Adsorption of selected toxic metals by modi?ed peanut shells
Srinath Chamarthy,1Chung W Seo 1*and Wayne E Marshall 2
1
Department of Human Environment and Family Sciences,Food and Nutrition Laboratory,North Carolina A&T State University,1601E Market Street,Greensboro,NC 27411,USA 2
United States Department of Agriculture,Agricultural Research Service,Southern Regional Research Center,1100Robert E Lee Blvd,PO Box 19687,New Orleans,LA 70179,USA
Abstract:The objective of this study was to modify peanut shells to enhance their adsorptive properties toward the metal ions cadmium (Cd 2 ),copper (Cu 2 ),nickel (Ni 2 ),lead (Pb 2 )and zinc (Zn 2 ).Milled peanut shells were initially washed with water or 0.1N NaOH or left unwashed.Following these treatments or lack of treatment,the shells were either left unmodi?ed or modi?ed by a heat treatment in the presence of either 1.0M phosphoric acid or 0.6M citric acid.Modi?ed peanut shells were evaluated either for adsorption ef?ciency or for adsorption capacity using the ?ve metal ions listed above.Adsorption ef?ciencies and capacities were compared with ef?ciencies and/or capacities for the commercial chelating or cation exchange resins Amberlite 200,Amberlite IRC-718,Duolite GT-73,and carboxymethylcellulose.For the adsorption ef?ciencies of individual metal ions,modi?ed peanut shells met or exceeded the adsorption values for cadmium,copper,nickel or zinc ions compared with the commercial resins Duolite GT-73and carboxymethylcellulose.In a solution containing all ?ve metal ions,modi?ed peanut shells met or exceeded the adsorption ef?ciencies for cadmium,copper and lead ions compared with Duolite GT-73,Amberlite IRC-718and carboxymethylcellulose.Adsorption capacities of modi?ed peanut shells met or exceeded the adsorption capacity of Duolite GT-73for lead ions only.Citric or phosphoric acid-modi?ed peanut shells showed a preference for Cu 2 and Pb 2 and appear promising as potentially inexpensive adsorbents for selected metal ions.#2001Society of Chemical Industry
Keywords:peanut shells;citric acid;phosphoric acid;metal ions;adsorbents
1INTRODUCTION
Peanut shells are low density,high volume agricultural waste which,in part,are used in animal feed formulations or are burned for energy.These are low value uses for the shells and additional product outlets need to be identi?ed to create higher value products.In order to add value to peanut shells,attention has focused on the utilization of peanut shells as adsor-bents.In this regard,peanut shells have been modi?ed by heat or chemical treatment to improve their ability to adsorb different metal ions or organic compounds.There have been several reports 1±7that have described peanut shell (hull)activated carbon for adsorption of various metal ions and organic com-pounds.In addition to their use as activated carbons,peanut shells have been used as is or undergone different modi?cations to enhance their ability to sequester metal ions from solution.Henderson et al 8reported that untreated peanut shells adsorbed sig-ni?cant amounts of Hg 2 ,Cu 2 ,Ni 2 and Zn 2 at low concentrations of the metals.A 1h exposure of the shells to these metal ion solutions signi?cantly increased their adsorption by decreasing the size of
the peanut shells.Azab and Peterson 9and Wartelle and Marshall 10used alkali treatment to increase Cd 2 and Cu 2 uptake,respectively,by peanut shells.Okieimen et al 11noted that treatment of peanut shells with the chelating agent EDTA improved subsequent Cd 2 and Pb 2 sorption.Recently,Wafwoyo et al 12reacted peanut shells in the presence of either phosphoric acid or citric acid to greatly increase the metal ion binding ability of the shells.
The present study is a continuation of research reported by Wafwoyo et al.12The objective of this study was to extend the results reported by Wafwoyo et al 12including:(1)adding a ?fth metal ion,namely Pb 2 ,to solutions containing the four metal ions,Cd 2 ,Cu 2 ,Ni 2 and Zn 2 ,(2)adding the commer-cial cation exchange resin,carboxymethylcellulose,for comparison purposes and (3)determining adsorption capacities for modi?ed peanut shells and commercial resins.
Phosphoric and citric acids were again chosen as modifying agents in this study because of the promis-ing metal ion adsorption results achieved by Wafwoyo et al 12using these modi?cation strategies.
(Received 26October 2000;revised version received 4January 2001;accepted 13February 2001)
*Correspondence to:Chung W Seo,Department of Human Environment and Family Sciences,Food and Nutrition Laboratory,North Carolina A&T State University,1601E Market Street,Greensboro,NC 27411,USA
Contract/grant sponsor:USDA-CSREES Capacity Building Program;contract/grant number:95-38814-1729Journal of Chemical Technology and Biotechnology
J Chem Technol Biotechnol 76:593±597(2001)
DOI:10.1002/jctb.418
2MATERIALS AND METHODS
2.1Peanut shells
Peanut shells were a gift from the Carolina±Virginia Peanut Growers Association(Suffolk,VA).They have
a bulk density of0.22gcmà3for a particle size range of
0.85to 2.00mm and consist predominantly of cellulose(45.3%).10Other major components include lignin(32.8%),hemicellulose(8.1%),protein(4.9%) and ash(2.3%).10
Sodium hydroxide,cupric sulfate,the nitrates of cadmium,lead,nickel and zinc,citric acid monohy-drate,acetic acid(glacial),sodium acetate(mono-basic),85%phosphoric acid,and nitric acid(trace metal grade)were purchased from Fischer Scienti?c (Fair Lawn,NJ).Plasma emission standards for the ?ve metal ions were obtained from Solutions Plus Inc (Fenton,MO).The commercial resins Amberlite IRC-718,Amberlite200,Duolite GT-73,and car-boxymethylcellulose were purchased from Supelco (Bellefonte,PA).Amberlite IRC-718and Duolite GT-73are chelating resins designed to be selective for certain metal ions,including Cu2 ,Pb2 ,Cd2 ,and Zn2 ,while Amberlite200and carboxymethylcellu-lose are general-purpose cation exchange resins.
2.2Preparation of peanut shells
The preparation of milled peanut shells and treat-ments for unwashed,base-washed and water-washed peanut shells were carried out as described by Wafwoyo et al.12Peanut shell modi?cations with 0.6M citric acid or1.0M phosphoric acid were also carried out according to the methods of Wafwoyo et al.12
2.3Metal ion solutions
The metal ions chosen for the study were Cu2 ,Pb2 , Cd2 ,Zn2 ,and Ni2 .All single metal ion and multiple metal ion solutions were made to a concen-tration of20m M in0.07M sodium acetate±0.03M acetic acid buffer,pH4.8.The multiple metal ions contained all?ve metal ions,each at20m M.To produce adsorption isotherms,single metal ion solu-tions of all?ve metals were made to concentrations of 5,10,15,20,25,30,35,40,45,and50m M in the sodium acetate±acetic acid buffer,pH4.8.
2.4Metal ion adsorption of modi?ed peanut shells and commercial resins
Metal ion adsorption was carried out according to the procedure used by Wafwoyo et al.12The pH of the solutions was measured at the beginning and end of the metal ion adsorption experiments and varied between4.6and5.0for any given experiment.
2.5ICP and statistical analysis
ICP analyses were performed on the?ltrates as described by Wafwoyo et al.12
The metal ions remaining in solution after adsorp-tion(equilibrium concentration)and the amount adsorbed(mmol)per g of sample were determined on a dry weight basis of adsorbent.Adsorption capacity was calculated using the Langmuir model, which in its non-linear form is given as:
X X m?K?C e = 1 K ?C e 1 Where:X=amount of solute adsorbed per unit weight of adsorbent;C e=equilibrium concentration of solute; X m=maximum adsorption capacity of adsorbent for solute and K=adsorption equilibrium constant. Solving eqn(1)by non-linear,least squares regres-sion analysis determines values for X m.These values were obtained by using a regression analysis program in Sigma Plot v4.0(SPSS Inc,Chicago,IL).In all cases,regression values(r2)were such that the Langmuir model predicted the data set at p<0.05. The adsorption data obtained were analyzed by a two-way ANOVA followed by Fischer's least signi?-cant difference(LSD)using the SAS system ANOVA procedure at a=0.05(v6.12from SAS Institute Inc, Cary,NC).
2.6Moisture determination
The moisture content of the peanut shells and commercial resins were determined as loss of weight on drying according to AAOC method24.003(a).13 Moisture values varied from1.4to7.2%for acid-modi?ed shells and7.7to59%for commercial resins.
3RESULTS AND DISCUSSION
Wafwoyo et al12clearly demonstrated that little,if any, metal ion adsorption occurred in the absence of phosphoric acid or citric acid modi?cation of peanut shells.Shells exposed to no washing,water washing or base washing alone adsorbed less than0.2mmolgà1 each of the?ve metal ions.For some of the samples,no metal ion adsorption was observed.Wafwoyo et al12 also observed little or no metal ion adsorption when non-modi?ed shells were placed in contact with a mixture of metal ions Cd2 ,Cu2 ,Ni2 and Zn2 . Lead ions were not included.On the basis of these published results,only acid-modi?ed shells were evaluated in the present study.
3.1Adsorption of individual metal ions Phosphoric acid modi?cation of unwashed,water-washed and base-washed shells gave products that showed little difference in their ability to sequester individual metal ions(Table1).Therefore,pre-treatment of shells with water or base may not be necessary for phosphoric acid modi?cation to be effective.However,citic acid modi?cation appears to be most effective with base-washed shells.The reason(s)for this observation is not known.Exposure to base removes more colored material from the shell than water washing.The removal of colored com-pounds and the concomitant changes that occur in the shell apparently enhance citric acid but not phosphoric acid modi?cation.Additionally,treatment with NaOH may saponify and remove surface lipids from the
S Chamarthy,CW Seo,WE Marshall
peanut shells,thus exposing more sites for the inter-action between citric acid and components(cellulose and hemicellulose)of the shell.In the case of phos-phoric acid,exposure to the acid may,to a limited degree,hydrolyze lipid and remove some surface lipid in a manner similar to NaOH.Citric acid,being a weaker acid than phosphoric acid,likely cannot remove lipid on the shell surface.
A comparison between the metal ion uptake ability of modi?ed shells and commercial cation exchange resins showed a number of cases where modi?ed shells met or exceeded adsorption by carboxymethylcellu-lose or Duolite GT-73(Table1).This observation was also made by Wafwoyo et al12when modi?ed shell adsorption was compared with Duolite GT-73only. For Ni2 ,Zn2 and Cu2 uptake,all unwashed and washed,phosphoric acid-treated shells met or ex-ceeded the adsorption of either carboxymethylcellu-lose or Duolite GT-73.Citric acid-modi?ed shells were less consistent in this regard.In all cases except one,the commercial resins adsorbed more cadmium and lead ions than the modi?ed shells.The two Amberlite resins showed signi?cantly higher uptake for all?ve individual metals compared with all of the modi?ed peanut shells.The apparent selectivity of any ion exchange resin for a given metal depends upon concentration,the presence of other species and pH.14±16Therefore,comparisons of the effectiveness of adsorbents should not be extrapolated beyond the systems for which they have been evaluated.
Shells modi?ed with citric or phosphoric acid, regardless of wash type,have a preference for copper and lead ions.
3.2Adsorption of multiple metal ions
The presence of multiple metal ions in solution reduced the adsorption of each metal ion(Table2) compared with only one metal ion in solution(Table 1)for all of the modi?ed peanut shell samples and most of the resins.This result was anticipated because of the competition among the metal ions for limited binding sites on the shells and resins.In general,base-washed,citric acid-modi?ed shells performed slightly better at metal ion adsorption than either unwashed or water-washed shells.For phosphoric acid modi?-cation,wash type had little effect on metal ion adsorption.These results are similar to the results found for acid-modi?ed shells in the presence of single metal ions(Table1).Phosphoric acid-modi?ed shells preferentially adsorbed Pb2 compared with citric acid-modi?ed shells.In the case of Cu2 adsorption, citric acid-modi?ed shells sequestered signi?cantly more metal ions than phosphoric acid-modi?ed shells. When comparing modi?ed shells with commercial resins,all of the phosphoric acid-and citric acid-modi?ed peanut shells adsorbed signi?cantly more Cd2 than Duolite GT-73or Amberlite IRC-718 when cadmium ions were present with other metal ions(Table2).All of the modi?ed shells were either similar to or better than carboxymethylcellulose in Cd2 uptake.For copper ion adsorption,base-washed,citric acid modi?ed shells were similar to Duolite GT-73and carboxymethylcellulose.All modi-?ed peanut shells adsorbed more copper ions than Amberlite200,except unwashed,phosphoric acid-modi?ed shells,which adsorbed a similar amount. Reaction with citric acid has been reported to increase adsorption of copper ions from solution by the lignocellulosic by-products corn?ber13and soybean hulls.15For Pb2 ,all types of phosphoric acid-modi-?ed shells had equivalent or greater adsorption than Amberlite IRC-718.For Ni2 and Zn2 ,none of the modi?ed peanut shells adsorbed as many metal ions as the commercial resins.
3.3Adsorption capacities for modi?ed peanut shells and commercial resins
When adsorption capacities for each of the?ve metals were determined for the modi?ed peanut shells, signi?cant differences were observed between samples that had been water-washed or base-washed, regardless of the type of acid modi?cation(Table3). Apparently,peanut shell adsorption capacities are not dependent upon a particular wash treatment
Table1.Adsorption ef?ciencies a of individual metal ions by modi?ed peanut shells and commercial resins Adsorbent
Metal ion
Cd2 Cu2 Pb2 Ni2 Zn2 Unwashed±citric acid0.31e0.58d0.35h0.27e0.31f Unwashed±phosphoric acid0.50cd0.66c0.77e0.33d0.46d Water-washed±citric acid0.33e0.56d0.37h0.27e0.26g Water-washed±phosphoric acid0.41e0.72c0.80e0.37c0.47d Base-washed±citric acid0.34e0.68c0.48g0.41c0.40e Base-washed±phosphoric acid0.43de0.69c0.66f0.30de0.47d Duolite GT-730.49d0.70c 1.15d0.37c0.54c Amberlite IRC-718 1.80a 2.19a 2.30a0.60b 2.03a Amberlite200 1.27b0.98b 1.84b 1.18a 1.30b Carboxymethylcellulose0.61c0.70c 1.53c0.27e0.34f a Values(mmoles gà1dry wt)in the same column with a common subscript are not signi?cantly different at a=0.05.Values are means of duplicate determinations where the standard error was <5.0%.
Values in boldface represent shell adsorption that meets or exceeds resin adsorption.
Adsorption of toxic metals by modi?ed peanut shells
before modi?cation.They also do not appear to be dependent on a particular acid modi?cation,although base-washed,phosphoric acid-modi?ed shells gener-ally appear to have greater capacity for select metal ions than base-washed,citric acid-modi?ed shells.
In almost all cases,the commercial resins had greater adsorption capacities than the acid-modi?ed peanut shells(Table3).The notable exception was in the adsorption capacity for Pb2 ,where citric acid-modi?ed shells had a greater capacity than Duolite GT-73.
4CONCLUSIONS
As originally shown by Wafwoyo et al12modi?cation of peanut shells with either phosphoric or citric acid increased shell adsorption toward metal ions.An extension of the work by Wafwoyo et al12has shown that phosphoric acid-modi?ed shells possessed gen-erally higher metal ion adsorption than citric acid-modi?ed shells when the two were compared with a single metal ion at a?xed concentration of20m M. In the presence of competing metal ions,metal ion uptake from solution by the modi?ed peanut shells depended strongly on the presence of the other metal ions.In several cases,modi?ed peanut shells had better or equal adsorption compared with commercial resins.Phosphoric acid-modi?ed samples generally had higher adsorption capacities than citric acid-modi?ed https://www.doczj.com/doc/fb6436484.html,mercial resins generally had higher adsorption capacities than the experimental samples.
This study indicates that inexpensive metal ion adsorbents can be developed by acid modi?cation of peanut shells,an agricultural waste.These adsorbents show a preference for copper and lead ions and also show greater adsorption ef?ciency for metal ions than some commercial resins.
ACKNOWLEDGEMENTS
We thank Dr Kanglin Lee(Department of Natural Resources and Environmental Design,North Carolina A&T State University)for his assistance in the statistical treatment of the data.The research is in partial ful?lment of a Masters of Science degree for one of us(SC)and was supported in part by a research grant from the USDA-CSREES Capacity Building Program(Grant#95-38814-1729).
Table2.Adsorption ef?ciencies a of individual metal ions by modi?ed peanut shells and commercial resins in a solution containing all?ve metal ions Adsorbent
Metal ion
Cd2 Cu2 Pb2 Ni2 Zn2 Unwashed±citric acid0.24bc0.28c0.26g0.13d0.13e Unwashed±phosphoric acid0.23bcd0.18de0.53e0.14d0.15de Water-washed±citric acid0.24bc0.28c0.29g0.13d0.18de Water-washed±phosphoric acid0.27b0.22d0.63d0.16dc0.20d Base-washed±citric acid0.22cd0.33b0.38f0.19c0.16de Base-washed±phosphoric acid0.23bcd0.22d0.62d0.16cd0.17de Duolite GT-730.09e0.33b0.97c0.34b0.35bc Amberlite IRC-7180.05e 2.02a0.55e0.41a0.38b Amberlite2000.36a0.15fe 1.33b0.40a0.47a Carboxymethylcellulose0.20cd0.37b 1.46a0.33b0.30c a Values(mmoles gà1dry wt)in the same column with a common subscript are not signi?cantly different at a=0.05.Values are means of duplicate determinations where the standard error was <5.0%.
Values in boldface represent shell adsorption that meets or exceeds resin adsorption.
Table3.Modi?ed peanut shell and commercial resin adsorption capacities a(X
m
)for individual metal ions Adsorbent
Metal ion
Cd2 Cu2 Pb2 Ni2 Zn2 Unwashed±citric acid0.36d0.62c0.84de0.29d0.31d Unwashed±phosphoric acid0.37d0.82c 1.09c0.30d0.32d Water-washed±citric acid0.32d0.69c0.92cd0.25d0.46d Water-washed±phosphoric acid0.50d0.46d0.70ef0.32d0.43d Base-washed±citric acid0.45d0.69c0.47g0.24d0.26d Base-washed±phosphoric acid0.48d0.74c0.52fg0.26d0.53cd Duolite GT-730.94c0.97bc0.59fg0.97c0.85c Amberlite IRC-718 2.26a 2.04a 1.45b 2.20a 2.36a Amberlite200 1.98b 1.42b 1.65a 1.51b 1.30b a Values(mmoles gà1dry wt)in the same column with a common superscript are not signi?cantly different at a=0.05.Values for X m were generated from non-linear least squares analysis of adsorption isotherms developed by means of the Langmuir model.
Values in boldface represent adsorption maxima that meet or exceed resin adsorption maxima.
S Chamarthy,CW Seo,WE Marshall
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Adsorption of toxic metals by modi?ed peanut shells