Effects of surface coatings on electrochemical properties and contaminant sorption of clay minerals Jie Zhuang a, * , Gui-Rui Yu b a Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717, USA b Institute of Geographical Science and Natural Resources Research, Chinese Academy of Sciences, P.O. Box 9717, 3 Datun Road, Chaoyang District, Beijing 100101, PR China Received 11 December 2001; received in revised form 13 May 2002; accepted 24 June 2002 Abstract Surface charges play a major role in determining the interactions of contaminants with soils. The most important sourcesofsoilchargesareclaymineralcolloids,whoseelectrochemicalpropertiesareusuallymodifiedbymetal-oxides andorganicmatterinnaturalenvironments.Inthisstudy,effectsofcoatingsoforganicmatterandFe-andAl-oxides on a series of electrochemical properties and heavy metal sorption of three clay minerals (kaolinite, montmorillonite andillite)predominantinnaturalsoilswereinvestigatedusingbatchtechniques.Theresultsindicatethatthecoatings increased the specific surface area of the clay minerals, except for the Al-oxide coated montmorillonite and organic mattercoated2:1clayminerals.Thesesquioxidecoatingsincreasedamountofpositivechargesbutdecreasednegative charges.Thiscausesgreatreductionofthenegativepotentialontheclaysurfaces,shiftofthezeropointofchargetoa higherpH,andpromotionoffluoridesorptionduetopresenceofmoreOH  andOH 2 ontheoxidesurfacesthanonthe claysurfaces.Incontrast,theorganiccoatingsignificantlyincreasedthenegativityofsurfacecharges,andthusthezero pointofchargeandzeta-potentialoftheclaysdroppeddown.Theorganiccoatingalsoinducedareductionoffluoride sorptionontheclays.Withrespecttothesorptionofleadandcadmium,thesesquioxidecoatingsproducedinsignificant effects. The experiments of lead/cadmium competitive sorption show that on both the oxide-coated surface and the original clay surface there exist different types of sites, each of which preferentially binds with a heavy metal. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Clay; Electrochemical properties; Surface coating; Sesquioxide; Organic matter; Contaminant sorption 1. Introduction Surfacereactionsofchargedparticlesareessentialto biogeochemicalcyclesoftraceelementsandthepathway of detoxification of these elements when present in nat- ural environments at hazardous concentrations. Many soil physical and chemical properties are directly or in- directly controlled by nature and amount of surface charge as well as their variation with soil solution characteristics (Greenland and Hayes, 1981; Barrow, 1987, 1996). Sources of surface charge in soils include both inorganic and organic components. Soil colloids, which are smaller than 2 lm in size and account for generallylessthan10%ofsoilinweight,bearmorethan 70–80%ofsoilcharges(Mattson,1931).Acombination of high charge and very small particle size results in a highchargedensityandfurthermoreanactivelyreactive surface of colloids. Thus, any change of soil colloidal propertiescanaffectaseriesofcontaminantreactionsin soil. The main components of soil colloids are layer silicates, oxides of Fe, Al and Mn, and organic materi- als. Larger phyllosilicate mineral grains, i.e., kaolinite montmorilloniteandillite,areusuallycoatedtoagreater Chemosphere 49 (2002) 619–628 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +1-302-831-2611/3365; fax: +1-302-831-0605. E-mail address: jzhuang@udel.edu (J. Zhuang). 0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0045-6535(02)00332-6