Journal of Waste Management
Volume 2015 (2015), Article ID 571213, 10 pages
http://dx.doi.org/10.1155/2015/571213

Research Article

Modeling the Sorption of Ni²⁺ and Co²⁺ on Saprist Peat Using the Response Surface Methodology

Emmanuel S. Asapo,1,2 Cynthia A. Coles,¹ and Leonard M. Lye¹

¹Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5
²Chemical and Polymer Engineering Department, Faculty of Engineering, Lagos State University, Epe Campus, PMB 1081, Lagos 106101, Nigeria

Received 16 November 2014; Accepted 26 December 2014

Academic Editor: Francesca Pagnanelli

Copyright © 2015 Emmanuel S. Asapo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A detailed study of the sorption of Ni²⁺ and Co²⁺ from simulated wastewater on saprist peat is presented. The significantly decomposed peat possessed a strong sorptive capacity that was maintained over a wide range of pH. With a metal concentration range of 50 to 200 mg/L, pH range of 3 to 10, peat dose of 2 to 40 g/L, and contact time of 12 to 24 h, batch experiments were conducted based on a four-factor Box-Behnken response surface design. The percentage removals of Ni²⁺ and Co²⁺ were analyzed using analysis of variance. Second order response surface models were developed with the significant factors and their interactions to predict the percentage sorption of Ni²⁺ and Co²⁺ independently. The prediction equations were verified with additional data not used in developing the equations. The study showed that the saprist peat could be a potential industrial metal adsorbent and the percentage of uptake of Ni²⁺ and Co²⁺ could be accurately predicted using the second order response surface models developed. Ni²⁺ uptake was greater for the two metals and reached a maximum value at just below a neutral pH and Co²⁺ uptake continued to increase from pH > 5, with higher uptake percentage at pH 10.

1. Introduction

Wastes from urban and industrial growth remain a major source of ground and surface water contamination [1] despite known scientific efforts and treatment technologies employed in most industries. Where mining is a major industry, metal pollution of waters is unlikely to be eliminated. Apart from their ecotoxicity and unlike organic contaminants, it is significant to note that metal contaminants are not biodegradable [2].

Nickel is a toxic trace element widely distributed in the environment [3, 4] and released by volcanoes, windblown dusts, and nickel based manufacturing processes. Soils typically contain 2 to 50 mg/kg of nickel and it can be readily absorbed by plant roots and mobilized in the plants [4]. Nickel compounds are human carcinogens to the extent that they are capable of releasing ionic nickel, which may become highly localized at critical tissue sites [3]. Above the daily permissible intake levels of 0.02 mg/L serious health impacts such as anemia, diarrhea, encephalopathy, hepatitis, and dysfunction of the central nervous system have been reported [5].

Cobalt occurs with copper, nickel, manganese, and arsenic, and small amounts are found in rocks, soil, water, plants, and animals. Average soil concentrations from 8 to 6.450 mg/kg have been reported at mine sites in Ontario, Canada [6]. Cobalt and cobalt compounds are possible human carcinogens and high intakes can lead to thyroid damage, heart problems, vomiting and nausea, and vision problems [3]. One study recommended that the interim maximum total Co concentration in a freshwater environment should not exceed 110 μg/L to protect aquatic life from acute effects of Co [7] because Co is highly toxic to zooplanktonic species [8, 9].

Adsorption remains an attractive technology in metal contaminated wastewater treatment but activated carbon is an expensive adsorbent. Thus inexpensive and alternate metals sorbents for wastewater treatment, such as rice husks [10], clays [11, 12], and chitosan [13], have been investigated [14, 15]. Peat, especially the poorly humified form, has been tested in the laboratory for the removal of mercury, cadmium, zinc, copper, iron, nickel, silver, lead, and antimony [16–19]. In central Estonia, an Oostriku peat bog was reported to contain 40% by weight of iron, lead, zinc, manganese, copper, and arsenic due to long exposure to metal-rich groundwater [20]. Since peat resources are often proximal to sites of effluent treatment this could ensure low cost treatment [21].

Peat provides a unique material for developing knowledge of the mechanisms and factors influencing trace metal binding onto natural organic constituents [16] and many reactions have been suggested as being responsible for the metal uptake in peat. These include ion exchange, surface adsorption (physical), chemisorption, complexation, and adsorption complexation [17, 18, 22, 23]. Multivalent cations may also form coordinate linkages with uptake dependent on the ionic radius of the metal [24]. The carboxylic and phenolic acid groups in peat are known to react with metals to release protons or at high pH their anion sites will attract available metals [18, 25], a form of ion exchange widely regarded as the main uptake mechanism. Limited knowledge of the uptake mechanisms could be one of the reasons for the scarcity of large-scale peat applications in the treatment of metal contaminated wastewaters.

Organic carbon content, pH, temperature, contact time, water holding capacity, particle size, peat texture, and quantity of peat used are some of the factors that may determine the metal removal efficiency. Interactions between these factors could also be significant in the understanding of the uptake chemistry. The main objective of this study was to establish the influential sorption parameters and, if significant, their interactions on the closely related Ni²⁺ and Co²⁺ ions. This paper presents the statistical analysis, establishes the optimum sorption conditions within the experimental range, and proposes an adjusted predictive regression model for uptake of the two metals.

The study employed batch sorption experiments and analyzed results using the Box-Behnken response surface design. The unique feature of the response surface design lies in its ability to locate the optimum region for the dependent response [26], in this case the % of metal removed. A saprist peat (obtained from a natural bog owned by Traverse Nurseries, near St. John’s, NL, Canada), of a homogeneous nature and containing predominantly oxygenated (carboxylic and alcoholic) and amine/amide functional groups and some cations but with no significant inorganic materials such as clays [27], was used. A summary of the physicochemical parameters of the saprist peat is presented in Table 1. A detailed characterization study has been reported in another study [27] with the presence of functional groups such as phenol, N-substituted aromatic, amine carbon, alcohol, carboxyl, and esters. Thus with this peat the metal uptake mechanism could be explained with less ambiguity. With the current mining of Ni and the recovery of Co in Newfoundland, the proximal availability of peat, especially the saprist type, if employed as a sorbent, could reduce Ni and Co loading to receiving waters.

Table 1: Summary of physicochemical parameters of saprist peat.

Parameter
Value

Degree of decomposition*
7-8H

pH (in deionized water)
4.2

Moisture content (%)
86

Fiber content (%)
69

Ash content (%)
9

Organic matter (%)
91

Dry bulk density (g/cm³)
0.28

pH 7.0 CEC** (meq/100 g)
70

Degree of decomposition is based on the von Post scale in which the percentage of undecomposed fiber in peat is used in the classification of a sample from 1H (least decomposed to 10H (most decomposed) [28]).

**CEC (cation exchange capacity) based on the calcium acetate/chloride method [29].

Other parameters were determined using the American Society for Testing and Materials (ASTM) for soils methods.

2. Materials and Methods

Saprist peat samples (between 7H and 8H on the von Post humification scale) were air-dried and sieved. Peat fractions ≤ 850 μm (90% of the peat) were further separated into fractions <450 μm and >450 μm for use as Ni and Co sorbents. The peat samples did not undergo any other treatment.

Nickel (II) and cobalt (II) solutions were prepared from analytical grade nitrate hexahydrates supplied by Anachemia Chemicals, Canada, by dilution in distilled water. The natural pHs of the Ni(II) and Co(II) solutions (5.5 and 5.8, resp.) were adjusted by adding 0.25 M sulphuric acid or buffer solution pH 10 containing carbonate, borate, and hydroxide of potassium buffer 0.05 M (Fischer Scientific, Canada) for a selection of pHs as indicated below and pH was measured with an ATI Orion model 3000 VWR brand pH/mV/temperature meter (VWR Scientific, Canada).

Preliminary batch experiments employed a two-level fractional factorial design with six factors (initial metal concentration, solution pH, contact time, peat dose, peat particle size, and agitation level). Analysis of variance (ANOVA) showed that agitation level and peat particle size were statistically insignificant at the 5% level and so these two factors were eliminated from further experiments. The metal adsorption behavior was also found to be nonlinear; hence a three-level Box-Behnken response surface design was chosen for further experimentation, to obtain predictive equations and to determine the optimum response within the range of the factors investigated.

Initial metal concentration, solution pH, contact time, and peat dose were the four factors selected for investigation in triplicate batch experiments. Samples of the saprist peat (0.08, 0.84, and 1.6 g) of ≤450 μm in size were combined with Ni²⁺ or Co²⁺ stock solutions (concentrations of 50, 125, and 200 mg/L) to give peat doses of 2, 21, and 40 g/L. Stock solutions were adjusted to pHs of 3, 6.5, and 10 before being added to the peat in 50 mL serum bottles. The samples were taped to a 5900 Eberbach reciprocal shaker and shaken for 12, 18, and 24 h, filtered with 45 μm quantitative filter paper (Anachemia Chemicals, Canada), and acidified with 0.5 M sulphuric acid, and the Ni²⁺ or Co²⁺ contents were determined with a Varian SpectrAA-55 flame atomic absorption spectrometer in air-acetylene flame. The data from these batch experiments were analyzed and regression models fitted to the data while the data from additional batch tests with metal concentrations of 100 and 150 mg/L, peat doses of 5, 10, 15, and 20 g/L, and pHs of 6 and 8 were used to validate the regression models.

Desorption of Ni²⁺ and Co²⁺ from the peat samples was investigated by the addition of 0.5, 1.0, and 2 M HCl to the peat-metal materials. The mixture was agitated for 2 h on a 5900 Eberbach reciprocal shaker and filtered using quantitative filter paper with 45 μm openings (Anachemia Chemicals, Canada). The filtrates were analyzed using flame atomic absorption spectrometry to determine the amounts of metals desorbed.

The combination of the four factors in the three-level design required 29 experimental runs for each metal and Design-Expert 8.0.4 (Stat-Ease, Inc.) was used for both experimental design and data analysis. The measured percentages of Ni²⁺ or Co²⁺ uptakes (the responses) are summarized in Table 2.

Table 2: Values of design variables and percentages of metals removed that were used in the Box-Behnken analysis.

Run number
Actual factors
Metal removed, %

Co²⁺
Ni²⁺

Conc. (mg/L)
Peat dose (g/L)
Time (h)
pH

1
50
2
18
6.5
45.0
98.0

2
200
2
18
6.5
30.0
58.7

3
50
40
18
6.5
93.8
98.8

4
200
40
18
6.5
84.6
99.6

5
125
21
12
3
77.2
93.0

6
125
21
24
3
78.7
94.4

7
125
21
12
10
94.6
23.7

8
125
21
24
10
94.8
26.2

9
50
21
18
3
86.4
94.0

10
200
21
18
3
73.6
79.7

11
50
21
18
10
96.2
32.8

12
200
21
18
10
93.5
23.4

13
125
2
12
6.5
36.3
69.9

14
125
40
12
6.5
87.9
99.6

15
125
2
24
6.5
33.4
70.8

16
125
40
24
6.5
88.9
83.6

17
50
21
12
6.5
89.6
95.5

18
200
21
12
6.5
78.8
80.5

19
50
21
24
6.5
90.4
96.1

20
200
21
24
6.5
74.0
87.3

21
125
2
18
3
33.1
31.7

22
125
40
18
3
87.3
97.3

23
125
2
18
10
54.9
17.5

24
125
40
18
10
95.7
30.4

25
125
21
18
6.5
81.3
78.8

26
125
21
18
6.5
78.6
88.9

27
125
21
18
6.5
80.7
91.4

28
125
21
18
6.5
80.2
90.2

29
125
21
18
6.5
79.4
99.7

The data were fitted to a second order polynomial function represented by the following equation for factors:

where is the linear effect of the th factor, is the quadratic effect, is the interaction or cross product effect between th and th factors, and is the random error term.

3. Results and Discussion

Before the addition of metals only 4 mg/kg Ni and no Co were detected to be naturally occurring in the peat [30]. Table 3 is the summary of the ANOVA for the response surface reduced quadratic model for the batch experiment results. For Ni²⁺ and Co²⁺ the values were 0.9431 and 0.9936, respectively, and the predicted values were 0.8215 and 0.9812, respectively, suggesting that the model for the two cations could be better at predicting responses for new conditions.

Table 3: ANOVA for response surface reduced* quadratic model for Ni²⁺ and Co²⁺.

Source
Sum of squares
Degrees of freedom
Mean square
value
value
prob. >
Comments

-Ni²⁺ conc.
616
1
617
10
0.0050

-peat dose
2204
1
2205
35
<0.0001

-pH
9413
1
9414
150
<0.0001

402
1
403
6
0.0194

694
1
694
11
0.0032

499
1
500
8
0.0102

8449
1
8450
134
<0.0001

Residual
1319
21
63

    Lack of fit
1096
17
65
1
0.4939
Not significant

    Pure error
222
4
56

Cor. total
23198
28

-Co²⁺ conc.
372
1
373
97
<0.0050

-peat dose
7777
1
7778
2030
<0.0001

-pH
726
1
727
190
<0.0001

26
1
26
7
<0.0194

45
1
45
12
<0.0032

24
1
24
6
<0.0102

2357
1
2357
615
<0.0001

216
1
216
56
<0.0001

Residual
77
20
4

    Lack of fit
72
16
4
4
0.0955
Not significant

    Pure error
4
4
1

Cor. total
11994
28

Reduced means factors and interactions were eliminated when their values were > 0.05.

The ANOVA (Table 3) eliminated statistically insignificant effects (contact time) and interactions. Contact time may have been insignificant in removing metals because of the rapidity of the reactions once the metals were exposed to the peat. The largest sum of squares values showed that pH had the strongest influence on Ni²⁺ sorption, peat dose moderately influenced Ni²⁺ uptake, and peat dose had the strongest influence on Co²⁺ uptake. The other factors and all interactions were of lesser importance.

The following equations are the initial second order quadratic models of the actual factors for Ni²⁺ and Co²⁺ sorption on the saprist peat, respectively:

After regression diagnostic checks on the regression models, the models were used to predict the uptake of Ni²⁺ and Co²⁺ at metal concentrations, peat doses, and solution pHs within the range of experimental values, with data not used in developing the models (50 to 200 mg/L of metal, pH 3 to 10, and peat doses of 2 to 40 g/L). The experimental and predicted values were compared for the two models ((2) and (3)) to determine the mean absolute percent errors (MAPEs), referred to as the original MAPEs.

When the experimental and predicted values were compared it was discovered that the equations could be improved by including the additional data to the model. This involved only a small change in the intercept term of the models obtained by adding (Ni²⁺) and taking (Co²⁺) the average of the absolute values of the deviations (actual minus experimental values) from the intercept term. Equations (4) and (5) show the corrected prediction equations and Table 4 presents the predicted values obtained from (4) and (5) and the actual values:

Table 4: Corrected predicted and actual % metals retained after 18 h contact/shaking time.

Metal conc. (mg/L)
Peat dose (g/L)
Predicted
Actual
Predicted
Actual
Predicted
Actual
Predicted
Actual

pH 3
pH 6
pH 8
pH 10

Ni²⁺
50
5
82.4
66.9
102.9*
86.5
86.6
72.4
48.2
32.6

50
10
86.7
74.5
103.4*
91.0
85.9
76.0
45.5
40.0

50
15
90.2
83.7
103.7*
96.4
84.2
77.1
41.8
40.5

50
20
92.3
91.5
103.0*
98.5
81.5
78.0
37.1
44.0

100
5
68.7
56.2
88.3
73.5
72.8
57.2
34.5
22.0

100
10
71.2
62.5
87.9
84.0
70.4
64.7
30.0
32.4

100
15
72.7
64.0
86.4
88.2
66.9
69.5
24.5
34.3

100
20
73.3
76.2
83.9
97.5
62.4
72.8
18.0
33.5

150
5
54.9
42.5
74.6
58.6
59.1
49.0
20.7
15.9

150
10
55.7
57.3
72.3
71.5
54.8
62.0
14.5
18.6

150
15
55.5
72.5
69.1
83.8
49.6
66.1
07.2
21.1

150
20
54.2
80.0
64.9
91.6
43.4
68.0
−1.0**
22.8

Co²⁺
50
5
46.5
46.0
47.8
50.0
53.3
54.0
62.4
62.6

50
10
61.3
62.5
61.8
61.5
66.8
69.0
75.4
74.8

50
15
73.5
73.1
73.3
76.0
77.8
80.1
85.9
91.0

50
20
83.3
80.5
82.3
83.0
86.3
86.4
93.9
94.0

100
5
39.5
40.2
42.3
43.2
48.7
57.2
58.8
60.0

100
10
54.3
56.5
56.3
54.0
62.2
61.7
71.8
72.5

100
15
66.5
67.3
67.8
58.0
73.2
71.5
82.3
80.8

100
20
76.3
76.2
76.8
74.0
81.7
82.0
90.3
96.5

150
5
34.2
42.5
38.4
38.5
45.8
47.0
56.8
59.5

150
10
48.9
57.3
52.4
53.2
59.3
60.0
69.8
68.6

150
15
61.2
72.5
63.9
63.5
70.3
71.1
80.3
75.5

150
20
70.9
80.0
72.9
71.5
78.8
80.0
88.3
88.0

MAPE for Ni²⁺, original
0.16

0.15

0.21

0.65

MAPE for Ni²⁺, corrected
0.16

0.13

0.16

0.36

MAPE for Co²⁺, original
0.07

0.09

0.06

0.06

MAPE for Co²⁺, corrected
0.06

0.002

0.03

0.03

The model predicted >100% Ni²⁺ uptake.

**The model predicted negative Ni²⁺ uptake.

The metal bonding capacity of the saprist peat is represented by the percentage of metal removed from solution. How the three noneliminated factors (pH, peat dose, and metal concentration) and their interactions affected the two metals uptakes is shown in Figures 1(a), 1(b), 2(a), and 2(b). On the -axis of the plots in the figures, the Design-Expert 8.0 generated two mid points in addition to the experimental mid points (125 mg/L for concentration and 21 g/L for peat dose). These additional mid points are at concentrations 162.5 mg/L (for points 125 and 200 mg/L) and 87.5 mg/L (for points 125 and 50 mg/L). For peat dose, the mid points are 30.5 g/L (for points 21 and 40 g/L) and 11.5 g/L (for points 21 and 2 g/L). Five design points representing the mid points used at various combinations of concentration, pH, and peat dose and their corresponding uptakes depicted by the green circles are also included in the plots.

(a)

(b)

Figure 1: (a) Ni²⁺ concentration-peat dose interaction plot for % Ni²⁺ removed. (b) Peat dose-pH interaction plot for % Ni²⁺ removed.

(a)

(b)

Figure 2: (a) Concentration-pH interaction plot for % Co²⁺ removed. (b) Peat dose-pH interaction plot for % Co²⁺ removed.

Figure 1(a) indicates that percentage Ni²⁺ removal is always lesser at large initial Ni²⁺ concentration and low peat dose (point 1), while at low peat dose and low concentration (point 2) the Ni²⁺ uptake is slightly less than the observed uptake at low concentration and large peat dose (point 3) and at large concentration and large peat dose (point 4) where greater uptake is observed (represented by the red line). Interaction is pronounced at low peat dose and low concentration (point 2) and at large peat dose and low concentration (point 3) combinations. As seen in Figure 1(a), the red and black lines nearly converge at points 2 and 3 while significantly diverging as the concentration is increased from 50 to 200 mg/L at low (point 1) and high peat doses (point 4).

Figure 1(b) shows the interaction between peat dose and pH for Ni²⁺ uptake. Larger Ni²⁺ uptake is observed at low pH and large peat dose (point 4). The uptake observed at low pH and low peat dose (point 3) is also larger than the uptakes at large pH and low peat dose (point 2) and large pH and large peat dose (point 1). The uptakes at the combinations of large pH and low peat dose and large pH and large peat dose are nearly the same.

The interactions effect requires the establishment of appropriate conditions depending on the desired uptakes. The percentage Ni²⁺ removal is greater at the larger peat dose except when combined with high pH and low Ni²⁺ concentration as shown in Table 3 with the pH having a greater influence than peat dose on Ni²⁺ removal.

For Co²⁺ uptake as seen in Figures 2(a) and 2(b) where the lines are almost parallel, the concentration-pH and peat dose-pH interactions are negligible (as also indicated in Table 3). Figure 2(a), however, shows greater Co²⁺ removal at the higher pH (represented by the red line) and at low Co²⁺ concentration (point 3). Figure 2(b) shows greater Co²⁺ removal at the higher pH (red line) and at high peat dose (point 3). Peat dose was seen to be the most significant factor in Co²⁺ removal (Table 3). When the Co²⁺ concentration increases, there are always small decreases in the percentage of Co²⁺ removal. The pH and metal concentration were seen to be less important than peat dose for their effect on Co²⁺ removal (Table 3). To summarize, Co²⁺ removal was seen to be greater with increase in peat dose, with increase in pH, and with decrease in Co²⁺ concentration.

Comparing uptake of the two metals by the saprist peat, it was the pH followed by the peat dose that was the most significant factor for Ni²⁺ uptake, and peat dose was the most significant factor for Co²⁺ uptake. In addition, greater percentage metal removal corresponded to lower pH for Ni²⁺ and higher pH for Co²⁺.

The second order polynomial equations (4) and (5) were good at predicting experimental results for parameter values within the range of experimental values, but not actually used in the model development for Ni²⁺ and Co²⁺ (Table 4) and this is depicted by the scatterplots (Figures 3 and 4).

Figure 3: Scatter plots for % actual and % corrected predicted Ni²⁺ adsorbed.

Figure 4: Scatter plots for % actual and % corrected predicted Co²⁺ adsorbed.

The dominant factor affecting Ni²⁺ uptake by the saprist peat was the solution pH. The highest Ni²⁺ uptake is calculated to occur in the range of pH 4.4 to 5.8 using (4). Table 5 also shows the best Ni²⁺ removal occurring at pH ~ 6. As pH increases from pH 3 to 5.5 deprotonation of the peat surface occurs and the H⁺ ions released from the peat could compete with Ni²⁺ ions for sorption sites and the cations could be sorbed by complexation. At pH > 5.5 a high concentration of hydroxyl groups will exist and will permit the formation of the reactive carboxylic anions and amine groups (RNH₂). These two groups are better reactive agents at higher pHs via the displacement of protons than Ni²⁺. As solution pH is increased above pH 5.5, more Ni²⁺ complexes are formed due to the involvement of more containing groups capable of donating electrons to the vacant orbital of the Ni²⁺ ions, thus replacing two displaced H⁺ ions (ion exchange). For Ni²⁺ it appears that complexation is favored at pHs < 5.5 and ion exchange is favored at pHs > 5.5.

Table 5: Two solutions for Ni²⁺ and Co²⁺ uptake by multiple response optimization.

Constraints and limits
Goal
Solutions

Conc., 50–200 mg/L
In range
~100% Ni uptake at 50 mg/L conc.

Peat dose, 2–40 g/L
Minimize
14 g/L peat dose and pH 5.3

pH 4.5–pH 6.5
In range
~94% Co removed at 50 mg/L,

27 g/L peat dose and pH 6.5

Conc., 50–200 mg/L
In range
~100% Ni uptake at 200 mg/L conc.

Peat dose, 2–40 g/L
Minimize
28 g/L peat dose and pH 4.9

pH 4.5–pH 6.5
In range
~90% Co uptake at 104 mg/L conc.,

30 g/L peat and pH 6.5

Since the dominant factor affecting Co²⁺ uptake was peat dose, higher peat doses may have provided more available active bonding sites due to a decreased sorption density and unsaturation of sorption sites. Thus the Co²⁺ uptake appears to have been governed by a different reaction mechanism compared to the Ni²⁺ uptake. Particle aggregation could also have been experienced as peat dose was increased, leading to hindered uptake and the need for greater space. At pH > 8, precipitation of the Co²⁺ complexes with available and active ligands could contribute to particle aggregation as well.

Metal concentration was the third significant factor affecting metal uptake and an increase in metal concentration resulted in a decrease in % Ni²⁺ and Co²⁺ removed. Comparing (4) and (5), however, the quadratic effect of concentration in (5) might also suggest a different uptake mechanism for Co²⁺ compared to Ni²⁺.

A multiple response optimization technique based on desirability functions using Design-Expert 8.0.4, for minimal use of peat as the goal (Table 5), was tested but not experimentally verified for two scenarios. This technique allows for the best combinations of responses and the choice of desirability is from zero (worst) to one (best). A desirability of one implies that a small increase or decrease in the factors will not render the quality of the response unacceptable [31]. This optimization approach suggested that more Ni²⁺ was retained under more acidic conditions and lower peat doses compared to Co²⁺.

The mechanisms of adsorption of Ni²⁺ and Co²⁺ are considered in terms of the functional groups present in the saprist peat. The phenolic OH, COOH, and C=O (oxygenated) functional groups originating in the humic and/or fulvic acids of saprist peat are good complexing agents. The binding of Co²⁺ and Ni²⁺ by these surface functional groups in the peat begins effectively at pH 3 and rises to an optimum value within the pH 4.4 to 5.8 range for Ni²⁺ and pH 10 for Co²⁺. These different values suggest different uptake reactions.

For Ni²⁺, complexes formed by oxygenated, amino, and imino functional groups [27] and ion exchange reactions following deprotonation of the carboxylic component in the saprist peat could have led to higher overall Ni²⁺ uptakes. At low pH electronic cleavages and rearrangement due to the influence of the acidic environment may have aided the incorporation of Ni²⁺ cations onto the peat surface by complexation. At pH < 5.0, more ligands with negative charge are exposed leading to an increase attack by the Ni²⁺ ions in solution [32]. As the pH is increased and the peat surface becomes more negatively charged, complexes formation via an exchange of deprotonated sites of the hydroxides of Ni²⁺ (Ni(OH)₂) may have dominated. At the optimum uptake pH, complexation and ion exchange reactions may have combined to give the large observed Ni²⁺ uptake.

The surface charge of peat is negative and, at alkaline solution pHs, humic and fulvic acids in the peat would be dissolved and could react with Co²⁺ to form soluble Co²⁺ complexes. Co²⁺ is known to strongly complex with these acids [33]. Unlike Ni²⁺, less Co²⁺ uptake by vacant sorption sites on the peat surface was experienced at low pHs when humic acid would have precipitated because access to the peat surface closely associated with H₃O⁺ could have been repelled [32]. As pH increases towards pH 10, complexation leading to the precipitation of the Co²⁺ complexes formed with the ligands in the peat dominated leading to the uptake of the ion.

Desorption experiments have identified ion exchange as an uptake mechanism in peat-metal chemistry [22, 34]. To assess ion exchange of Ni²⁺ and Co²⁺ with the saprist peat, desorption tests with 0.5, 1.0, and 2.0 M HCl were conducted. For a peat dose of 40 g/L and at pH 10, >50% of initially sorbed metal was desorbed (Table 6). At peat doses of 2 and 21 g/L and at pH 3, Ni²⁺ and Co²⁺ were not significantly desorbed, while, at pH 5.5 and 8, Co²⁺ was not significantly desorbed but >50% of Ni²⁺ was desorbed. Within the acid range of pH of desorption, the metal ions sorbed in a peat matrix are exchanged for the hydroxonium ion. Thus, where desorption was observed, ion exchange is suggested as an initial route through which the metal ion was sorbed. Ion exchange appears to have been more important for Ni²⁺ uptake than for Co²⁺ uptake.

Table 6: % desorption of Ni²⁺ and Co²⁺ at a peat dose of 40 g/L and solution pH of 10.

Initial cation conc. (mg/L)
Ni²⁺
Co²⁺

12.5
200
12.5
200

% desorption with 0.5 M HCl
78.5
93.5
50.6
69.1

% desorption with 1.0 M HCl
82.4
96.9
61.7
72.5

% desorption with 2.0 M HCl
92.0
97.5
67.1
83.5

4. Conclusion

The conventional experimental approach of investigating reaction factors is to change one variable at a time but that does not allow an optimized formulation to be developed as interactions among the variables are neglected. This study employed the Box-Behnken design in the optimization of the saprist peat sorbent for the uptake of Ni²⁺ and Co²⁺. The second order polynomial equations obtained were good at predicting experimental results that were not used in the model development but were within the range of tested values as shown in Table 3 for Ni²⁺ and Co²⁺ and depicted by the scatterplots in Figures 3 and 4. The models showed better results for Co²⁺ compared to Ni²⁺. Peat being a nonrenewable material should be economically used if employed as a metal sorbent. This study utilized peat at a dose far less than the usual 20 : 1 solution to soil ratio employed in soil sorption studies; thus minimal use was achieved.

Statistical analyses indicated that it is unlikely that Ni²⁺ and Co²⁺ uptake are explained by a single reaction mechanism as the effect of each factor and interactions among the factors differed for Ni²⁺ and Co²⁺. However, metal concentration, peat dose, and pH were the same three main factors affecting sorption for each metal. Time of contact was statistically insignificant as reactions were initiated as soon as sufficient contact between the peat and the metal was established. The saprist peat could be a potential industrial metal sorbent.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are grateful to the National Sciences and Engineering Research Council of Canada (NSERC) for financial support. The laboratory support from Ms. Geraldine Kennedy at the Chemistry Department is highly appreciated. Finally, Traverse Nursery, Torbay, NL, Canada, is acknowledged for the supply of the untreated peat.

References

G. Annadurai, R. S. Juang, and D. J. Lee, “Adsorption of heavy metals from water using banana and orange peels,” Water Science and Technology, vol. 47, no. 1, pp. 185–190, 2003.

Q. Fei and W. Bei, “Single- and multi-component adsorption of Pb, Cu, and Cd on peat,” Bulletin of Environmental Contamination and Toxicology, vol. 78, no. 3-4, pp. 265–269, 2007.

International Agency for Research on Cancer, Chromium, Nickel and Welding, IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans, International Agency for Research on Cancer, Lyon, France, 1990.

H. Çiftçi, A. Ölçücü, and T. Çiftçi, “The determination of nickel in some plants with reversed-phase high performance liquid chromatography (HPLC),” The International Journal of Science & Technology, vol. 2, no. 2, pp. 105–108, 2007.

G. Sheng, S. Yang, J. Sheng, D. Zhao, and X. Wang, “Influence of solution chemistry on the removal of Ni(II) from aqueous solution to titanate nanotubes,” Chemical Engineering Journal, vol. 168, no. 1, pp. 178–182, 2011.

Ontario Fact Sheet, Ministry of Environment Programs and Initiatives, 2001

N. K. Nagpal, “Water quality guidelines for cobalt,” Tech. Rep., Ministry of Water, Land, and Air Protection, British Columbia, Canada, 2004, http://www.env.gov.bc.ca/wat/wq/BCguidelines/cobalt/cobalt_tech.pdf.

B. K. Das and A. Kaviraj, “Individual and interactive lethal toxicity of cadmium, potassium permanganate and cobalt chloride to fish, worm and plankton,” Geobios, vol. 21, no. 4, pp. 223–227, 1994.

B. S. Khangarot and P. K. Ray, “Investigation of correlation between physicochemical properties of metals and their toxicity to the water flea Daphnia magna Straus,” Ecotoxicology and Environmental Safety, vol. 18, no. 2, pp. 109–120, 1989.

U. Kumar and M. Bandyopadhyay, “Sorption of cadmium from aqueous solution using pretreated rice husk,” Bioresource Technology, vol. 97, no. 1, pp. 104–109, 2006.

J. H. Potgieter, S. S. Potgieter-Vermaak, and P. D. Kalibantonga, “Heavy metals removal from solution by palygorskite clay,” Minerals Engineering, vol. 19, no. 5, pp. 463–470, 2006.

R. Celis, M. G. Hermosín, and J. Cornejo, “Heavy metal adsorption by functionalized clays,” Environmental Science and Technology, vol. 34, no. 21, pp. 4593–4599, 2000.

I. N. Jha, L. Iyengar, and A. V. S. Prabhakara Rao, “Removal of cadmium using chitosan,” Journal of Environmental Engineering, vol. 114, no. 4, pp. 962–974, 1988.

S. Babel and T. A. Kurniawan, “Low-cost adsorbents for heavy metals uptake from contaminated water: a review,” Journal of Hazardous Materials, vol. 97, no. 1–3, pp. 219–243, 2003.

A. Türkman, Ş. Aslan, and I. Ege, “Treatment of metal containing wastewaters by natural zeolites,” Fresenius Environmental Bulletin, vol. 13, no. 6, pp. 574–580, 2004.

I. Twardowska and J. Kyziol, “Sorption of metals onto natural organic matter as a function of complexation and adsorbent-adsorbate contact mode,” Environment International, vol. 28, no. 8, pp. 783–791, 2003.

P. A. Brown, S. A. Gill, and S. J. Allen, “Metal removal from wastewater using peat–review paper,” Water Research, vol. 34, no. 16, pp. 3907–3916, 2000.

Y. S. Ho, D. A. J. Wase, and C. F. Forster, “Batch nickel removal from aqueous solution by sphagnum moss peat,” Water Research, vol. 29, no. 5, pp. 1327–1332, 1995.

D. Couillard, “The use of peat in wastewater treatment,” Water Research, vol. 28, no. 6, pp. 1261–1274, 1994.

K. Syrovetnik, M. E. Malmström, and I. Neretnieks, “Accumulation of heavy metals in the Oostriku peat bog, Estonia: determination of binding processes by means of sequential leaching,” Environmental Pollution, vol. 147, no. 1, pp. 291–300, 2007.

E. C. Lima, B. Royer, J. C. P. Vaghetti et al., “Adsorption of Cu(II) on Araucaria angustifolia wastes: determination of the optimal conditions by statistic design of experiments,” Journal of Hazardous Materials, vol. 140, no. 1-2, pp. 211–220, 2007.

Z. Zhipei, Y. Junlu, W. Zengnui, and C. A. Piya, “A preliminary study of the removal of Pb²⁺, Cd²⁺, Zn²⁺, Ni²⁺, and Cr⁶⁺ from wastewaters with several Chinese peats,” in Proceedings of the 7th International Peat Congress, pp. 18–23, Dublin, Ireland, 1984.

P. R. Bloom and M. B. McBride, “Metal ion binding and exchange with hydrogen ions in acid-washed peat,” Soil Science Society of America Journal, vol. 43, no. 4, pp. 687–692, 1979.

L. Ringqvist and I. Öborn, “Copper and zinc adsorption onto poorly humified Sphagnum and Carex peat,” Water Research, vol. 36, no. 9, pp. 2233–2242, 2002.

R. H. Crist, J. R. Martin, J. Chonko, and D. R. Crist, “Uptake of metals on peat moss: an ion-exchange process,” Environmental Science and Technology, vol. 30, no. 8, pp. 2456–2461, 1996.

D. C. Montgomery and G. C. Runger, Applied Statistics and Probability for Engineers, John Wiley & Sons, New York, NY, USA, 3rd edition, 2003.

E. Asapo and C. Coles, “Peat characterization and uptake of nickel (II) and cobalt (II) in a saprist peat column,” Adsorption Science and Technology, vol. 30, no. 5, pp. 369–381, 2012.

W. Stanek and T. Silc, “Comparisons of four methods for determination of degree of peat humification (decomposition) with emphasis on the von Post method,” Canadian Journal of Soil Science, vol. 57, no. 2, pp. 109–117, 1977.

B. H. Sheldrick, Analytical Methods Manual 1984, LRRA Contribution Number 84-30 6/1–3, Research Branch, Agriculture Canada, 1984.

E. S. Asapo, An adsorption study of nickel and cobalt on saprist Newfoundland peat [Ph.D. thesis], Memorial University of Newfoundland, St. John's, Canada, 2011.

D. C. Derringer, “A balancing act: optimizing a product's properties,” Quality Progress, vol. 27, no. 6, pp. 51–58, 1994.

N. Singh and R. Gadi, “Removal of Ni (II) and Cu (II) from their solutions and waste water by nonliving biomass of Pseudomonas oleovorans,” Hydrology Current Research, vol. 3, no. 1, 2012.

X. L. Li, C. L. Chen, P. P. Chang, S. M. Yu, W. S. Wu, and X. K. Wang, “Comparative studies of cobalt sorption and desorption on bentonite, alumina and silica: effect of pH and fulvic acid,” Desalination, vol. 244, no. 1–3, pp. 283–292, 2009.

B. S. Gupta, M. Curran, S. Hasan, and T. K. Ghosh, “Adsorption characteristics of Cu and Ni on Irish peat moss,” Journal of Environmental Management, vol. 90, no. 2, pp. 954–960, 2009.