metals Article
A Case Study of Landfill Leachate Using Coal Bottom Ash for the Removal of Cd2+, Zn2+ and Ni2+ Julia Ayala and Begoña Fernández * Laboratorio de Metalurgia, Escuela de Minas Energía y Materiales, Universidad de Oviedo, Independencia 13, 33004 Oviedo, Spain;
[email protected] * Correspondence:
[email protected]; Tel.: +34-98-5104235 Academic Editor: Hugo F. Lopez Received: 3 October 2016; Accepted: 16 November 2016; Published: 29 November 2016
Abstract: The removal of Cd2+ , Zn2+ and Ni2+ by coal bottom ash has been investigated. In single metal system, metal uptake was studied in batch adsorption experiments as a function of pH (2–3), contact time (5–180 min), initial metal concentration (50–400 mg/L), adsorbent concentration (5–40 g/L), particle size, and ionic strength (0–1 M NaCl). Removal percentages of metals ions increased with increasing pH and dosage. Removal efficiency at lower concentrations was greater than at higher values. The maximum amount of metal ion adsorbed in milligrams per gram was 35.4, 35.1 and 34.6 mg/g for Zn2+ , Cd2+ and Ni2+ , respectively, starting out from an initial solution at pH 3. Simultaneous removal of Zn2+ , Cd2+ and Ni2+ ions from ternary systems was also investigated and compared with that from single systems. Cd2+ uptake was significantly affected by the presence of competing ions at pH 2. The results obtained in the tests with landfill leachate showed that bottom ash is effective in simultaneously removing several heavy metals such as Ni, Zn, Cd, As, Mn, Cu, Co, Se, Hg, Ag, and Pb. Keywords: bottom ash; heavy metal; removal; landfill leachate; wastewater treatment
1. Introduction Some industrial activities like mining, smelting and refining of non-ferrous metals, fertilizer industries, the manufacture of batteries, and paper industries discharge heavy metal wastewaters directly or indirectly into the environment. The presence of these metals has a potentially damaging effect on human and other biological systems because heavy metals are not biodegradable and tend to accumulate in living organisms [1]. The most widely used techniques for removing metal ions from wastewater include chemical precipitation, ion exchange, reverse osmosis and solvent extraction and adsorption [2–14]. Several studies have reported that significant amounts of heavy metals are removed from solution by adsorption on fly ash [15–29]. Coal bottom ash consists of heavy particles that fall to the bottom of coal power plants. According to the American Coal Ash Association (ACAA), the U.S. industry generated 14.4 million metric tons of bottom ash during 2013 [30]. Bottom ash properties are influenced by the design and type of power plant, the source and feed of fuels and by the type of coal and secondary fuels. The chemical composition of bottom ash is determined by the chemistry of the coal and the combustion process. Bottom ash is mainly composed of silica, alumina, and iron with small amounts of calcium, magnesium, sulfate, etc. The chemical composition of bottom ash is similar to that of fly ash, but typically contains greater quantities of carbon. The properties of bottom ash are similar in form and composition to fine aggregates like sand and gravel, making them useful for applications in the construction industry. This by-product Metals 2016, 6, 300; doi:10.3390/met6120300
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is predominantly used for the following applications: road base and sub-base layers under rigid and flexible pavements, structural fill, backfill, aggregate for concrete, asphalt and masonry, abrasives, etc. [31–35]. Bottom ash is also often used as a sorbent for pollutants in synthetic water. However, few studies have been conducted using this by-product to remove Ni, Cd and Zn from real wastewaters [36–43]. According to Ecoba, about 2.4 million metric tons of bottom ash were used in the construction industry in Europe (EU 15) in 2008 Joint EURELECTRIC/ECOBA [44]. The aims of this study are to examine the removal of Zn2+ , Cd2+ and Ni2+ ions from an aqueous solution by bottom ash and to investigate the feasibility of using a low-cost treatment for the removal of heavy metals from landfill leachate. The removal of heavy metal ions by bottom ash was evaluated under various conditions such as pH, contact time, initial concentrations, particle sizes and bottom ash dosages. This paper also presents the results of the removal of Zn2+ , Cd2+ and Ni2+ from multicomponent solutions and landfill leachate. An important environmental problem that arises in the landfills of abandoned mines is the leachates discharge that can contaminate soil and groundwater with heavy metals. Utilization of some industrial wastes, such as bottom ash, for the treatment of these wastes, entails many benefits in both economic and environmental terms. Materials used in this study come from local industries in northern Spain. 2. Materials and Methods 2.1. Materials The coal bottom ash used in this research study comes from a power plant burning very low quality coal, mine tailings and biomass using a fluid bed technology. This by-product was dried in an oven at 105 ◦ C overnight before use. It was characterized by means of two different instrumental techniques: X-ray fluorescence (Phillips PW2404, Phillips, Eindhoven, The Netherlands) and X-ray diffraction analysis (Phillips PW1430 system with Cu Kα radiation, Phillips, Eindhoven, The Netherlands). The diffractometer was operated at 45 kV and 40 mA, over the range of 2θ from 5◦ to 80◦ , with a detector speed of 1◦ /min. The loss on ignition (LOI) was calculated by heating a pre-weighed dry sample to 900 ◦ C for 3 h, using a gravimetric technique to analyze the sulfate content. The bottom ash was screened to give different particle sizes: >3.15 mm, 3.15–1 mm, 1–0.5 mm, 0.5–0.3 mm, 0.3–0.1 mm, and Zn > Ni ; Mohan et al. [20] reported that the uptake behavior of fly ash 2+ 2+ 2+; and, for Cetin (19), 2+ 2+ 2+. However, ashheavy for heavy cations follows the order: > ;Zn > .Ni for metalmetal cations follows the order: Cd2+ >Cd Zn and, for Cetin (19), Zn2+ Zn > Ni However, the the adsorption studies carried by M. Visa [27] toheavy estimate heavy metal removal from adsorption studies carried out byout M. Visa et al. [27]ettoal. estimate metal removal from wastewater 2+ ≥ Cd 2+ ,>while wastewater using activated flythat ashthe showed that follows the efficiency follows order: Pb2+ Zn2+ ≥Ricou Cd2+, using activated fly ash showed efficiency the order: Pb2+the > Zn while Ricou et al. studied the removal of fly heavy by of flyflyash and a mix of fly the ash/lime, et al. [47] studied the[47] removal of heavy metals by ash metals and a mix ash/lime, reporting order: 2+ > Zn2+ reporting the Ni > order: Cd2+ . Ni2+ > Zn2+ > Cd2+. 3.3.3. 3.3.3. Effect Effectof ofDosage Dosage The Theeffect effectdosage dosagewas wasstudied studiedby byshaking shaking400 400mg/L mg/L solution with different different doses doses of of bottom bottom ash, ash, ranging rangingfrom from(5–40 (5–40g/L), g/L), for 180 min at pH 2 and 3. ItItwas that the the percentage of metal removal gradually increased as the dose adsorbent wasobserved observed that percentage of metal removal gradually increased as of the dose of 2+, Cd increased 5 to 40 g/L5(Figure 4).(Figure The uptake from Zn2+from , Cd2+ Ni2+2+and solutions was 67.5%, adsorbentfrom increased from to 40 g/L 4). The uptake Znand Ni2+ solutions was 47.5% 54.7%, at an adsorbent concentration of 5 g/L, to 100% at 20 at g/L 67.5%,and 47.5% and respectively, 54.7%, respectively, at an adsorbent concentration of 5increasing g/L, increasing to 100% 20 when the initial solutions hadhad a pH of 3. thethe case of of a solution g/L when the initial solutions a pH of In 3. In case a solutionwith withpH pH2,2,when whenthe theadsorbent adsorbent concentration concentrationincreased increasedfrom from55to to40 40mg/L, mg/L, the loading capacity of bottom ash increased increased from from 22.8% 22.8% 2+ 2+ 2+ 2+ 2+ 2+ to andfrom from 5.6% 5.6% to to 73.5% 73.5% for for Ni Ni . . to96.3% 96.3%for forZn Zn , from 10.8% to 92.8% for Cd ,,and
Figure 4. Effect of Bottom ash dosage on the removal of heavy metals. Figure 4. Effect of Bottom ash dosage on the removal of heavy metals.
3.3.4. Effect of Ionic Strength 3.3.4. Effect of Ionic Strength The effect of ionic strength on metal removal was studied employing a heavy metal The effect of ionic strength on metal removal was studied employing a heavy metal concentration concentration of 300 mg/L, varying the background electrolyte of the solution (0–1 M NaCl) with an of 300 mg/L, varying the background electrolyte of the solution (0–1 M NaCl) with an adsorbent adsorbent concentration of 10 g/L at pH 2 and 3. concentration of 10 g/L at pH 2 and 3. In the tests performed with solutions at pH 3, it was found that the presence of different In the tests performed with solutions at pH 3, it was found that the presence of different concentrations of NaCl did not decrease the effectiveness in removing heavy metals. At pH 2, concentrations of NaCl did not decrease the effectiveness in removing heavy metals. At pH 2, however, however, the results indicate that small amounts of NaCl (0.01 to 0.1 M) decreased the removal of the results indicate that small amounts of NaCl (0.01 to 0.1 M) decreased the removal of heavy metals, heavy metals, while increasing NaCl to 1 M enhanced the amount of metal removed (Figure 5). while increasing NaCl to 1 M enhanced the amount of metal removed (Figure 5).
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Figure 5. 5. Effect Effect of of ionic ionic strength strength on on the the removal removal of of Zn, Zn, Cd Cd and and Ni. Ni. Figure Effect Figure 5. of ionic strength on the removal of Zn, Cd and Ni.
3.3.5. Effect Effect of of Particle Particle Size Size 3.3.5. A series seriesofof ofexperiments experiments were carried out to examine examine the effect effect of particle particle size employing employing A were carried out out to examine the effect of particle size employing a solutionaa series experiments were carried to the of size solution concentration concentration of 300 300 mg/L and aaash bottom ash concentration concentration of(Figure 10 g/L g/L 6). (Figure 6). 6). concentration of 300 mg/L and a bottom concentration of 10 g/Lof solution of mg/L and bottom ash 10 (Figure
Figure 6. Cont.
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Figure 6. 6. Metal ofof particle size at different initial pH:pH: (a) Zn; (b) Figure Metal removal removalby bybottom bottomash ashasasa afunction function particle size at different initial (a) Zn; Cd; and (c) Ni. (b) Cd; and (c) Ni.
It is clear from the results that the uptake of heavy metal ions increased with decreasing bottom It is clear from the results that the uptake of heavy metal ions increased with decreasing bottom 2+ removal efficiency increased from 22% and 15% to 100% for an initial solution ash particle size. Cd2+ ash particle size. Cd removal efficiency increased from 22% and 15% to 100% for an initial solution at pH 3 and pH 2, respectively, as the particle sizes decreased from >3.15 mm to 3.15 mm to
