Accepted Manuscript Combined performance of biochar sorption and magnetic separation processes for treatment of chromium-contained electroplating wastewater Sheng-ye Wang, Yan-kui Tang, Kun Li, Ya-yuan Mo, Hao-feng Li, Zhan-qi Gu PII: DOI: Reference:
S0960-8524(14)01419-9 http://dx.doi.org/10.1016/j.biortech.2014.10.007 BITE 14050
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
9 August 2014 30 September 2014 1 October 2014
Please cite this article as: Wang, S-y., Tang, Y-k., Li, K., Mo, Y-y., Li, H-f., Gu, Z-q., Combined performance of biochar sorption and magnetic separation processes for treatment of chromium-contained electroplating wastewater, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Combined performance of biochar sorption and magnetic separation processes for treatment of chromium-contained electroplating wastewater
Sheng-ye Wang1, Yan-kui Tang1*, Kun Li1, Ya-yuan Mo1, Hao-feng Li1, Zhan-qi Gu2
1. College of Environment Science and Technology, Guangxi University, Nanning 530004, China. 2. Department of Environmental Protection of Guangxi Zhuang Autonomous Region, Naning 530028, China.
*
Corresponding author: phone: +86-13977187116; fax: +86-0771-3273440
E-mail address: [email protected]
1
Abstract Magnetic biochar was prepared with eucalyptus leaf residue remained after essential oil being extracted. Batch experiments were conducted to examine the capacity of the magnetic biochar to remove Cr (VI) from electroplating wastewater and to be separated by an external magnetic field. The results show that the initial solution pH plays an important role on both sorption and separation. The removal rates of Cr (VI), total Cr, Cu (II), and Ni (II) were 97.11%, 97.63%, 100% and 100%, respectively. The turbidity of the sorption-treated solution was reduced to 21.8 NTU from 4075 NTU after 10 min magnetic separation. The study also confirms that the magnetic biochar still retains the original magnetic separation performance after the sorption process.
Keywords: Separation; Electroplating wastewater; Leaf residue; Magnetic biochar; Sorption.
2
1. Introduction
There are more than 10 thousand electroplating factories in China from which more than 4 billion cubic meters Cr-contained wastewater is discharged yearly. Looking for economically feasible and environmentally benign techniques for electroplating wastewater treatment appears to be of great concern. Among those practiced techniques, adsorption has evolved as a front line of defense for heavy metals in wastewater (Mohan et al., 2014b). Various inexpensive adsorbents prepared from biomass have been developed. Among them, biochar is thought of as a potential adsorbent due to its large specific surface and the inherent porous structures (Mohan et al., 2011). Biochar is usually obtained from the carbonization of sludge, manure, peels, barks, leaves etc and has been used for hydrocarbons (Oleszczuk et al., 2012), dyestuff (Sun et al., 2013) and metal ions removal (Lu et al., 2012). Nevertheless, the drawback of its application is the difficulty to separate the tiny biochar particles from the treated wastewater. Currently, magnetic separation has been found to be one of the promising approaches of solid–liquid separation because it is more selective and efficient (and often much faster) than centrifugation or filtration (Yavuz et al., 2006), which can easily recover the magnetic adsorbent from treated water by using an external magnetic field, requiring no further separation treatment. So far, magnetic separation has been applied into many fields (Olsvik et al., 1994; Uhlen, 1989; Wang et al., 2004). The methods of conversion of biochar into their magnetic derivatives have
3
been summarized (Safarik et al., 2012) and much work has been done on the magnetically modified biochar for removing dye, oil, and heavy metals from aqueous water. However, little data are available on its application for the treatment of actual electroplating wastewater and the knowledge about the combined performance of biochar sorption and magnetic separation processes is unclear. Eucalyptus urophylla, a kind of fast growing tree, is widely cultivated primarily as a pulpwood in the south of China. Its leaves, however, are found to be steam distilled to extract eucalyptus oil and the oil yield is 0.53% (fresh weight) with cineole being the major isolate (57.7%) in the previous study (Cimanga et al., 2002), indicating that this kind of eucalyptus could be one source of eucalyptus essential oil production. The utilization of the residue that remains after the oil extraction would not only provide an added value of eucalyptus leaves but also help to solve agricultural solid waste disposal problems with a positive impact in local and national economies. The objective of this study was to prepare magnetic biochar with eucalyptus urophylla leaf residue and investigate the performance of the as-prepared biochar in removing Cr ions from the actual Cr-contained electroplating wastewater by sorption and magnetic separation processes.
2. Materials and methods
2.1. Materials
2.1.1. Wastewater
4
Simulated wastewaters containing Cr ions were prepared with deionized water from K2Cr2O7 (AR grade). Their pH was adjusted by HNO3 (0.1 M) and NaOH (0.1 M) solutions. The actual Cr–contained electroplating wastewater was collected from chromium plating processing.
2.1.2. Preparation of adsorbent
Eucalyptus (Eucalyptus urophylla) leaves were obtained from the campus of Guangxi University in Nanning, China. After being washed, the leaves were crushed for essential oil extraction. The residue collected was carbonized at 400 oC for 1 h after being dried at 120 oC; Then the carbonized product was impregnated with ZnCl2 (ZnCl2 :C=2:1), exposed to 700 oC for 2 h and washed several times with HCl (0.1 mol L-1) followed by deionized water until the pH was 6.0–6.5. Then the biochar was sieved after being dried and 100–160 mesh carbon was used for magnetization (shown in Fig. 1a). The as-prepared biochar (20 g) was then suspended in a beaker with 200 mL of deionized water. FeCl3·6H2O (20 g) and FeSO4·7H2O (11.1 g) was added to another beaker with 600 mL of deionized water and stirred until they were dissolved completely. These two solutions were then mixed and stirred at room temperature (20–25 oC) for 20 min. Thereafter 10 M–NaOH (aqueous) was added drop wise into the mixed suspension until the pH was 10–11. After being stirred for 1 h, the suspension was boiled for 1 h and filtered. Then the filtrate was washed with deionized water and ethanol several times and dried at 70 oC for 12 h in a hot air
5
oven.
2.2. Methods
2.2.1. Adsorbent characterization
The physico-chemical characteristics of the biochars were determined using standard procedures. The carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents of magnetic eucalyptus leaf residue biochar (MELRC) and eucalyptus leaf residue biochar (ELRC) were determined using a Euro–EA 3000 (HEKAtech Italy). Ash content was determined by incinerating (air-treating) about 1 g of the sample at 650 oC for 12 h in an electrical furnace. Values of pH were measured in a 1:10 suspension of the biochar in pure water. BET surface area and pore volume were determined by a surface analyzer (NOVA 4200e, USA) with N2 adsorption. The functional groups of MELRC were analyzed using Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet Corporation, USA). The surface morphologies of the biochars were carried out by field emission scanning electron microscope (FESEM, Hitachi SU8020, Japan). X–ray diffraction (XRD) pattern was obtained on a powder X–ray diffractometry (Rigaku Co. Ltd. 4153B172, Japan). Hysteresis measurements were carried out on a Vibrating Sample Magnetometer (VSM, Lakeshore 7410, USA).
2.2.2. Sorption system
Batch sorption studies were carried out by taking 25 mL of the Cr (VI) solution 6
and desired weight of MELRC into 100 mL conical flasks. After agitating at 150 rpm in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through 0.45 µm pore size nylon membrane filters. The effects of initial solution pH (1.0–6.0), contact time (5–960 min), temperature (25–55 oC), MELRC dose (4–50 g L-1) and coexisted ions [Ni (II) and Cu (II)] on Cr sorption onto MELRC were discussed. Cr (VI) sorption was investigated in concentrations of 150 mg L-1 and 200 mg L-1. This range was selected based on the Cr (VI) concentrations found in the Cr–contained electroplating wastewater. The Cr (VI) concentrations in the solution were analyzed by the colorimetric 1, 5-diphenylcarbazide method. The TCr (Total Cr), Ni (II) and Cu (II) concentrations were determined using an inductively coupled plasma emission spectrometer (ICP–OES, Perkin–Elmer Optima 5300DV, USA).
2.2.3. Magnetic separation system
The separation of MELRC from liquid was studied by batch experiments. As is shown in Fig. 1b, one hundred milliliters of deionized water was taken into plastic cups (Φ=4.5 cm), and their pHs were adjusted by adding dilute solution of HCl or NaOH. Then a desired weight of MELRC was added into each cup. After stirring the solution for about 10 minute with an agitator at a speed of 60 rpm, the cups were then put on the magnets (Dongsheng Magn. Mater. Co., Ltd, China) with a magnetic field of 0.5 T. When the separation process is done, 20 mL of the water sample 2 cm above the bottom in each cup was collected by an injector through a pipe for turbidity
7
measurement using a turbid meter (HACH 2100N, USA). The effects of initial solution pH (1.0–6.0), separation time (1–60 min), and concentration of adsorbent (2.0–30 g L-1) were discussed.
2.2.4. Statistical analysis
All treatments were repeated twice or more to observe the reproducibility. Average values and standard deviations (SD.) were calculated. Data were tested for statistical significance with one way analysis of variance (ANOVA). A value of p0.05). The value of pH has a big influence on the forms of Cr (VI) (CrO42−, Cr2O72− and HCrO4−) (Gupta et al., 2013a). At solution pH of 1–3, HCrO4− is the predominant Cr (VI) species, and it is more favorable for sorption due to its low sorption free energy. However, this form shifts to Cr2O72− and CrO42− as the pH increases (Yuan et al., 2009) and the increasing concentration of OH- ions compete with Cr (VI) anions for the sorption sites and decrease the removal of Cr (VI). Moreover, Cr (VI) is more easily reduced to Cr (III) at a low pH in the presence of biochar, which could also be a reason for the high removal efficiency of Cr (VI). Similar results have been reported for the Cr (VI) adsorption by activated carbon (Suksabye et al., 2007). It is also showing that when the value of pH was 1, the efficiency of Cr (VI) was higher than that of TCr, whereas the efficiency of TCr became higher than Cr (VI) as the pH grew. It is probably because at a low pH, Cr (VI) easily reduces to Cr (III) and a strong competition by [H3O]+ reduces Cr (III) adsorbed onto MELRC as the surface of MELRC is highly protonated (Gupta et al., 2013b). In addition, the final pH increased only from 5.64 to 6.56 when the initial pH varied from 2 to 6, indicating that MELRC has a buffering capacity (Tang et al., 2013). Since the pH value of Cr–contained electroplating is around 3, the adsorption studies below were conducted by adjusting
12
the initial solution pH to 3. The influence of MELRC dose (4–60 g L-1) was studied at a contact time of 720 min, initial solution pH of 3.00, temperature of 25±0.5 oC and Cr (VI) concentration of 200 mg L-1 (250 mg L-1 of TCr). The results are shown in Fig. S5. The removal efficiency of Cr (VI) increased sharply from 46.80 to 98.04% as the sorbent dose increased from 4 to 40 g L-1. However, further increase in sorbent dose had no significant effect on the removal of Cr (VI). Similar trend was found in the removal of TCr. Therefore 40 g L-1 was chosen as the optimum adsorbent dose for the removal of Cr ions from the electroplating wastewater.
3.2.3. Effect of contact time and temperature
Sorption experiments of effect of contact time were carried out at different temperatures (25 oC, 40 oC and 55 oC) with an initial solution pH of 3.02, dose of 20 g L-1, Cr (VI) concentration of 200 mg L-1 (250 mg L-1 of TCr), and contact time range of 5–960 min. The results shown in Fig. S6 reveal a two–step kinetic process for all temperatures: a rapid initial sorption and a much slower rate of sorption. The rapid sorption removed a majority of Cr ions within the first 30 min, which is probably due to the existence of adequate vacant sites and a great concentration gradient between Cr ions in aqueous and MELRC solid phases (Liu et al., 2010). After this period, active sites or functional groups for Cr sorption on the surface of MELRC became less available (Ye et al., 2010). For 25 oC and 40 oC, the sorption equilibriums for both Cr (VI) and TCr were about to be attained at 720 min. But for 55 oC, a slow
13
uptake still existed before the sorption was done. A significant change (increase) in the Cr removal was found when the temperature increased from 25 to 55 °C (p
S0960-8524(14)01419-9 http://dx.doi.org/10.1016/j.biortech.2014.10.007 BITE 14050
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
9 August 2014 30 September 2014 1 October 2014
Please cite this article as: Wang, S-y., Tang, Y-k., Li, K., Mo, Y-y., Li, H-f., Gu, Z-q., Combined performance of biochar sorption and magnetic separation processes for treatment of chromium-contained electroplating wastewater, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Combined performance of biochar sorption and magnetic separation processes for treatment of chromium-contained electroplating wastewater
Sheng-ye Wang1, Yan-kui Tang1*, Kun Li1, Ya-yuan Mo1, Hao-feng Li1, Zhan-qi Gu2
1. College of Environment Science and Technology, Guangxi University, Nanning 530004, China. 2. Department of Environmental Protection of Guangxi Zhuang Autonomous Region, Naning 530028, China.
*
Corresponding author: phone: +86-13977187116; fax: +86-0771-3273440
E-mail address: [email protected]
1
Abstract Magnetic biochar was prepared with eucalyptus leaf residue remained after essential oil being extracted. Batch experiments were conducted to examine the capacity of the magnetic biochar to remove Cr (VI) from electroplating wastewater and to be separated by an external magnetic field. The results show that the initial solution pH plays an important role on both sorption and separation. The removal rates of Cr (VI), total Cr, Cu (II), and Ni (II) were 97.11%, 97.63%, 100% and 100%, respectively. The turbidity of the sorption-treated solution was reduced to 21.8 NTU from 4075 NTU after 10 min magnetic separation. The study also confirms that the magnetic biochar still retains the original magnetic separation performance after the sorption process.
Keywords: Separation; Electroplating wastewater; Leaf residue; Magnetic biochar; Sorption.
2
1. Introduction
There are more than 10 thousand electroplating factories in China from which more than 4 billion cubic meters Cr-contained wastewater is discharged yearly. Looking for economically feasible and environmentally benign techniques for electroplating wastewater treatment appears to be of great concern. Among those practiced techniques, adsorption has evolved as a front line of defense for heavy metals in wastewater (Mohan et al., 2014b). Various inexpensive adsorbents prepared from biomass have been developed. Among them, biochar is thought of as a potential adsorbent due to its large specific surface and the inherent porous structures (Mohan et al., 2011). Biochar is usually obtained from the carbonization of sludge, manure, peels, barks, leaves etc and has been used for hydrocarbons (Oleszczuk et al., 2012), dyestuff (Sun et al., 2013) and metal ions removal (Lu et al., 2012). Nevertheless, the drawback of its application is the difficulty to separate the tiny biochar particles from the treated wastewater. Currently, magnetic separation has been found to be one of the promising approaches of solid–liquid separation because it is more selective and efficient (and often much faster) than centrifugation or filtration (Yavuz et al., 2006), which can easily recover the magnetic adsorbent from treated water by using an external magnetic field, requiring no further separation treatment. So far, magnetic separation has been applied into many fields (Olsvik et al., 1994; Uhlen, 1989; Wang et al., 2004). The methods of conversion of biochar into their magnetic derivatives have
3
been summarized (Safarik et al., 2012) and much work has been done on the magnetically modified biochar for removing dye, oil, and heavy metals from aqueous water. However, little data are available on its application for the treatment of actual electroplating wastewater and the knowledge about the combined performance of biochar sorption and magnetic separation processes is unclear. Eucalyptus urophylla, a kind of fast growing tree, is widely cultivated primarily as a pulpwood in the south of China. Its leaves, however, are found to be steam distilled to extract eucalyptus oil and the oil yield is 0.53% (fresh weight) with cineole being the major isolate (57.7%) in the previous study (Cimanga et al., 2002), indicating that this kind of eucalyptus could be one source of eucalyptus essential oil production. The utilization of the residue that remains after the oil extraction would not only provide an added value of eucalyptus leaves but also help to solve agricultural solid waste disposal problems with a positive impact in local and national economies. The objective of this study was to prepare magnetic biochar with eucalyptus urophylla leaf residue and investigate the performance of the as-prepared biochar in removing Cr ions from the actual Cr-contained electroplating wastewater by sorption and magnetic separation processes.
2. Materials and methods
2.1. Materials
2.1.1. Wastewater
4
Simulated wastewaters containing Cr ions were prepared with deionized water from K2Cr2O7 (AR grade). Their pH was adjusted by HNO3 (0.1 M) and NaOH (0.1 M) solutions. The actual Cr–contained electroplating wastewater was collected from chromium plating processing.
2.1.2. Preparation of adsorbent
Eucalyptus (Eucalyptus urophylla) leaves were obtained from the campus of Guangxi University in Nanning, China. After being washed, the leaves were crushed for essential oil extraction. The residue collected was carbonized at 400 oC for 1 h after being dried at 120 oC; Then the carbonized product was impregnated with ZnCl2 (ZnCl2 :C=2:1), exposed to 700 oC for 2 h and washed several times with HCl (0.1 mol L-1) followed by deionized water until the pH was 6.0–6.5. Then the biochar was sieved after being dried and 100–160 mesh carbon was used for magnetization (shown in Fig. 1a). The as-prepared biochar (20 g) was then suspended in a beaker with 200 mL of deionized water. FeCl3·6H2O (20 g) and FeSO4·7H2O (11.1 g) was added to another beaker with 600 mL of deionized water and stirred until they were dissolved completely. These two solutions were then mixed and stirred at room temperature (20–25 oC) for 20 min. Thereafter 10 M–NaOH (aqueous) was added drop wise into the mixed suspension until the pH was 10–11. After being stirred for 1 h, the suspension was boiled for 1 h and filtered. Then the filtrate was washed with deionized water and ethanol several times and dried at 70 oC for 12 h in a hot air
5
oven.
2.2. Methods
2.2.1. Adsorbent characterization
The physico-chemical characteristics of the biochars were determined using standard procedures. The carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents of magnetic eucalyptus leaf residue biochar (MELRC) and eucalyptus leaf residue biochar (ELRC) were determined using a Euro–EA 3000 (HEKAtech Italy). Ash content was determined by incinerating (air-treating) about 1 g of the sample at 650 oC for 12 h in an electrical furnace. Values of pH were measured in a 1:10 suspension of the biochar in pure water. BET surface area and pore volume were determined by a surface analyzer (NOVA 4200e, USA) with N2 adsorption. The functional groups of MELRC were analyzed using Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet Corporation, USA). The surface morphologies of the biochars were carried out by field emission scanning electron microscope (FESEM, Hitachi SU8020, Japan). X–ray diffraction (XRD) pattern was obtained on a powder X–ray diffractometry (Rigaku Co. Ltd. 4153B172, Japan). Hysteresis measurements were carried out on a Vibrating Sample Magnetometer (VSM, Lakeshore 7410, USA).
2.2.2. Sorption system
Batch sorption studies were carried out by taking 25 mL of the Cr (VI) solution 6
and desired weight of MELRC into 100 mL conical flasks. After agitating at 150 rpm in a mechanical shaker for a desired time, the conical flasks were withdrawn and the mixtures were filtrated through 0.45 µm pore size nylon membrane filters. The effects of initial solution pH (1.0–6.0), contact time (5–960 min), temperature (25–55 oC), MELRC dose (4–50 g L-1) and coexisted ions [Ni (II) and Cu (II)] on Cr sorption onto MELRC were discussed. Cr (VI) sorption was investigated in concentrations of 150 mg L-1 and 200 mg L-1. This range was selected based on the Cr (VI) concentrations found in the Cr–contained electroplating wastewater. The Cr (VI) concentrations in the solution were analyzed by the colorimetric 1, 5-diphenylcarbazide method. The TCr (Total Cr), Ni (II) and Cu (II) concentrations were determined using an inductively coupled plasma emission spectrometer (ICP–OES, Perkin–Elmer Optima 5300DV, USA).
2.2.3. Magnetic separation system
The separation of MELRC from liquid was studied by batch experiments. As is shown in Fig. 1b, one hundred milliliters of deionized water was taken into plastic cups (Φ=4.5 cm), and their pHs were adjusted by adding dilute solution of HCl or NaOH. Then a desired weight of MELRC was added into each cup. After stirring the solution for about 10 minute with an agitator at a speed of 60 rpm, the cups were then put on the magnets (Dongsheng Magn. Mater. Co., Ltd, China) with a magnetic field of 0.5 T. When the separation process is done, 20 mL of the water sample 2 cm above the bottom in each cup was collected by an injector through a pipe for turbidity
7
measurement using a turbid meter (HACH 2100N, USA). The effects of initial solution pH (1.0–6.0), separation time (1–60 min), and concentration of adsorbent (2.0–30 g L-1) were discussed.
2.2.4. Statistical analysis
All treatments were repeated twice or more to observe the reproducibility. Average values and standard deviations (SD.) were calculated. Data were tested for statistical significance with one way analysis of variance (ANOVA). A value of p0.05). The value of pH has a big influence on the forms of Cr (VI) (CrO42−, Cr2O72− and HCrO4−) (Gupta et al., 2013a). At solution pH of 1–3, HCrO4− is the predominant Cr (VI) species, and it is more favorable for sorption due to its low sorption free energy. However, this form shifts to Cr2O72− and CrO42− as the pH increases (Yuan et al., 2009) and the increasing concentration of OH- ions compete with Cr (VI) anions for the sorption sites and decrease the removal of Cr (VI). Moreover, Cr (VI) is more easily reduced to Cr (III) at a low pH in the presence of biochar, which could also be a reason for the high removal efficiency of Cr (VI). Similar results have been reported for the Cr (VI) adsorption by activated carbon (Suksabye et al., 2007). It is also showing that when the value of pH was 1, the efficiency of Cr (VI) was higher than that of TCr, whereas the efficiency of TCr became higher than Cr (VI) as the pH grew. It is probably because at a low pH, Cr (VI) easily reduces to Cr (III) and a strong competition by [H3O]+ reduces Cr (III) adsorbed onto MELRC as the surface of MELRC is highly protonated (Gupta et al., 2013b). In addition, the final pH increased only from 5.64 to 6.56 when the initial pH varied from 2 to 6, indicating that MELRC has a buffering capacity (Tang et al., 2013). Since the pH value of Cr–contained electroplating is around 3, the adsorption studies below were conducted by adjusting
12
the initial solution pH to 3. The influence of MELRC dose (4–60 g L-1) was studied at a contact time of 720 min, initial solution pH of 3.00, temperature of 25±0.5 oC and Cr (VI) concentration of 200 mg L-1 (250 mg L-1 of TCr). The results are shown in Fig. S5. The removal efficiency of Cr (VI) increased sharply from 46.80 to 98.04% as the sorbent dose increased from 4 to 40 g L-1. However, further increase in sorbent dose had no significant effect on the removal of Cr (VI). Similar trend was found in the removal of TCr. Therefore 40 g L-1 was chosen as the optimum adsorbent dose for the removal of Cr ions from the electroplating wastewater.
3.2.3. Effect of contact time and temperature
Sorption experiments of effect of contact time were carried out at different temperatures (25 oC, 40 oC and 55 oC) with an initial solution pH of 3.02, dose of 20 g L-1, Cr (VI) concentration of 200 mg L-1 (250 mg L-1 of TCr), and contact time range of 5–960 min. The results shown in Fig. S6 reveal a two–step kinetic process for all temperatures: a rapid initial sorption and a much slower rate of sorption. The rapid sorption removed a majority of Cr ions within the first 30 min, which is probably due to the existence of adequate vacant sites and a great concentration gradient between Cr ions in aqueous and MELRC solid phases (Liu et al., 2010). After this period, active sites or functional groups for Cr sorption on the surface of MELRC became less available (Ye et al., 2010). For 25 oC and 40 oC, the sorption equilibriums for both Cr (VI) and TCr were about to be attained at 720 min. But for 55 oC, a slow
13
uptake still existed before the sorption was done. A significant change (increase) in the Cr removal was found when the temperature increased from 25 to 55 °C (p
