz. According to the X-ray diffraction results, there were few carbonate minerals in the coal waste slurry, as supported by the chemical analysis data, which show low percentages of CaO and MgO. Other properties of the coal tailing and coal waste slurry are also given in Table 1. It should be noted that the ζ-potential results show that the particles in coal waste slurry are negatively charged. The solid particle size and its distribution are closely related to the settling characteristics of the coal waste slurry.27 Table 2 shows the size composition of the coal tailing obtained by wet
flocculating rate = (A − B)/A × 100% where A is the OD550 (optical density at 550 nm) of the control, while B is the OD550 of the sample supernatant. The effects of the pH and bioflocculant concentration were investigated. 2.2.2. ζ-Potential Measurements. ζ-Potential measurements of the demineralized coal and minerals (kaolinite, illite, and quartz) were carried out. The kaolinite, illite, and quartz were obtained from the B
DOI: 10.1021/acs.energyfuels.6b03052 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Characterization of the Coal Waste Slurrya constituent
bioflocculants caused restabilization of the colloids in the kaolin suspension.29,30 Fine flocculants are expected to not only neutralize a portion of the surface charge reducing repulsion but also adsorb with loops and tails reaching into the solution, which results in bridging with other particles. According to the theoretical calculations and flocculation tests, the realization of flocculation does not require high doses of flocculant or the complete coverage of particle surfaces by the flocculant.28,31,32 Approximately 50% coverage of the solid surface by flocculant agrees with the optimum dosage of flocculant for the highest settling rate.33 Steric repulsion caused by the complete coverage of the particle surface by the flocculant would lead to stable suspension.28 Similar flocculating rates were obtained at coal waste slurry concentrations of 30 and 50 g/L, while the flocculating ability was lower at 10 g/L. This may be attributed to the increase in density in the coal waste slurry, which increases the mutual repulsive forces between particles.34 Particles adhere to each other during the destabilization of fine particle suspensions via contact and collisions, which result in the growth of flocs. Brownian motion arising from thermal energy gives rise to collisions between particles moving toward each other in the suspending fluid.25 The maximum flocculating rate (83.09%) was achieved at a bioflocculant dosage of 80 mg/ L when the coal waste slurry concentration was 30 g/L. Compared to other bioflocculants, a higher bioflocculant dosage was needed to achieve a higher flocculating rate.35,36 3.2.2. Effect of pH. The pH is important in the flocculating processes because it affects the stability of the suspended particles and the floc formation.24,37 The flocculation tests studying the effect of the pH were conducted over the pH range 2.2−11.2. Suspensions of various types of suspended solids, such as coal waste slurry, quartz, illite, and kaolinite, were tested. The initial bioflocculant concentrations were 40 mg/L for the coal waste slurry and 30 g/L for the various mineral suspensions. Figure 2 shows the effect of the pH on the
solid
fixed carbon (air dried), % volatile matter (air dried), % total ash content (air dried), % sulfur content (total, air dried), % bomb calorific value (gross, at constant volume, air dried), cal/g main mineral component
22.55 ± 0.38 38.41 ± 0.17 31.70 ± 0.45 0.54 ± 0.02 5017 ± 17 kaolinite, illite, and quartz coal waste slurry
parameter
7.8 ± 0.1 −28.21 ± 1.56 6.85 ± 0.41 1.07 ± 0.01
natural pH ζ potential at pH 7.8, mV solids by weight, % pulp density, g/cm3
The data are presented as the means ± the standard deviation of the means (x ± SDM).
a
Table 2. Size Distribution and Ash Content of the Coal Tailing Samplea size (mm)
wt (%)
0.5−0.25 0.25−0.125 0.125−0.075 0.075−0.045
