INTR()DUCTION In the tropieal and subtropieal regions of China large areas of aeid 50115 ean be found(Hseung andLi,1987).Beeause their surfaee ehar罗5 depend on 5011 pH,they are ealled variable eharge 50115(Yu,1997).Differing from eonstant ehar罗50115,these 50115 ean adsorb eations and anions simultaneously(Yu,1997).Variable eharge 50115 adsorb CI一mainly through eleetrostatie attraction(Blaek andV吸飞ring,1979;Jia:id Kong,1992:Wang et al.,1987;Yu,1997),and with adsorption of Cl一some speeifie foreem即also be involved(Wang andyu,1998:Yu,1997:Zhang et al.,1998).从乞ng et al.(1987)found thatin a variable eharge 5011 system,adsorPtion of Cl一deereased with inereasing PH.In addition,the ironoxide eontent of the 5011 and the aeeompa盯ing eations and anions eould have affeeted the adsorPtion(矶陌ng et al.,1987;Xu and Ji,2002).FOr example,with variable eharge 50115,CI一adsorPtion deereasedremarkably with the eo一existenee of 50遗一(Xu and Ji,2002). Low一moleeular一weight(LMW)organie aeids secreted from plant roots and produeed through thedeeomposition of plant residues have oeeurred widely in 50115(Jones,1998;Strobel,2001;Han andJordan,1995:Shen et al.,1997).Although the eoneentration of LMW organie aeids in ordinary 50115m盯be as low as less thanl拼molL一1 to 1 mmolL一1(Strobel,2001),in the rhizosphere it eouldbe as higli as 10一20 mmolL一’(Jones,1998).These organie aeids have been found to perform ma理funetions in 50115 ineluding root nutrient ae妙isition,mineral weathering and metal detoxifieation(Jonesand Brassington,1998).Besides their direet effeet on plant growth,anions of these organic aCids werealso adsorbed by 50115,partieularly variable eharge 50115,and oxides(Hue,1991;Jones and Brassington,1998;Parfitt et al.,1977).FOr variable eharge 50115,this adsorption resulted in a ehange ofsurfaeeeharge(Xu et al.,2003)‘This ehange in 5011 surfaee eharge should affeet the adsorption of Cl一Howe*r,up tothe present little information 15 avallable regarding the effeet of organie aeids on adsorption of CI一byvariable eharge 50115.Beeause ehloride 15 a neeessary nutrient element for Plants,a study of this nature。hould be of signifieanee in Praetiee.Therefore,the objeetive of this investigation was to evaluate theeffeet of two LMW organie aeids,eitrie aeid and oxalie aeid,on CI一adsorPtion by three variable charge50115.MAI,ERIALS AND METHODS Three variable eharge 50115 from South China,a lateritie red 5011 from Guangzhou(2309‘N,113018‘E),a latosol from Xuwen,Guangdong Provinee(20020‘N,110010‘E)and a red 5011 from Jinxian,Jiangxi Provinee(28023‘N,116017‘E)were used.5011 samples for eaeh 5011 type were air一dried andground to pass a 60一mesh sieve.Then they were eleetro一dialyzed tillH/A卜saturated,dried at 60 OCwith infrared light,ground and Passed through the 60一mesh sieve again.Seleeted proPerties of the 50115are given in Table 1.Two LMW organie aeids,eitrie aeid and oxalie aeid,were seleeted for the studybeeause they oeeur naturally in 50115.The organie aeids were ehemieal Pure reagents·压、、BLEISeleeted properties of the 50115 used5011Parent material AI敲mmol kg一110.30 0 .60 231CECcmol kg一18 .917 .156 .050 .M.*20全,Dom;nant el二minera,s·,一9 kg一1_ O Gv) HGoIG(l,K KKRed 5011 Quaternary red el即4.74Latosol Basalt 5.44Lateritie red 5011 Granites滩04 .8704 .2 41,1156.4565a)Exchan罗able AI.b)Dithionite一eitrate一biearbonate(DCB)mcth od.c)K,硫olinite;G,gibbsite;H,hematite;v,vermieulite;Go,goethite;I,hydrousmiea. 5011 samPles 2.50 9 eaeh were weighed into eentrifuge bottles with 2 rePlieates,and 25 mL of a mixedsolution eontaining KCI with or without organie aeid was added to eaeh bottle.The amount of organieaeid added was based on data rePorted in the literature(Jones,1998:Strobel,2001).The suspensionwas shaken in a eonstant一temperature water bath at 28 OC(士1 OC)for 2 h.After standing for 22h,the solution was eentrifuged at 3 000 r min一1 for 5 minutes to separate the solid phase.The CI一in thesolution was deteroined by the ehloride eleetrode method(Yu and Ji,1993).Then Cl一adsorption wasea
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