Removal of Hexavalent Chromium from Aqueous Solution using Natural Adsorbents – Column Studies

Removal of Hexavalent Chromium from Aqueous Solution using Natural Adsorbents – Column Studies

Shreyashi Sarkar

Resesarch Scholar Chemical Engineering Department

University of Calcutta

92, A.P.C. Road, Kolkata 700009, West Bengal, India

Sudip Kumar Das

Professor

Chemical Engineering Department University of Calcutta

92, A.P.C. Road, Kolkata 700009, West Bengal, India

Abstract The packed bed column study was investigated using coconut shell and neem leaf for the removal of Cr(VI) from aqueous solution. The breakthrough characteristics for Cr(VI) removal was reported. The effect of operating parameters like influent Cr(VI) concentration, bed height of the packed column and the flow rate of aqueous solution on the breakthrough curve was studied. Bohart-Adams, Yoon-Nelson, Thomas model,Yan et al. model and Wolborska model were applied to the experimental data for their suitability.

Keywords Hexavalent chromium removal, Column study, Yoon-Nelson model, Thomas model, Adams-Bohart model Introduction

1. INTRODUCTION

   The major sources of chromium in water bodies by the human activities are mainly from industrial waste water discharges such as mining, cement, steel and paint industries. Such types of industries discharge Cr(VI) contained water. Cr(VI) has been reported to be toxic to animals, humans and also it is known as carcinogenic (Raji and Anirudhan, 1998). Excessive accumulation of Cr(VI) in human being causes ulcerations, dermatitis and allergic skin reactions. The concentration of Cr(VI) in industrial wastewater remains in the range from 0.5 to 270 mg/L (Casarett and Doul, 1980). According to EPA the maximum tolerance limit of Cr(VI) in drinking water 0.1 mg/L (MINAS, 2001). Tolerance limit of Cr(VI) in drinking water according to Ministry of Environment and Forest, Government of India is 0.01 mg/L (MINAS, 2001).

   Several conventional technologies are used to remove the heavy metal from the aqueous solution such as membrane separation, electrochemical precipitation, ion exchange, reverse osmosis, ultrafiltration and adsorption. Major drawbacks of these processes are either high cost or discharge of sludge formed. Ion-exchange is considered a better alternative technique but its operating cost is high. All other techniques have higher operating cost. Hence among these techniques the adsorption method with using low-cost adsorbent is cost effective and extremely useful. Fixed bed column study is more effective for industrial purpose than batch adsorption process. A wide variety of agricultural waste such as rice husk, coconut shell, sawdust, wheat husk, wheat and rice straw abundantly available can be used as low- costadsorbent. Here, coconut shell and neem leaf are used as

   the adsorbent for Cr(VI) removal in continuous mode operation. The effect of initial pH, flow rate of metal solution, influent metal ion concentration and bed height are studied. Available models, i.e., Thomas model, Yoon-Nelson model, Adams-Bohart model, Yan et al. model, Wolborska model are applied to the experimental data to find out the suitable model. The coconut shell and neem leaf are widely available throughout India. India being a 3rd world country these adsorbent are suitable for the treatment on of Cr(VI) containing wastewater for small to any sized industries.

2. EXPERIMENTAL

   Preparation of adsorbent

   In the present study coconut shell and neem leaf were used as the adsorbent for the removal of Cr(VI) ions from aqueous solution. Initially adsorbents were collected from local area near Kolkata, West Bengal, India. Initially coconut shell was crushed in roll crusher and then it was grinded. Neem leaves and coconut shell were treated with 0.1N NaOH to remove lignin-based colour materials followed by 0.1N H2SO4 (Singha and Das, 2011). Then the adsorbents were washed with distilled water several times and dried at 105Cfor 6 h. Next the adsorbent was sieved to pass through a

   -44 +52 mesh screen. After sieving the adsorbent was kept by in an airtight container in a decicator.

   Metal solution

   1000 mg/L of Cr(VI) solution was prepared by dissolving 2.83 gm of potassium dichromate (K2Cr2O7) in double distilled water. Then stock solution of Cr(VI) was diluted to obtain required standard solution containing 5, 10, 15, 20 mg/L of Cr(VI). Throughout the experiment the glass wares were washed with 20% nitric acid followed by washing with deionized water and then the glass wares were dried in oven at 383K.

   Reagents and instrument used

   The analytical grade reagents were used and obtained from E. Merck Limited, Mumbai, India. WTW pH meter (Multi 340i/SET, Germany) was used to determine the pH of the solution. UV-vis Spectrophotometer (Dr 5000, HACH, U.S.A.) was used to determine the Cr(VI) content in standard and treated solution.

   Experimental procedure

   Adsorption experiment was conducted in packed bed column at ambient temperature. Required amount of adsorbent was packed in glass columns to produce desired height. The glass columns had an internal diameter of 2 cm and height of 50 cm. Initially pH of Cr(VI) was adjusted by adding 0.1 N HCl. Cr(VI) solution was pumped through the packed bed column in a down flow direction at a desired flow rate by a peristaltic pump (Cole-Parmer, model-7535-04, USA). Samples were collected from the exit of the column at desired time intervals and 1,5 diphenyl-cabazide is added to produce the complex agent which is analyzed by UV-vis Spectrophotometer to determine the concentration of Cr(VI) (Standard Methods for Examination of Water and Wastewater, 1998). Experimental setup is shown in Figure 1.

   Figure 1 Schematic diagram of experimental set up of column study

   1. Reservoir tank with metal solution; 2 Peristaltic pumps; 3. Porous sheet; 4. Glass beads; 5.Adsorbents; 6. Filter; 7. Treated effluent storage.

3. RESULTS AND DISCUSSIONS

   Characterization of the adsorbent

   Scanning electron microscopic image of the used adsorbents were carried out to see its surface structure. SEM images of adsorbent represented in Figures 2 and 3. The surface of coconut shell and neem leaf is porous and irregular in nature.

   Figure 2. Scanning electron micrograph (SEM) of coconut shell

   Figure 3. Scanning electron micrograph (SEM) of neem leaf

   The FTIR studied indicated that the main functional groups like aliphatic -CH , aliphatic acid, aromatic NO2, alkane and sulphonyl chlorides stretching are responsible for Cr(VI) binding on neem leaves and similarly phosphate ester, aliphatic acid, aromatic nitro, alkane SO3 and sulphonic S=O stretching are important for Cr(VI) binding on coconut shell (Singha et al., 2011).

   The surface areas are measured in Micromeritics surface area analyzer (ASAP 2010, USA). The point of zero charge was determined by solid addition (Srivastava et al. 2006). Other physical properties of the adsorbents are shown in Table 1.

   Table 1 Different physical characteristics of bioadsorbent

   Adsorbents	Coconut shell	Neem leaf
   Surface area (m2/g)	0.52	0.57
   Bulk Density (g/cm3)	0.82	0.71
   Dry matter (%)	93.84	91.67
   Moisture content(%)	6.16	8.33
   Point of zero charge	6.62	6.94
   Ash content(%)	9.23	13.58

   Effcts of operating parameters

   – 2-

   – 2-

   – 2-

   – 2-

   In our earlier study (Singha and Das, 2011) the effects of pH on the Cr(VI) removal onto coconut shell and neem leaf are reported for batch process. It was observed that for both adsorbents the maximum adsorption observed at pH 2. Cr(VI) exist in different forms in aqueous solution depending on pH. The chromate represented as H2CrO4, HCrO4 , CrO4 , HCr2O7 and Cr2O7 in solution as a function of pH and concentration [Gurgel et al., 2009].

   –

   –

   4

   4

   2 2

   2 2

   2 – 2 –

   2 – 2 –

   At pH 1, chromium ions exist in the form of HCrO4 , while in the pH range 26 different forms of chromium ions such as Cr2O7 , HCrO4 ,and Cr3O10 ,coexists, of which HCrO4 predominates. As the pH increases this form shifts to CrO4 and Cr2O7 (Singha and Das 2011). At low pH values, the Cr(VI) exists as univalent form, HCrO – and needs only one exchange site. As the pH increased, the overall surface charge on the adsorbents became negative and adsorption decreased.

   The adsorption experiments were operated at flow rate of 10 ml.min-1, 20 ml.min-1 and 30 ml.min-1.The breakthrough curve is a plot of dimensionless ratio of concentration (Ct/C0) versus time and is shown in Figures 4 and 5. The better performance was observed at lower flow rate (10 ml.min-1). At higher flow rate breakthrough occurred more rapidly than the lower flow rate. These figures indicated that with increase in flow rate the breakthrough time decreases. This behaviour occurred because at higher flow rate the Cr(VI) solution had less contact time, i.e., residence time within the column. At lower flow rate the Cr(VI) solution had more time to contact with the adsorbent in the column and hence higher removal of metal ions observed. The best adsorption capacity of the fixed bed column obtained at 10 ml min-1 flow rate. Similar results are obtained by other researchers (Hanen and Abdelmottaleb,2013 ; Sivaprakash et al.,2010).

   Figure 4 Effect of flow rate for Cr(VI) adsorption on coconut shell at constant bed height and influent metal ion concentration

   Figure 5 Effect of flow rate for Cr(VI) adsorption on neem leaf at constant bed height and influent metal ion concentration

   The effects of bed height were investigated for bed height of 5, 10 and 15 cm at a constant flow rate and influent metal ion concentration. The breakthrough curves are shown in Figures 6 and 7.The removal of heavy metal is entirely dependent on the bed height. From these graphs it was observed that the breakthrough time decreases as the bed height decreases. At higher bed height breakthrough occurred more slowly than the lower bed height because at higher bed height aqueous solution got more binding sites of the adsorbent and this behaviour produced higher removal of metal ions in the column. Similar observations are obtained by other researchers (Hasfalina et al., 2012; Nwabanne et al., 2012).

   The effects of metal ion concentration were investigated at different initial Cr(VI) concentration of 10 mg/L, 20 mg/L and 30 mg/L. The breakthrough curves are shown in Figures

   8 and 9. From the breakthrough graphs it observed that adsorption capacity of the packed bed increases with the decrease in metal ion concentration. At high influent metal ion concentration adsorption capacity decreased due to earlier saturation of active surfaces. At lower influent metal ion concentration the adsorption capacity increased because the metal ions would react with active binding sites. The breakthrough time increased as the influent metal ion decreased (Deepali, 2011).

   Figure 6 Effect of bed height of column for removal of Cr(VI) on coconut shell at constant flow rate and influent metal ion concentration

   Figure 7 Effect of bed height of column for removal of Cr(VI) on neem leaf at constant flow rate and influent metal ion concentration

4. MODELLING OF COLUMN STUDY Maximum adsorption capacity of the adsorbent in a

fixed bed column is calculated by following equations

Figure 9 Effect of influent metal ions concentration for the removal of Cr(VI) on neem leaf at constant bed height and flow rate

Bohart-Adams (1920) model

Bohart and Adams developed to find the adsorption capacity and adsorption rate constant of the continuous column operation (Bohart and Adams, 1920).The expression

qe

v

1000w

ttotal

(C0

• Ct )dt

  (1)

  given by Bohart and Adams for an adsorption column is given as follows (Aksu and Gönen, 2004),

  C z

  AB 0 AB 0

  AB 0 AB 0

  0 ln

  t k C t k N

  (3)

  q qtotal

  (2)

  Co

  v

  emax w

  The intercept and slope of the linear plot of ln(Ct/C0) versus time were used to determine the value of adsorption capacity N0 and kinetic constant kAB. The values of kAB, N0 and statistical parameters are tabulated in Table 2. It is observed that the rate constant (kAB) and saturation concentration (N0) is dependent on flow rate and metal ion concentration. It is also observed that the value of rate constant (kAB) increases with the increasing flow rate.

  Figure 8 Effect of influent metal ions concentration for the removal of Cr(VI) on coconut shell at constant bed height and flow rate

  Table 2 Bohart -Adams model parameters

  0.6409

  Flow rate v (ml/min)	Influent concentration C0 (mg/L)	Bed height (cm)	Rate constant, kAB X 104 ml/( min. mg)	Saturation concentration, N0 (g/L)	R2
  Adsorbent : Coconut shell
  10	10	5	5.68	11.48	0.7999
  20	10	5	5.55	14.03	0.7869
  30	10	5	13.92	13.91	0.6586
  10	20	5	2.4	18.665	0.8155
  20	20	5	2.4	40.65	0.8825
  30	20	5	3.75	21.963	0.9280
  10	30	5	0.433	70.27	0.9368
  20	30	5	0.933	58.20	0.9096
  30	30	5	1.26	45.73	0.9810
  10	30	10	0.933	37.74	0.9285
  20	30	10	0.8	48.64	0.9077
  30	30	10	1.233	24.90	0.9750
  10	30	15	0.8	32.7108	0.9637
  20	30	15	19.4	40.16	0.8630
  30	30	15	0.4033	48.28	0.9555
  Adsorbent : Neem leaf
  10	10	5	3.5	12.97	0.8796
  20	10	5	3.8	21.10	0.8228
  30	10	5	10.3	13.10
  10	20	5	2.2	15.95	0.8770
  20	20	5	2.1	34.24	0.8737
  30	20	5	2.7	21.95	0.9387
  10	30	5	0.4	48.97	0.9421
  20	30	5	0.8	50.32	0.9017
  30	30	5	0.9	43.14	0.9815
  10	30	10	1.2	24.41	0.9688
  20	30	10	0.7	38.37	0.9300
  30	30	10	0.9	23.84	0.9848
  10	30	15	1.1	20.27	0.9618
  20	30	15	0.7	28.63	0.8920
  30	30	15	0.5	28.55	0.9636

  Yoon-Nelson (1984) model

  The basic assumption of this model is the rate of decrease in the probability of adsorption for each adsorbate molecule depends on the adsorbate adsorption. The Yoon-

  Table 3 Yoon-Nelson model parameters

  Flow rate v (ml/min)	Influent concentration C0 (mg /L)	Bed height (cm)	Time, (min)	Rate constant kYN (1/min)	R2
  Adsorbent : Coconut shell
  10	10	5	475.0072	0.0069	0.8623
  20	10	5	359.4598	0.0072	0.8581
  30	30	5	191.2437	0.0170	0.7370
  10	20	5	332.679	0.0065	0.8791
  20	20	5	377.849	0.0063	0.9304
  30	20	5	89.48	0.0126	0.9520
  10	30	5	633.398	0.0020	0.9826
  20	30	5	237.68	0.0046	0.9403
  30	30	5	66.497	0.0061	0.9672
  10	30	10	1027.048	0.00349	0.9604
  20	30	10	524.57	0.0035	0.9475
  30	30	10	149.84	0.0031	0.9230
  10	30	15	1419.0813	0.0028	0.9773
  20	30	15	691.978	0.0027	0.9192
  30	30	15	257.4705	0.0020	0.9708
  Adsorbent : Neem leaf
  10	10	5	654.9772	0.0044	0.9284
  20	10	5	663.2368	0.0038	0.8277
  30	10	5	216.7615	0.0130	0.7286
  10	20	5	340.952	0.0063	0.9238
  20	20	5	391.551	0.0058	0.9240
  30	20	5	102.75	0.0091	0.9620
  10	30	5	543.63	0.0022	0.9675
  20	30	5	258	0.0040	0.9330
  30	30	5	61.96	0.0056	0.9830
  10	30	10	864.66	0.0045	0.9746
  20	30	10	507.64	0.0034	0.9630
  30	30	10	84.34	0.0053	0.9790
  10	30	15	1134.7027	0.0037	0.9580
  20	30	15	625.1	0.0030	0.9358
  30	30	15	161.533	0.0030	0.9721

  Thomas (1948) model

  Thomas model has been widely used for heavy metal removal in fixed bed column studies (Thomas, 1948). The mathematical form of Thomas model is expressed as follows (Malkoc et al., 2006),

  Nelson equation is as follows (Zeinali and et al., 2010),

  Ct 1

  (5)

  C

  C

  t exp(kYNt kYN)

  (4)

  C0 1 exp[( kThqe x ) k C t]

  C0 Ct

  Q Th 0

  The intercept and slope of the linear plot of ln(Ct/C0-Ct) Linearized form of Thomas model is expressed as follows,

  versus time were used to determine the value of breakthrough

  parameters and kYN. Parameters and the values of R2 are

  ln( Ct

  1)

  kTH qe x

• k C t

(6)

tabulated in Table 3. From the tabulated data it is observed

C Q

TH 0

0

that time required for 50% breakthrough decreases as the flow rate increases but the value of rate constant kYN increases with the increasing flow for Cr(VI) removal on coconut shell and neem leaf respectively. From the R2 values it can be concluded that Yoon-Nelson model is appropriate to describe the column operation of Cr(VI) removal on coconut shell and neem leaf respectively. Similar result obtained by other researcher (Saadi et al., 2013).

The model parameters and the values of R2 are tabulated in Table 4. From the table it is observed that the value of calculated qe from Thomas model is greater than the value of experimental qe. The value of qe increases as the adsorbent dose increases but the value of KTh decreases for the same. It is also observed that value of qe decreases as the flow rate of aqueous solution increases.

Table 4 Thomas model parameters

Yan et al. (2011) model

Mass transfer model is used to describe the breakthrough curve of the fixed bed column study (Yan et al. 2011). This model proposed a better approach to describe the breakthrough curve than the Thomas model. Linearized form

Table 5 Yan et al. model parameters

Wolborska (1989) model

The mathematical expression of Wolborska model is similar to Adams-Bohart model. The linearised form of Wolborska model is given as follows (Wolborska, 1989),

Ct z

of Yan et al. model is given as follows,

ln C

kABC0t kAB N0 v

(8)

Ct K

Q2

K C0

o

ln Y C0 ln Y ln t

(7)

The model parameter values are tabulated in Table 6. These

C0 Ct Q KY qY m Q

values depend on the flow rate and bed height. The value of b

The value of rate constant (KY), adsorption capacity (qY) and R2 are tabulated in Table 5. From the table it is observed that the adsorption capacity (qY) increases with the increase in flow rate and bed height. The rate constant (KY) value increases with the increase in flow rate.

increases with the increase in flow rate but decreases with the increases in bed height.

Table 6 Wolborska model parameters

1. SAFE DISPOSAL OF USED ADSORBENTS

   The leaching possibility of the Chromium ions from the used adsorbents restricts its dumping in the open environment. The Cr(VI) loaded adsorbents were incinerated at 700o C to form ash and then 5 mg of the ash samples were mixed with 25 ml of deionized water to give a liquid – solid ratio of 5:1 (Nag et al., 2015). After continuous gentle stirring for 24 h the filtrate was analyzed for Cr(VI) ions. It was observed that Cr(VI) did not leach from the ash sample. Hence, this ash may be used for road filling particularly in the nearby rural areas or to be placed in the agricultural field.

2. CONCLUSIONS

Fixed-bed column studies have been carried out for the removal of Cr(VI) ions using coconut shell and neem leaf

.

1. An effective removal of Cr(VI) is observed by a fixed bed column using coconut shell and neem leaf as an low- costadsorbent.

2. The removal efficiency of the adsorbent depends on the pH and flow rate of the metal solution, influent metal ion concentration and the bed height of the packed column. The removal process is effective at pH 2 for Cr(VI) removal on coconut shell and neem leaf respectively.

3. Maximum amont of metal removal is observed at lower flow rate. But at higher flow rate breakthrough time decreases which finally produces insufficient amount of metal removal.

4. At higher bed height maximum amount of heavy metal is removed than the lower bed height at a constant flow rate.

5. At low metal ion concentration adsorption capacity of the packed column increases. At higher metal ion concentration lower amount of metal removal is observed.

6. Bohart-Adams model, Yoon-Nelson model, Thomas model, Yan et al. model and Wolborska model are used to describe the performance of the fixed be column study. Thomas model and Yoon-Nelson model fit well with the experimental data for predicting the breakthrough curves for Cr(VI) removal compare to the other models.

7. Incineration of the used adsorbents to form non-leachable ash is the safe disposal suggested method.

ACKNOWLEDGEMENT

S. Sarkar thanks UGC for fellowship [Project no. F1- 17.1//2012-13/RGNF-2012-13-SC-WES-24757/(SA- III/Website)].

Authors are thankful to DST, Govt. of West Bengal for providing nancial assistance [Sanction Number: 211(Sanc.)/ST/P/S&T/13G-1/2013 dt. 06.06.2014].

NOMENCLATURE

C0 initial Cr(VI) concentration (mg/L) Ct Cr(VI) concentration at time t (mg/L) kAB kinetic constant (ml/min.mg)

kTh rate constant (ml/min.mg)

kYN rate constant (1/min)

KY rate constant (ml/mg.min)

N0 adsorption capacity (g/L)

qe,max maximum adsorption capacity (mg/g ) qY maximum adsorption capacity (mg/g) x mass of the adsorbent (g)

t breakthrough time (min)

z bed depth (cm)

Greek letters

Wolborska model constant (L/min)

time required for 50% adsorbate breakthrough (min)

v flow rate of metal solution (ml/min)

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