Article pubs.acs.org/IECR
Feasibility of Combining Reverse Osmosis−Ferrite Process for Reclamation of Metal Plating Wastewater and Recovery of Heavy Metals Seungjoon Chung,† Seungjin Kim,‡ Jong-Oh Kim,§ and Jinwook Chung*,† †
R&D Center, Samsung Engineering Co. Ltd, 415-10 Woncheon-Dong, Suwon, Gyeonggi-Do 443-823, Republic of Korea Environmental Engineering Department, Samsung Engineering Co. Ltd, 500 SAMSUNG GEC, Sangil-Dong, Gangdong-Gu, Seoul, 134-728, Republic of Korea § Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seongdong-Gu, Seoul, 133-791, Republic of Korea ‡
ABSTRACT: The feasibility of reverse osmosis (RO) separation combined with ferrite reaction was investigated for the reclamation and recovery of heavy metals from metal plating wastewater. Disc tube-type RO modules were used for simultaneous purification and concentration of wastewaters containing zinc and chromium ions. A subsequent ferrite reaction was performed to recover zinc ions from RO concentrates. The operating conditions of the RO and ferrite reaction were determined in several lab-scale experiments with several types of model wastewater. Pilot-scale RO test results revealed that the zinc plating wastewater treated by one-pass RO and the chrome plating wastewater treated by two-pass RO were acceptable for preplating rinsewater. After ferrite reaction, 99.7% of zinc ions were recovered from the second-stage RO concentrate in the form of zinc ferrite, while significant amounts of chromium ions were retained in the concentrate. As a result of economic analysis, we suggested a retrofitting option, including a combined RO and ferrite process, for a wastewater treatment facility in a local metal plating company.
1. INTRODUCTION Metal plating processes employ many water rinsing steps to remove contaminants and chemicals from metal surfaces. Thus, wastewater from the metal plating industry contains solvents, oil and grease (OG), organics, heavy metals (e.g., chromium, copper, zinc, lead, nickel, and iron), and other cations and anions, depending on the types of processes.1 Generally, metal plating facilities are located in urban areas and are operated by numerous small companies, which have limited capital and personnel. The wastewater from this industry is typically discharged to the sewer, contaminating the sewer sludge with heavy metals. Consequently, regulations concerning the discharge of such wastewater are becoming more stringent.2 Many technologies, such as hydroxide precipitation, adsorption, ion exchange, membrane filtration, electrodialysis, and electrocoagulation, have been studied for the removal of heavy metals from wastewater.3−10 Among these technologies, hydroxide precipitation is the most common technique for the treatment of metal plating wastewater, based on its simplicity and inexpensiveness. However, this technique requires large amounts of chemicals for coagulation and generates chemical sludge, incurring additional disposal costs. Reverse osmosis (RO) is a pressure-driven membrane process that separates purified water and a concentrated solution, forming a barrier to ions. RO has received significant attention leading to many research works on the removal of heavy metals from industrial wastewater, resulting from its high efficiency and low cost.1,9,11,12 Two types of RO modules have been used for heavy metal removal. The predominant type is a spiral-wound RO (SW© 2014 American Chemical Society
RO) module, in which membrane sheets are packed tightly to maximize the available membrane area per module. The disc tube type RO (DT-RO) module is a relatively new design, with open channel flow distribution for operation at higher pressures and at reduced fouling/scaling conditions compared with the SW-RO module. Thus, this type of module is beneficial in removal of contaminants from wastewater with severe scaling and fouling. EPA reported that this type of module was very effective in landfill leachate treatment.13 Furthermore, it would be applied in the reduction stage of the concentrate from the SW-RO is limited, minimizing waste volume.11,14 The ferrite process removes heavy metals from wastewater by the formation of ferrites, iron oxide compounds containing other metal ions, and the subsequent separation with magnetic filtration. The advantages of this process are as follows: (1) heavy metals in the sludge are less mobile than in hydroxide form; (2) heavy metals incorporated into ferrites are easily separated from sludge in a magnetized field; (3) the recovered ferrite compounds can be used in various industrial applications.15−18 The combination of RO and the ferrite process has many environmental (decreased chemical and water use, less and more convenient management of sludge) and economic (recovery of valuable products, such as ferrite and purified water for rinsing) benefits. Nevertheless, the concept of this Received: Revised: Accepted: Published: 15192
June 18, 2014 September 8, 2014 September 8, 2014 September 8, 2014 dx.doi.org/10.1021/ie502421b | Ind. Eng. Chem. Res. 2014, 53, 15192−15199
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic of ferrite reactor.
which was in excess of required amount for zinc ferrite formation reaction. The reaction time was set to 0 when ferric chloride, sodium hydroxide, and oxygen gas were injected into the reactor at a given temperature. The acidity (pH) and the redox potential (Eh) of the solution were measured at various time intervals. The solid and the liquid fractions were separated after the ferrite reaction, and the solid fraction was characterized using X-ray diffractometer (XRD; D/MAX2500, Rigaku Co., Japan). 2.3. Pilot Test with Combination of RO and Ferrite Processes. Wastewater from a local metal plating manufacturer (J) in the Daehwa Industrial Complex (Daehwa-dong, Daejeon, Korea), which produces 40 000 tons of precision devices annually, such as bolts, tapping screws, and rivets for the automobile industry, was selected as the target wastewater for a pilot plant with a capacity of 3 m3 d−1. This company produces 200 tons of wastewater daily140 tons d−1 from rinsing, 30 tons d−1 from zinc plating, and 30 tons d−1 from chrome plating, as shown in Figure 2a. Two types of wastewater were treated during the continuous RO experiments. Two different streams of wastewater, one from chrome plating wastewater and the other from zinc plating wastewater mixed with the preplating rinse wastewater (called zinc plating wastewater for convenience) were obtained from the pipeline connections. Strongly acidic (pH 2−3) wastewaters were adjusted to an optimal pH (6−7) for RO with sodium hydroxide. The schematics of the pilot-scale processes for zinc plating and chrome plating wastewater are depicted in Figure 2b and c. The wastewater was pretreated with a multimedia filter (EPUSA, US) and microfilter (UNA-620A, Asahi-Kasei Co., Japan), and it was delivered to the RO module at a pressure of 60−90 atm. The pilot plant was operated for 20 days. The collected RO concentrate was used for heavy metal recovery experiments under the conditions determined by the batch
process has not been studied extensively, necessitating a feasibility study for its treatment of metal plating wastewater. In this study, we investigated the feasibility and performed the economic analysis of this process in the treatment of wastewater from a metal-plating company by pilot-scale tests.
2. MATERIAL AND METHODS 2.1. RO Cell Tests. RO membrane (FT-30, DOW Filmtec) cell tests were performed to compare the rejections of zinc and chromium ions under various feed concentrations and pHs. The RO cell test unit was a stainless steel (SUS 316) cylinder with 76 mm in diameter and 240 mL in effective volume. A magnetic stirrer was installed in the cell tester to reduce the concentration polarization at the surface of membrane. A plunger type high pressure pump (Meta HK 121-25S) was used to drive the feed stream up to 26 L h−1 in volumetric flow rate and 60−90 atm in hydraulic pressure. The model wastewater was prepared by mixing ZnSO4·7H2O and K2Cr2O7 (Shinyo Pure Chemicals Co.) with deionized water. The concentrations of zinc and chromium ions ranged from 100 to 500 and 100 to 1000 mg L−1, respectively. The pH of the model wastewater was adjusted to 2−12. 2.2. Batch Experiments for Ferrite Reaction. The effects of reaction time, reaction temperature, pH, and Zn/Fe molar ratio on the formation of zinc ferrite were examined to determine the optimal conditions for the ferrite process in batch experiments. The schematics of the experimental apparatus for the ferrite process are shown in Figure 1. A 1-L five-neck flask was used as the reactor and placed in a thermostat. A mechanical stirrer with variable speed drive was used to mix artificial wastewater thoroughly during the ferrite reaction. The reactor, filled with artificial wastewater at specific concentrations, was heated in a thermostat, and the head space was purged with nitrogen gas. The oxygen gas from gas cylinder was delivered to reaction chamber (1 L) at 200 mL min−1, 15193
dx.doi.org/10.1021/ie502421b | Ind. Eng. Chem. Res. 2014, 53, 15192−15199
Industrial & Engineering Chemistry Research
Article
Figure 2. Schematic of (a) metal plating processes with types of wastewater produced, (b) RO-ferrite process for zinc plating wastewater, and (c) RO−ferrite process for chrome plating wastewater.
outlet. The flow of water through the membrane and into the channel created by the polymer mesh is shown in Figure 3b. The concentrations of zinc and chromium were measured by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Ultima ICP, Horiba Jobin-Yvon Inc. US). pH and Eh were measured with a pH meter (HI 8418, Hanna Instruments, US) and an oxidation−reduction potential meter (CM-7B, Towa, Japan), respectively. Energy dispersive X-ray spectroscopy (EDX, Oxford Instrument, US) was used to analyze the solid products that were recovered by ferrite reaction.
experiments with model wastewater. Zinc ferrite was recovered with a magnetic separator (HMO-350, permanent magnet and conveyor type, Hyundai Magnet Co., Korea). The ferrite process was performed to treat 1 ton of concentrated wastewater in batch mode once a week. The initial prototype was based on schematics for disc-tube reverse osmosis (DT-RO) unit provided by Pall Co. (Type 02191, USA). The water flow path can be seen in Figure 3a and is such that water flows through the feed inlet port, down around the outside of the stack of internal components, and then back up in a zigzag fashion, flowing parallel to each membrane stack and hydraulic disc face. This way the water crosses over the face of each individual RO membrane. The concentrated brine reject stream then flows out through the concentrate outlet. As the brine stream flows over the RO membranes, some of the water passes through the membrane face and into the volume between the two membranes that are maintained by the polymer mesh. The polymer mesh acts as a spacer between the two membrane sheets but is porous and allows the purified water to flow laterally into the center of the module, where it then flows vertically up to the permeate
3. RESULTS AND DISCUSSION 3.1. Determination of Operating Conditions for RO. Figures 4a and b show the fluxes and salt rejection in model wastewater simulating zinc plating wastewater and chrome plating wastewater at various pHs. The flux reached the highest at pH 6−8, as with zinc plating wastewater. The rejection of chromium ion was the highest at pH 8−10, whereas that of zinc ions was kept constant over the entire pH range. The differences would be caused by different speciation of zinc and chromium ion. Zinc species mainly exist in the form of 15194
dx.doi.org/10.1021/ie502421b | Ind. Eng. Chem. Res. 2014, 53, 15192−15199
Industrial & Engineering Chemistry Research
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Zn(OH)2 at weak alkaline pH. At low to neutral and high alkaline pH, it is either ionized to Zn2+ or Zn(OH)42−, respectively, increasing the osmotic pressure.3,19,20 The speciation of chrome at various pHs differed from that of zinc, because it existed as ions (HCrO4−, CrO42−, Cr2O72−) at nearly all pHs. As shown in Figure 4c and d, the flux declined as the concentration of zinc and chromium ions increased due to the rise in osmotic pressure at higher ion concentrations and the constant operating pressure (60 atm). The rejection rates of zinc ions are remained over 99% at zinc concentrations of 100− 500 mg L−1, but those of the chrome ion fell significantly at concentrations 400−1000 mg L−1. The higher rejection of zinc species would be caused either by different concentration polarization behavior or by the fouling of Zn(OH)2 on the membrane surface. Differences in charges and molecular weight according to the speciation would exhibit the higher concentration polarization for chrome species than that for zinc species. The fouling of Zn(OH)2 would lead to the higher rejection rate of zinc species than that of chrome species. 3.2. Determination of Operating Conditions for Ferrite Reaction. Redox potential values (Eh) and change in pH during the ferrite reaction are shown in Figure 5a. The Eh decreased from 0 to −400 mV as the reaction proceeded, and it reverted immediately to near 0 after 30 min. These results suggest that zinc and ferric ions were reacted to form to zinc ferrite, and that the reaction was completed after 30 min. Figure 5b shows the relative main peak intensity in the XRD patterns of the precipitate according to Zn/Fe molar ratios. The relative main peak intensity increased as the molar ratio rose
Figure 3. Diagram of (a) cross-sectional view and (b) water flow of DT-RO module (based on information from the Pall Corporation).
Figure 4. Effect of pH on flux and rejection rates for (a) zinc-containing model wastewater at [Zn] = 100 mg L−1 and for (b) chrome-containing model wastewater at [Cr] = 200 mg L−1. Effect of concentration on flux and rejection for (c) zinc-containing model wastewater at pH = 7 and (d) chrome-containing model wastewater at pH = 7. 15195
dx.doi.org/10.1021/ie502421b | Ind. Eng. Chem. Res. 2014, 53, 15192−15199
Industrial & Engineering Chemistry Research
Article
Figure 5. Changes in (a) Eh and pH during the formation of Zn-ferrite, (b) relation between relative X-ray main peak intensity and saturation magnetization as a function of Zn/Fe molar ratio, (c) X-ray diffraction of Zn-ferrite by reaction time(A) 10; (B) 20; (C) 30 minand (d) X-ray diffraction of Zn-ferrite at various temperatures(A) 60; (B) 70; (C) 80 °C.
from 0.2 to 0.6 and reached the maximum at molar ratios of 0.5−0.6. These results indicate that the optimal Zn/Fe molar ratio for the formation of zinc ferrite is 0.5−0.6. Also, as shown in Figure 5c, the XRD analysis confirmed that unreacted or intermediate species disappeared after 30 min and that the final product was identified as ZnFe2O4. The XRD patterns of zinc ferrite at different temperatures (60, 70, and 80 °C) are shown in Figure 5d. At reaction temperature of 60 °C, unreacted products and intermediates exist. At 70 °C, the peak intensities of Zn-ferrite increases, while those of unreacted or intermediate products decreases. At 80 °C, well-defined peaks of zinc ferrite are observed, and those of undesired products are almost negligible. Thus, temperature for ferrite reaction was chosen as 80 °C. 3.3. Pilot-Scale Test of a Combined RO and Ferrite Process. RO permeate is for preplating rinsewater, and RO concentrate is used to recover heavy metals through the ferrite process. The water quality analyses results are summarized in Tables 1 and 2. Zinc plating wastewater did not contain chromium ion, and the concentration of zinc was approximately 40 mg/L after being mixed with preplating rinse wastewater. The acceptable water quality criteria for reuse as preplating rinsewater was that it has electrical conductivity less than 300 μS cm−1 (service water 220−290 μS cm−1) with the trace amount of zinc and chromium ions (
