1 INTRODUCTION
Water pollution by heavy metals is an important economic and environmental issue in numerous parts of the world [1]. Among these heavy metals, chro mium (Cr) is a common contaminant in surface water and ground water resulting from numerous industrial activities such as the preservation of wood, textile dye ing, leather tanning, electroplating and metal finishing [2]. The element exists mainly in the Cr(III) and Cr(VI) valence states, although Cr (0), Cr (II), and Cr(V) have also been observed. Both of the common Cr(VI) anions, chromate ( ) and dichromate (Cr2 ) are strong oxidants, and chromate is a known carcinogen and a suspected mutagen and ter atogen. By contrast, Cr (III) toxicity is negligible because it often forms insoluble hydroxides at circum neutral pH (5.5 and 7.5) [3]. Accordingly, chromium containing waste waters must be treated to lower the Cr(VI) to allowable limits before discharging into the environment. The Cr(VI) removal rate depends signif icantly on the pH of the solution, because the solubility of precipitate is strongly reliant on pH [4, 5]. Cr(VI) removal was insignificant except when the solution pH was low enough to dissolve the passive oxide layers [5]. Conventional methods utilized to remove the Cr(VI) from industrial waste waters include reduction followed 1The article is published in the original.
CrO42– O72–
by chemical precipitation [6], activated carbon adsorp tion [7], electrochemical precipitation [8], ion exchange [9], solvent extraction [10], reverse osmosis (RO) [11], etc.
In recent years, membrane manufactures have developed RO membranes with chromium rejections of 91–96% [12, 13]. However, most of the current desalination plants have to implement the additional treatment steps such as pH adjustment of feed water, posttreatment of RO permeate with ion exchange or several pass stage of permeate in order to improve chromium rejection. In addition, several process con figurations have been proposed to obtain the low chro mium concentration of the permeate from RO plant [14, 15].
AG and SWHR RO membranes produced by FILMTEC Co. offered advantages over traditional cellulose acetate (CA) RO membranes. The most important of advantages of RO were better rejection of dissolved solids and organics, increased productivity at lower operating pressures, great structural stability, and the ability to produce two to three times more purified water per unit area than CA (cellulose ace tate) membranes. Furthermore, these membranes combine higher flux efficiency with a larger area pack aged in the same volume and format as conventional 8 inch elements allowing for a substantial reduction of investment costs, as well as lower operating costs due to reduced pressure and fouling tendency. In the case PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
Removal of Chromium Ions from Waste Waters Using Reverse
Osmosis AG and SWHR Membranes
1Aysel Çimena*, Fevzi K l çela, and Gül in Arslanb
aKaramano lu Mehmetbey University, Faculty of Science, Department of Chemistry, 70200 Karaman, Turkey bSelcuk University, Faculty of Science, Department of Chemistry, 42031 Konya, Turkey
*email: ayselcimen42@hotmail.com Received February 18, 2013
Abtract—The aim of this work is to investigate removal of chromium from waste waters. The effect of pH and
concentration of the feed water and operating pressure on the chromium rejection were also investigated. In the study; the reverse osmosis (RO) technique and the sea water high rejection (SWHR) and high rejection brackish water (AG) membrane were used for the separation process. Results of the study indicated that chro mium rejection mostly depends on the membrane type, pH of the feed water and operating pressure. Also pH of the feed water was found to be 3 for the effective removal of chromium. Furthermore the rejection effi ciency of the membranes was found to be in the order of AG > SWHR. For two membranes, chromium rejec tion increased with increasing operating pressure. Finally, waste water sample containing 7542 mg/L (with 100 mg/L) of chromium was treated by using RO technique with AG membrane. RO could be efficiently used (with >91% rejection) for the removal of chromium from waste water sample.
Keywords: chromium removal, reverse osmosis, membrane, heavy metals.
DOI: 10.1134/S0036024414050045
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of the low energy consuming elements, the part of the operating cost related to energy consumption has been roughly reduced by 30 to 50% compared with conven tional RO membranes [16]. The lowcost RO mem brane units, the medium and low pressure, could be used economically for separation and recovery of chromium from waste water. RO membranes have been chosen in this study because of the outstanding features of the above mentioned.
The removal of chromium by RO is affected by sev eral factors, i.e., pH, pressure, feed water flow rate, initial concentration, etc. [17]. These factors were described separately in results and discussion section. Many methods have been tried to remove chromium from waste water [2, 6, 9]. These methods were also used remove of boron [18] arsenate and arsenide [19] and chromium [14, 15, 17, 20] from waste water. Removal of chromium by using RO technique and AG membrane is 95%.
The present study was designed to investigate and compare the chromium removal efficiencies of two
different RO membranes (AG and SWHR) using model solutions containing chromium as single solute by reverse osmosis pilot plant. The effect of pH, con centration of feed water and operating pressure on the chromium rejection was also investigated. The experi mental parameters and membranes for the best chro mium removal are determined.
EXPERIMENTAL MATERIALS
Reverse osmosis pilot plant. (Prozesstechnick
GmbH) used in this study consists of a diaphragm pump controlled with a frequency converter (flow range: 1.8–12 L/min, pressure range: max 40 bar), feed tank with heating/cooling jacket (5 L capacity), membrane housing for both spiral wound and flat sheet membranes, different emptying and pressure valves (Fig. 1).
Membranes. The membranes (SWHR and AG)
having 44 cm2 exposed area with a flatsheet configu ration were studied. The most relevant characteristics of these membranes are summarized in Table 1. Experimental data are obtained by using these mem branes are shown in Tables 2–5.
METHODS
The potassium dichromate solutions were prepared in distilled water by diluting the prepared stock solu tions (1000 mg/L) to desired concentrations. K2Cr2O7, NaOH and HCl were obtained from Merck Co. (Darmstad, Germany). All chemicals were the analyt ical grade reagents. In specific experiments, composi tion of the feed water and operating pressure were cho sen as below:
—Feed solutions are a chromium solution with dif ferent concentration (50, 100, 500, and 1000 mg/L);
—Feed solutions (50, 100, 500, and 1000 mg/L) were treated with AG, SWHR, SE, and SG mem branes at the different operating pressure ranging from 15 to 35 bars;
—Feed solutions (50, 100, 500, and 1000 mg/L) were treated with AG and SWHR membranes at pH 1–6 under operation pressure of 20 bar;
—Samples were taken from feed solution and per meate solution at the 1st, 20th, 80th, and 140th min utes (separation/time ratio).
At the beginning of each experiment, pH of the feed water (1 L) was adjusted to the desired pH level by addition of 0.1 M NaOH or 0.1 M HCl and it was placed in the feed water tank. The system was operated in the permeate recycle mode. In order to prevent a change in the chromium concentration in the feed tank, permeate and concentrate (retentate) phases were recycled in to feed tank. A new membrane was used for each experiment after conditioning the mem brane at least 3 h under the experimental conditions. Then the measuring sequence was started. Every hour,
V3 DP I M1 PI 01 V2 V1 V4 TI 01 P1 II 01 B1 Cooling heating Permeate
Fig. 1. Flow diagram of the reverse osmosis plot plant, M1:
membrane Housing, B1: feed tank with heating/cooling jacket, V1 and V2: emptying valve, V3: pressure regulation valve, V4: spring loaded valve, PI01: pressure gauge, DP1: differential pressure indicator, LI01: level indicator on the feed tank, TI01: temperature indicator.
Table 1. Characteristics of the membranes used
Characteristics SWHR AG
Configuration Flatsheet Flatsheet
Max temperature, °C 45 50
Max pressure, bar 83 41
Salt rejection, % 99.6 99
samples of permeate and sample of feed water were taken and their chromium concentrations were deter mined by flame atomic absorption spectrometer (ContrAA 300, Analytikjena). The experiments were performed at 20 ± 1°C. According the experimental result, the chromium rejection (R) was calculated as [18, 19]:
where c and cw are the chromium concentrations of the permeate and feed water, respectively.
Waste water obtained from chromiumcoating appli cation. The studies were carried out as described in
experiments section. Experiments were performed under conditions: 20°C (temperature), 15, 20, 25, 30, 35 bar (pressure), pH 1–6; 50, 100, 500, and 1000 mg/L concentrations). It was found that the best condition for removal of chromium was pH 3; concentrations 100 mg/L; pressure 20 bar; temperature 20°C. The application of RO on waste water obtained from chro miumcoating in Konya was performed in these con ditions.
Instruments. The concentration of chromium and
cations in the samples was determined by ContrAA 300 Atomic Absorption Spectroscopy (ContrAA 300, Analytikjena). The wavelength utilized for the deter mination of chromium was 357 nm. Linearity for chromium was observed in the concentration range of 10–1000 mg/L. In addition, coefficient of regression (R2) and limit of detection (LOD) for chromium were 0.999 and 2.935 mg/L, respectively. pH of the samples was determined by an Orion ion meter with combined pH electrode.
RESULTS AND DISCUSSION
Effect of feed water concentration. The rejection of
Cr(VI) as a function of feed water concentration is shown in Fig. 2. It is seen that the concentration of feed water has no significant effect on the rejection. A possible explanation for this result might be related to that permeates water concentration increases with increasing the feed water concentration. Figure 2 also shows that chromium rejection is mainly affected by membrane type. Permeate flux for membranes was found to be in the order AG > SWHR. Chromium % rejection for membranes was found to be in the order AG > SWHR. The mean rejections for AG and SWHR were 96.606 and 87.83, respectively, [21]. These results showed that chromium rejection evidently depends upon membrane type, but it is not dependant on the feed concentration. According to obtained order from chromium (%) rejection and flux values, it has been seen that AG membrane could be preferred to the other membrane in removal of chromium.
Effect of pH of feed water. The dependence of chro
mium rejection upon pH of the feed water is presented in Fig. 3 for AG and SWHR membranes. For two membranes, the highest chromium rejection was
R %, = [1–c/cw] 100,×
obtained at pH 3. Chromium rejection for membranes was found to be in the order AG > SWHR. The mean rejections for AG and SWHR membranes were 92.93 and 86.4, respectively. These results showed that chro mium rejection evidently depends upon membrane type and pH of the feed water.
Effect of operating pressure. Figure 4 shows the
effect of operating pressure on the chromium rejec
Table 2. Rejection values (%) corresponding to the con
centration of feed (c, mg/L) of chromium(VI) ions (pH of feed water: 3, operating pressure: 20 bar, temperature: 20°C)
c AG SWHR
1000 96.606 87.80
500 96.605 87.81
100 96.604 87.82
50 96.603 87.83
Table 3. Rejection values (%) versus pH (chromium concen
tration of feed water: 100 mg/L, operating pressure: 20 bar, temperature: 20°C) pH AG SWHR 1 83.611 69.606 2 92.359 85.171 3 96.525 95.388 4 96.083 89.623 5 93.885 88.578 6 95.134 90.146
Table 4. Rejection values (%) versus operation pressure (p, bar) (100 mg/L, pH 3 of feed water, temperature: 20°C)
p AG SWHR 15 85.701 65.596 20 86.611 69.605 25 87.906 74.838 30 89.845 80.545 35 90.968 86.004
Table 5. Rejection values (%) versus operation time (τ, min)
(chromium concentrations of waste water: 100 mg/L, pH 3 of feed water, operating pressure: 20 bar, temperature: 20°C)
τ AG SWHR
20 91.678 86.401
80 94.603 86.701
140 96.662 87.152
tion. Chromium rejections for AG and SWHR mem branes increased with increasing operating pressure [22–24]. Due to the increase in pressure value, we observed increase in flux value. The amount of solvent acrossing the membrane increased because of the increase in pressure, which acts as driving force. This situation results more adsorption of chromium in membrane and decrease in concentration of chro mium in permeate [22, 23]. This may also be due to the decrease of pore size of the membrane surface as a result of salt accumulation over time [14, 15]. Higher chromium rejection was observed when operating pressure was increased. Chromium rejection for mem branes was found to be in the order of AG > SWHR. The mean rejections for AG and SWHR membranes were 88.21 and 75.32, respectively. Permeate flux for membranes was found to be in the order AG > SWHR. According to obtained order from chromium % rejec tion and flux values, it has been seen that AG mem brane could be preferred to the other membrane in removal of chromium.
In addition, operating pressure also increased per meate flux and was found in the order AG > SWHR. Permeate flux for AG and SWHR membranes were 4.65 and 3 L/(m2 h), respectively (Fig. 5). Higher operating pressure resulted in higher volume of per meate water [22]. Permeate flux is important because higher flux gives the short operation time, which reduces the cost of RO system.
Waste water obtained from chromiumcoating appli cation. RO technique was applied for the removal of
chromium in waste water obtained from chromium coating application. The highest rejection and perme ate flux were obtained by using AG membrane. Waste water was taken from Konya (Turkey) with chromium
concentrations of 100 mg/L. Prior to RO application, pH of the waste sample was adjusted to 3 at which the highest chromium rejection was obtained. Figure 6 shows the time dependence of chromium rejection for waste sample. The mean chromium rejections for AG and SWHR were recorded as 97.851 and 87.854%, respectively. It increased with increase in time in AG and SWHR membrane (Fig. 6). Permeate flux for waste water sample obtained from chromiumcoating application is shown in Fig. 7. As seen in Fig. 7, per meate flux increased during initial 3 h, which may indicate that dynamic membrane conditions were not achieved at initial 3 h. Then the flux of the permeate reached a steady state value. Permeate fluxes for waste water sample were recorded as 4.6–13.9 L/(m2 h). 90 1000 600 200 c, mg/L 85 95 100 Rejection, % 1 2
Fig. 2. Dependency of chromium rejection on the chro
mium concentration of feed (c) for AG (1) and SWHR (2)
membranes; pH 3 of feed water, operating pressure: 20 bar, temperature: 20°C. 80 6 2 0 pH 70 90 100 Rejection, % 4 1 2
Fig. 3. Dependency of chromium rejection on pH of feed
water for AG (1) and SWHR (2) membranes; pH 3 of feed water, operating pressure: 20 bar, temperature: 20°C.
70 35 20 15 p, bar 60 80 90 Rejection, % 25 1 2 30
Fig. 4. Dependency of chromium rejection on the operat
ing pressure (p) for AG (1) and SWHR (2) membranes, 100 mg/L, pH of feed water: 3, temperature: 20°C.
The chemical composition of the waste water obtained from chromiumcoating application was determined by three times analyses [n = 3] and the results are given in Table 6.
In Fig. 8, the effect of concentration of utilized car rier, AG and SWHR membrane, versus the separation process. Thus to establish the concentration of the optimum carrier AG and SWHR membrane were tested with 100 mg/L. A good separation of Cr(VI) was observed at 100 mg L–1 concentration of AG mem brane.
High chromium rejection (close to 95%) and low permeate concentration (<1 mg/L) were obtained at pH 3 by single stage RO with AG membrane in this study. The present study was performed on waste water obtained from chromiumcoating application but AG
membrane has got the highest rejection and permeate flux can be used for the removal of chromium from natural water by RO technique.
CONCLUSION
In this study, removal of chromium from waste water by using reverse osmosis with AG and SWHR membranes was investigated. The effect of pH and concentration of the feed water and operating pressure on the chromium rejection were also investigated. Rejections of AG and SWHR were determined from experimental result and compare with each other. The obtained results can be concluded as follows.
1. The metal quantities from feed and permeate solutions were determined by atomic absorption spec 1 35 20 15 p, bar 0 2 5 Permeate flux, L/(m2 h) 25 1 2 30 4 3
Fig. 5. Dependency of permeate flux on the operating
pressure; chromium concentration of feed water: 100 mg/L, pH 3 of feed water, temperature: 20°C.
88 200 50 0 τ, min 84 92 Rejection, % 96 150 100 1 2
Fig. 6. Dependency of chromium rejections on the operat
ing time (τ); chromium concentrations of waste water: 100 mg/L, pH 3 of feed water, operating pressure: 20 bar.
6 200 50 0 τ, min 4 10 8 Permeate flux, L/(m2 h) 150 275 100 ~ ~
Fig. 7. Dependency of permeate fluxes on the operating
time (τ) for waste water sample obtained from chromium coating application with AG membrane. (chromium con centrations of waste water: 100 mg/L, pH 3 of feed water, operating pressure: 20 bar, temperature: 20°C.
15 200 50 0 t, min 10 20 25 c, mg/L 100 1 2 150 250
Fig. 8. Change over time of amount of Cr(VI) ions in feed
tank; amount of Cr(VI) ions in feed tank was measured by AAS in 100 mg/L feed solution. (pH 3 of feed water, oper ating pressure: 20 bar, temperature: 20°C; (1) and (2) see Fig. 2.
trometer (AAS). It was observed that good result were obtained at low concentration values of Cr(VI) 50 and 100 mg L–1, respectively, in which the AG and SWHR membranes, strength in membrane allowed Cr(VI) carrying in good conditions from the feed to permeate solution, accordingly to the separation efficiency (AG > SWHR) as separation/time ratio.
2. Removal of chromium by RO depends greatly on the pH of the feed water. For two studied membranes it was found that chromium can be effectively removed at pH 3.
3. Removal of chromium increases when increas ing the operating pressure.
4. The rejection of chromium does not depend upon the feed water concentration.
5. Removal efficiency is also depending on type of membrane.
6. The highest rejection (95%) and permeate flux were obtained by using AG membrane. Therefore, it is recommended to use AG membrane for the removal of chromium from natural water by RO technique.
ACKNOWLEDGMENTS
The authors thank to the Scientific Research Project Commission of Karamano lu Mehmetbey University for financial support (BAP–Grant number 101M10).
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Table 6. The chemical composition of the waste water
obtained from chromiumcoating application (n = 3)
Ionic species c, mg/L Cd 0.1946 Cr 7542 Cu 76 Fe 828.6 Ni 1.55 Pb 48.6 Zn 5.813
