Table of Contents
Journal of Waste Management
Volume 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

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 [1619]. 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.

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.

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+.

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.

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.

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

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.

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.

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