title: Modeling the effect of pH on biosorption of heavy metals by citrus peels.
abstract: Biosorption by materials such as citrus peels could be a cost effective technique for removing toxic heavy metals from wastewater. Orange peels, lemon peels and lemon-based protonated pectin peels (PPP) had Langmuir sorption capacities of 0.7-1.2 mequiv./g (39-67 mg/g) of Cd per biosorbent dry weight. A potentiometric titration was interpreted using a continuous pK(a) spectrum approach. It revealed four acidic sites with pK(a) values of 3.8, 6.4, 8.4 and 10.7, and a total site quantity of 1.14 mequiv./g. Sorption isotherms of untreated citrus peels showed an unusual shape with two plateau values. Protonated pectin peels on the other hand showed a typical Langmuir behavior with a higher sorption capacity than untreated peels. At lower pH, metal binding was reduced due to increased competition by protons. This was modeled using pH-sensitive isotherm equations. It was not necessary to assume four binding sites; using one site with pK(a) 3.8 and a quantity of 1.14 mequiv./g was sufficient. It was possible to accurately predict metal uptake at one pH using the metal binding constant determined at a different pH. A 1:1 stoichiometry model fit the sorption isotherms shape better than a 1:2 stoichiometry. For constant pH, the 1:1 stoichiometry reduces to the Langmuir model.
authors.affiliation: Department of Civil & Environmental Engineering, University of Alaska Fairbanks, PO Box 755900, Fairbanks, AK 99775, USA. ffsos@uaf.edu
authors.forename: Silke
authors.initials: S
authors.lastname: Schiewer
authors.forename: Santosh B
authors.initials: SB
authors.lastname: Patil
mesh_terms: D000327:Adsorption; D018533:Biomass; D002957:Citrus / Q000737:chemistry*; D005638:Fruit / Q000737:chemistry; D019216:Metals, Heavy / Q000032:analysis* / Q000737:chemistry; D008956:Models, Chemical; D013499:Surface Properties; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry; D018508:Water Purification / Q000379:methods*
chemical_list: D019216:Metals, Heavy; D014874:Water Pollutants, Chemical