Adsorption of zinc and cadmium by soils and synthetic hydrous metal oxides : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University

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Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author.

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A

HYDROUS METAL OXIDES

thes is presented in part ial fulfilment of th e requirements for the degree

DOCTOR OF PHILOSOPHY in Soil Science at Mas sey Unive rs ity

Antony Hugh Col eby Rob erts

' · _• 11981

o f

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i

ABS TRACT

This s tudy invo lved an inves tigation into the adsorp tion of Zn and Cd by soils and synthetic hydrous metal ox ides , and the effect of several factors on th e adsorption reac tions in con trolled laboratory experiments .

Ini tial ly , in order to determine low concentra tions of Zn and Cd in solution , a concentration pro cedure involving so lvent ex traction was developed . The procedure utilised a chela ting agent , di thizone , and an organic so lvent , carbon tetrachloride . Zinc and Cd were back-ex trac ted f rom the organic solvent into hydrochloric acid to achieve a ten- fold increas e in concen tration .

For soils and Fe gel , adsorp tion o f both Zn and Cd was charac terised by an ini tial , rapid removal from solution which was followed by a much s lower , continuing decreas e in solution concentration .

Isoth erms ( 40 hr⁾ for Zn and Cd adsorp tion f rom 3 x 10-� NaCl by soils , synthetic hydrous ferric oxide ⁽Fe gel⁾ and allophane , were des crib ed us ing two Langmuir equat ions . Values for the derived free energi es of adsorption , for each region of adsorp tion , were s imilar for both Zn and Cd on all soils and on Fe gel . Allophane , however , had

higher f ree energies of adsorption than either the soils or Fe gel . For a l l adsorb en ts , Langmuir cons tants derived from adsorp tion da ta indicated tha t the adsorb ing surface contained a relatively small numb er of s ites with a high free energy of adsorp tion and a much larger numb er o f s i tes with a lower adsorp tion energy . Iron gel appeared to provide a satisfactory model for des crib ing Zn and Cd adsorp tion by soils .

Amounts o f Zn and Cd adsorb ed by Fe gel increas ed as pH increased . Adsorp tion o f Zn and Cd by Fe gel in each Langmuir region appeared to b e

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affected s imilarly by changes in pH , al though the pH50 values ( the pH at which 50% o f the ini tial amount added was adsorbed) were higher for Cd than for Zn .

Experi�ental data ob tained in the s tudy are consis tent with a two- mechanism model for b o th Zn and Cd adsorption . I t is proposed tha t the

firs t mechanism involves the forma t ion of a b identate compl ex .

Adsorp tion o f Zn and Cd into this region invo lves the releas e o f two pro tons for each Zn or Cd ion adsorb ed , res ul ting in a bond o f higher energy than when Zn or Cd is adsorbed by the s econd mechanism . In this lat ter cas e , it is proposed that a monodentate complex is formed wi th one proton released for each ca tion adsorb ed .

In addi t ion to proton releas e , data in support o f the two-mechanism system was ob tained in iso topic exchangeab ility and desorption s tudies . For example , the mole ra tios ( i . e . , numb er o f pro tons released for every Zn or Cd ion a dsorb ed) for Zn and Cd were non-integer values . Fo r Zn the mol e ra t ios ranged b etween 1 . 31 and 1 . 6 7 , and for Cd from 0 . 80 to 1 . 1 2 , a t pH 6 . 4 . There was no obvious trend in mole ra tios for Zn either with increasing amounts of Zn adsorb ed or increas ing pH , but for Cd mole ra tios increased with increas ing amounts of Cd adsorbed or increas ing pH . The iso topic exchangeab ility of Zn was s imilar a t all levels of Zn adsorb ed , but decreased with increasing pH ( pH 5 . 85 - 6 . 6 5) from 58% exchangeab ility to 27% , possibly due to an increased propor tion of more-tigh t ly bound Zn . Cadmium , by contras t , had a lower exchange- ability at low levels of Cd adsorb ed ( 55 - 7 6%) than at h igher l evels ( 80 - 85%) but was moreexchangeab l e (55 - 85%) than Zn at equivalent pH values and l ower surface coverages , indica ting that a greater proportion of adsorb ed Cd was less t igh t ly bound compared to Zn . S equential

desorp t ion o f Zn by calcium ions followed by copper ions showed that a

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iii

proportion of Zn ( 1 2 - 24%) was re tained by the surfaces agains t

desorp tion , further ind icat ing the presence of two adsorp tion mechanisms of dif ferent b inding s treng th . Only at low levels of adsorb ed Zn did the amount of desorb ed Zn clos ely approximate the amounts o f Zn cal cula ted lfrom Langrnuir cons tants) to b e in reg ion II . These desorp tion data ,

toge ther with the exchangeab ility data for Zn , point to the possib le l imitations in the use of Langmuir equat ions for quant ifying the amount of Zn adsorb ed by each mechanism.

The Langrnuir iso th erm s tudies , together with pro ton releas e , exchangeab ility and desorp t ion data , indicated two mechanisms o f adsorption for Zn and for Cd . However , the greater exchangeab ility and fewer pro tons released per mol e adsorb ed suggest that more Cd is held by the mechanism involving monodenta t e bonding than is the cas e for Zn .

Zinc or Cd was no t obs erved to be ab sorbed or "occluded" by

synthetic ferric oxide gel or go ethite . There was evidence to sugges t that Zn and Cd migh t dif fuse into cracks or defects in the crys tal s truc ture o f natural goe thi t e , developed by grinding . Al though some fraction of adsorbed ^Znor Cd was non-exchangeab le and non-desorbab le (by copper) for all three adsorb ents , this Zn and Cd was no t in the absorbed phase .

Long t erm ( up to thirty year) addit ions of superphosphate fer t i liser to three soils d id no t produce m easurab le accumulations of Zn or Cd in the soils . The calculated addi t ions agreed well with ac tual increas es measured in to tal Zn , but not with ac tual increas es in to tal Cd .

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ACKNOWLEDGEMENTS

I t is with heartfelt grati tude that I acknowledge the many people who have made the comp let ion of th is study possib le :

Firs t ly and mo st importantly , Dr. Max Turner , for his unfai ling supply o f advice , guidance , de terminat ion and perseverance especially over th e lat ter part o f the s t udy .

P ro fessor Keith Syers , whose ins ight and advi ce was o f t en keenly sough t throughout the s t udy .

Mr . Russell Tillman who cheerfully as sisted in several phases o f the s tudy .

Th e staff and pos t-graduate students , pas t and p res ent , o f the Soil S cience Department all o f whom cont ributed in a variety o f ways .

The University Grants Commi t tee and Mas sey Univers i ty for the necessary funding .

Mrs . D . Syers for typing this thesis .

Las t ly , b ut not leas t , my parents wi thout who se phys i cal and moral support the s tudy would certainly never have been comple t ed .

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CHAPTER TWO

SOIL DES CRIPTIONS , SOIL PROPERTI ES AND GENERAL METHODS 2 . 1

2 . 2

2 . 3

2 . 4

Introduction Soil Descrip tions

2 . 2 . 1 Soil and s i t e information 2 . 2 . 2 Preparation of soils General Methods .

2 . 3 . 1 Adsorption s tudies

2 . 3 . 2 Preparation o f synthe tic hydrous metal oxide gels . . .

2 . 3 . 3 Analysis for Zn and Cd 2 . 3 . 4 Soil properties

Resul ts and Discuss ion

2 . 4 . 1 Comparison of Zn and Cd adsorp tion by soils 2 . 4 . 2 Relationship of Zn and Cd adsorp tion

to soil properties . . . .

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CHAPTER THREE DEVELOPMENT AND EVALUATION OF A METHOD FOR SIMULTANEOUS EXTRACTION AND CONCENTRATION OF Zn AND Cd FROM SOIL EXTRACTS

3 . 1 3 . 2

3 . 3

3 . 4

3 . 5

Introduc tion . . . Solvent Ex traction 3 . 2 . 1 APDC/MIBK 3 . 2 . 2 Dithi zone

3 . 2 . 2 . 1 Dithi zone/chlo ro form

3 . 2 . 2 . 2 Dith i zone/carbon tetrachloride A Method for S imul taneous Zn and Cd Determina tion 3 . 3 . 1 Procedure . . . .

Assessment o f the Performance o f the Procedure 3 . 4 . 1 Effect o f ex t ract ion pH .

3 . 4 . 2 Recovery of Zn and Cd from water

3 . 4 . 3 Recovery of Zn and Cd f rom soil ex tracts 3 . 4 . 4 Variability of the procedure

Results and Discus s ion . ^. . . . . . CHAPTER FOUR

ADS ORPTION OF Zn AND Cd BY SOILS AND SOIL COMPONENTS 4 . l

4 . 2

Introduc t ion . Metho ds . . .

4 . 2 . 1 Kinetic s t ud ies

4 . 2 . 2 Time-dependent and equi librium adsorption isotherms

4 . 2 . 3 40-hr adsorp t ion iso therms 4 . 2 . 4 Native-adsorbed Zn and Cd .

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4 . 3

4 . 2 . 5 Experiments with synthe tic Fe gel and allophane . . . . 4 . 2 . 5 . 1 Adsorption experimen ts 4 . 2 . 5 . 2 Effect of pH on Zn and Cd

adsorp tion by Fe gel Resul ts and Discussion .

4 . 3 . 1 Effect of time o f equi l ib ration on Zn and Cd adsorption . . . ^.

4 . 3 . 2 Evalua tion of Zn and Cd adsorption iso therms 4 . 3 . 2 . 1 Evaluation o f iso therms using

the Langmuir equation

4 . 3 . 2 . 2 Free energies of adsorption

4 . 3 . 3 Comparison of 40 , 1 20 , 1 44 and 1 9 2 hr adsorpt ion iso th erms for Zn and Cd adsorption by Makerua peaty silt loam topsoil and Okaihau gravelly

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1 25 1 34

c lay subsoil . . . 1 36

4 . 3 . 4 40 -hr adsorption isotherms 1 39

4 . 3 . 5

4 . 3 . 4 . 1 k and � G values for Zn and Cd adsorpt ion by soils

4. 3 . 4 . 2 b values for Zn and Cd adsorpt ion by soils Zn and Cd adsorp t ion by synthetic hydrous metal oxides

4 . 3 . 5 . 1 Des crip tion o f Zn and Cd adsorpt ion by Fe gel us ing Langmuir equations 4 . 3 . 5 . 2 40-hr adsorption iso therms

for Fe gel and allophane 4 . 3 . 5 . 3 The effe c t o f pH on Zn and

Cd adsorp t ion by Fe gel 4 . 3 . 6 Summary

CHAPTER FIVE

1 4 2

1 45 . . . 1 4 7 14 7 1 4 9 1 5 7 1 60

MECHANISMS FOR Zn AND Cd ADSORPTION BY SYNTHETIC Fe GEL

5 .¹ Introduction . . . 1 6 3

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5 . 2

5 . 3

5 . 4

Methods . . .

5 . 2 . 1 Effect of pH on Zn and Cd adsorp tion by 5 . 2 . 1 . 1 Zn adsorp tion by Fe gel ,

wi th no pH control

5 . 2 . 1 . 2 Adsorp tion of Zn and Cd by Fe unde r con trolled pH condi t ions 5 . 2 . 2 Proton release .

5 . 2 . 3 Exchangeab i lity o f Zn and Cd ads orb ed by Fe gel

.

5 . 2 . 4 Desorp t ion of Zn adsorbed by Fe ge l Resul ts and Discuss ion

5 . 3 . 1 Effect of pH on Zn and Cd adsorption

5 . 3 . 2

5 . 3 . 1 . 1 Zn adsorp tion by Fe gel under uncont ro l l ed pH condi tions 5 . 3 . 1 . 2 Zn and Cd adsorpt ion by Fe gel

under con trolled pH conditions Pro ton releas e

5 . 3 . 3 Isotopic exchangeab ili ty o f Zn and Cd adsorb ed by Fe gel

5 . 3 . 4 Deso rp t ion o f adsorbed Zn from Fe gel by CaC12 and CuC1 2 solutions General Discussion .

5 . 4 . 1 S ummary of evidence for the existence of two mechanisms for adsorption of Zn and Cd by Fe gel

5 . 4 . 2 Mechanisms proposed for Zn and Cd adso rpt ion by Fe gel . . .

CHAPTER S IX

Fe gel

. gel

TIME-DEPENDENT ADSORPTION OF Zn AND Cd BY HYDROUS FERRI C OXIDES 6 . 1 Intro duct ion . . . . .

6 . 2 Materials . . . ^. . . . .

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6 . 3 6 . 4

Metho ds

Resul ts and Discus sion 6 . 4 . 1

6 . 4 . 2

Adso rp tion of Zn and Cd by hydrous ferric oxides Desorption of Zn and Cd from hydrous ferric oxides

CHAPTER S EVEN THE EFFECT OF PHOSPHATE FERTILISER ADDI TIONS ON Zn AND Cd CONTENTS OF SOILS

7 . 1 7 . 2

7 . 3

Intro duction . . .

Methods for Det ermining Ext ractab l e and Total S o il Zn and Cd Contents

7 . 2 . 1 Ext ractable Zn and Cd

7 . 2 . 2 To t al soil Zn and Cd con t ents Resul ts and Discuss ion

7 . 3 . 1 Meas ured and predi c ted increases in s o i l Zn and Cd from s up e rphosphat e fertiliser addition

SUMMARY . . . BIBLIOGRAPHY APPENDICES

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Figure 1 . 1 1 . 2 1 . 3

1 . 4

1 . 5

1 . 6 1 . 7 1 . 8 1 . 9 1 . 1 0 2 . 1

2 . 2 2 . 3

2 . 4

2 . 5 2 . 6

LIST OF FIGURES

Dis t ribution o f ions and potential at a nega tively charge d interface according to Helho l t z . . . Distrib ution o f ions and po t ential at a negatively charged interface according to the Gouy-Chapman model . Distribut ion o f ions and po t ential at a negatively cha rged int erface according t o the Gouy-Chapman- S tern model . . . . ^. ^. . . ^. . . . . ^. . . . Dis t rib ut ion o f ions and po t ential a t a nega tively charged interface according to the Grahame mo del . ^. Di s tribution o f charge and po t ential at a negatively charged s urface showing charge reversal arising from sup erequivalent adsorption o f cat ions . . . pH t i t rat ion curves for goethi t e in NaCl solut ions of di f ferent ionic s treng ths . . . . .

Solub l e Zn species in solution in equilib rium wi th adsorbed Zn

Zinc speciation in ch lo ride media

Class ification of iso therms for solute adsorp tion Effect o f pH on amount of solute adsorb ed by surfaces o f hydrous metal oxides

Isotherms ( 40hr) fo r the adsorp t ion of Zn hy Okaihau (a) and Tokomaru (b) topsails from 3 x 1 0-2M NaCl . Iso therm ( 40h r) for the adsorption o f Zn by Egmont brown loam topsoil from 3 x 1 0-2M NaCl

Isoth erms ( 40hr) for the adsorp t ion of Zn from

3 x 1 0- 2M NaCl by Okaihau ( a) , Tokomaru (b) , Makerua (c) and Egrnont ( d) sub soils . ^. . . . . Iso therms (40h r) for the adsorp tion o f Cd by Okaihau

(a) and Tokomaru (b) topsails f rom 3 x 1 o-2M NaCl . Isotherm ( 40hr) for the adsorption o f Cd by Egmont brown loam topsoil from 3 x 1 o- 2M NaCl . . . . ^. . Isotherms ( 40hr) for the adsorp t ion o f Cd from 3 x 1 0- 2M NaCl b y Tokomaru ( a) , Okaihau (b) , Egmon t ( c) and Makerua ( d) sub soils.· . . . . .

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40

42

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Figure 4 . 1

4 . 2

4 . 3

4 . 4

4 . 5 4 . 6

Recipro cal time plo t o f Zn in solution during adsorp t ion o f added Zn by Okaihau gravelly clay subsoil . ^. . . . . ^. . . . Effect o f f inal pH on amount o f Zn adsorbed by Fe gel f rom 3 ^x 10- 2M NaCl after 40hr . , at differen t addi tions of Zn . . . ^. Effect o f t ime o f equil ib ration o f low ini tial solut ion concentrations of Zn and Cd for adso rp tion of Zn and Cd by Ymkerua topsoi l and Okaihau subsoil Effect o f t ime o f equilib rat ion o f high final

solution concentrat ions o f Zn and Cd fo r adsorp tion of Zn and Cd by Makerua topsoil and Okaihau subsoil Effect o f t ime o f equilib rat ion on solut ion Zn

concen trat ions for Makerua topso il and Okaihau subsoil . Effect o f t ime o f equilib ration on solution Cd

concent rat ions for Makerua topsoil and Okaihau subsoil . 4 . 7 Recipro cal plo ts o f amoun ts o f Zn adso rb ed by

Makerua peaty s il t loam topsoil agains t equilib rium 4 . 8

4 . 9

4 . 1 0

4 . 1 1

4 . 1 2

4 . 1 3

4 . 1 4

Zn concent rat ions .

Reciprocal plo ts o f amounts o f Zn adso rbed by Okaihau gravelly clay subsoil agains t equilib rium Zn conc entrations . . . ^. ^. . ^. Recipro cal plots o f amounts o f Cd adsorbed by

Makerua p eaty s i l t loam topsoil agains t equilib rium Cd concent rations . . . ^. . ^. . . . . .

Recipro cal plo ts o f amoun ts o f Cd adsorbed by Okaihau gravelly clay s ubsoil agains t equilib rium Cd concent rations . ^. ^. . . . ^. . . ^. . Plo ts o f Zn adsorp tion (40hr) by Okaihau gravelly clay s ub s o il showing actual (c), reso lved (region I

(a) and region I I (b)) and recomb ined (o) iso therms Plots of Cd adsorp t ion (40hr) by Okaihau gravelly c lay subsoil showing a c tual (c), reso lved (region I (a)

and region II (b)) and recomb ined (o) isotherms . Recipro cal plot o f adsorp t ion data (40hr) for Zn

adsorp t ion by Fe gel showing l inear relationship a t low final so lution concentra tions . . . ^.

Recipro cal p lo t o f adsorpt ion data (40hr) for Cd adsorp t ion by Fe gel showing l inear relationship at low final solution concentrations . . . .

xiii Page

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Figure 4 . 1 5

4 . 1 6

4 . 1 7

4 . 1 8

5 . 1

5 . 2

Isotherms for the adsorption o f Cd by Fe gel (a) and allophane (b ) from 3 x 1 0-2M NaCl after 40hr at pH 6 . 40 over the final concentration range of 0-60 �mol Zn 1 -1 . .

Isotherms for the adsorp tion of Cd by Fe gel (a) and allophane (b ) from 3 x 1 0-2M NaCl

after 40hr at pH 6 . 40 over the final concentration range of 0 - 30 �mol Cd 1 -1 . . . ^. . Zinc adsorpt ion iso therms ( 40hr) det ermined at different pHs for the adsorption of Zn by Fe gel from 3 x 10- 2M NaCl over the final solution concentra t ion range of 0-46 �mo l Zn 1 - 1 . . . . Cadmium adsorption iso therms ( 40hr) determined at different pHs for the adsorption o f Cd by Fe gel from 3 x 1 0- 2M NaCl over the final solution concentra tion range of 0-30 �mo l Cd 1 - 1 . . . . Flow diagram showing the metho d o f sequent ial desorption o f Zn by 1 x 10-2M CaC12 and

1 x Io-2M Cuc12 solutions . .

Adsorp t ion ( 40hr) o f Zn by Fe g el from 3 x Io-2M NaCl at three d i fferent pHs . . . ^.

Page

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1 58

1 59

1 7 3 1 76 5 . 3 Effect o f pH ( cont rolled) , on the adsorption o f

Zn and C d by F e gel i n 3 x 1 0- 2M NaCl 1 7 8 5 . 4 Effect o f pH on percent of init ia l Zn o r Cd added ,

adsorb ed by Fe gel in 3 x 10- 2M NaCl . . . 1 79 5 . 5 Base consumed in maintaining a cons tant pH

( pH 6 . 40 ) during Zn (a) and Cd (b) adsorp tion by Fe gel f rom 3 x 1 0-2M NaCl , as a funct ion

of the amount o f Zn or Cd adsorbed . . . 1 83 5 . 6 Deso rp t ion o f adsorbed Zn from Fe gel by 1 x 1 0- 2M

CaC1 2 ( a ) , 1 x 10-2M CuC12 after desorp tion by CaC12 (b) and total amount of Zn deso rb ed by

CaC12 ⁺ CuC12 ( c ) . . . 1 90 6 . 1

6 . 2 6 . 3

6 . 4 6 . 5

X-ray d i ffract ion peaks for a natural goe thite sampl e f rom B iwab i k , Minnesota . . . ^. . . Time-dependent adsorp t ion o f Zn b y Fe gel (a) ' natural goeth i t e (b) and synthetic go ethi te (c) . Time-dependent adsorption o f Cd by Fe gel (a) ' natural go eth i t e ( b ) and synthetic goethi te ( c) . Desorption o f Zn from hydrous ferric oxides . Desorp t ion o f Cd from hydrous ferric oxides .

20 1

. . . . 20 3

204 2 1 2 2 1 3

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LIS T OF TABLES Table

1 . 1 Na tura lly occurring Zn-containing minerals 1 . 2 Zinc contents o f primary rocks . .

1 . 3 Cadmium contents of primary rocks

1 . 4 Range of to tal Zn contents of soils from

1 . 5 1 . 6 1 . 7 1 . 8

2 . 1 2 . 2 2 . 3

2 . 4 3 . 1

var ious countries

' Availab l e ' Zn content o f a range o f s oils To tal Zn and Cd cont ent of sewage s ludges Zinc and Cd con tents o f rock phosphates Zinc and Cd con tents of some superphos phate

fertil is ers . . . .

Site data fo r so ils us ed in the s tudy

Ins trument parame ters for Zn and Cd analysis by atomic ab sorpt ion spectropho tometry . . . Amoun ts of Zn and Cd adsorb ed by soils , and the

ratio of Zn to Cd adso rb ed from 3 x 1 0-2M NaCl af ter 40hr . at two f inal solution concentrations Soil propert ies of possib le importance in

Zn and Cd adsorption . . . .

Effect o f ext raction pH on recovery o f 0 . 0 38 ^llmol Zn and 0 . 0 22 l.Jmol Cd from de-ionised water

3 . 2 Effect o f l evel o f addition of Zn and Cd on recovery from de-ionised wa ter

3 . 3 3 . 4

Recovery o f Zn and Cd added to calcium chloride ext racts of soils . ^. . ^.

Within- and b e tween-day variab ility in amounts of Zn and Cd extract ed from soils by calcium chloride 4 . 1 Initial additions of Zn and Cd used to s tudy 4 . 2

adsorp tion kinetics . .

Effect o f t ime of equilib ration on the adsorpt ion parameters for Zn adsorption by Makerua and

Okaihau s oils . . . ^. . . . ^. . . . ^. . . . .

XV

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Table 4 . 3

4 . 4

4 . 5

4 . 6

4 . 7

4 . 8

4 . 9

4 . 1 0

4 . 1 1

5 . 1

5 . 2 5 . 3

5 . 4

5 . 5

Effect o f t ime of equilib rat ion on the adsorp tion parame ters for Cd ads orp tion by Makerua and Okaihau soils . ^. . . . .

Comparison o f slopes and intercep ts f i t ted by regression analys is and "by inspection" for Zn and Cd adsorp tion by Okaihau tops o il

Langrnuir parameters (b , k) and free energies of adsorpt ion (6G) for Zn adsorp tion by Okaihau, Tokornaru, Egmont and Makerua topsails and subso ils Langmuir parameters (b , k) and free energies of ads orp t ion (6G) for Cd adsorption by Okaihau , Tokomaru, Egmont and Makerua topsails and subsoils Ranges o f adsorpt ion maxima (b) and adsorption energy contents (k values ) for Zn and Cd adsorp tion by so ils , as repo rted in the l i t erature . . . . Comparison o f

order Fe wi th Cd by so ils .

% organic carbon and short-range b val ues for adsorp t ion o f Zn and Adsorp t ion parame t ers for Zn and Cd adsorp tion by Fe gel and allophane gel from 3 x 10-2M NaCl

(pH 6 . 40 , 40hr) . . . .

Comparison of Langmuir parameters and free energies of adsorp t ion for Zn and Cd adsorption by Fe gel and soils . . . .

Parameters describ ing Zn and Cd adsorp tion ( 40hr) by Fe g el from 3 x 1 0- 2M NaCl at two pH values . Effect o f t ime o f equilib ration on the iso topic exchange ab i l i ty o f Zn and Cd adso rb ed by Fe gel pH50 values for Zn and Cd adsorp tion by Fe gel A summary of pub l ished data

effec ts of pH on percen tage and Cd b y iron oxides . . .

relating to the adsorp tion o f Zn Mole ratios o f b ase consumed to Zn and Cd

adsorb ed , as a funct ion of Zn and Cd concentration and pH , for Fe gel in 3 x 1o-2M NaCl ^. ^. ^• ^. ^. Iso topic exchangeab i lity o f Zn and Cd adsorbed on Fe gel as affected by pH and l evel o f addi t ion

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Table Page

5 . 6 Dis tribut ion o f Zn adsorbed by Fe.gel and desorption agains t 1 x 10- 2M CaC1 2 and

1 x 10- 2M CuC1 2 . ^. ^. ^. ^. ^. ^. ^. ^. ^. ^· ^· ^· . . . 1 9 1 5 . 7 Proportion o f more-t igh tly held Zn and Cd ,

expressed as a percentage o f t o tal adsorption as given by desorption , iso topic exchange and

Langmuir adsorp tion data . . . 1 9 4 6 . 1 So lution Zn concentrations and Index o f

Comp let ion o f adsorp tion at various t imes following Zn addition to Fe gel , natural

go ethite and synthetic goethite . . . . . . 206 6 . 2 Solut ion Cd concen trat ions and Index of

Complet ion o f adsorption , at various times fol lowing Cd additions to Fe gel , na tural go ethite and synthetic go ethite .

6 . 3 Amounts and p ercentages o f Zn deso rbed

. . . 20 7 by 1 x 1 o- 2M cac12 and 1 x 10- 2 CuC1 2 from

hydrous ferric ox1des . . . . . . 2 10 6 . 4

7 . 1

7 . 2

Amounts and percentages of Cd desorb ed by 1 x 1 0-2M CaC1 2 and 1 x 1 0-2M CuCl 2 from

hydrous ferric oxides . . . 2 1 1 H 20-extractable , CaC12-extrac t ab l e , CuC1 2-ex tractab l e and total Zn contents o f so ils treated

with long-term superphosphate additions . . . 2 1 8 H 2o-extractab le , CaC1 2-ex tractab l e , CuC1 2-extractab l e

aRd tota+ C d cont ents o f soils t reated wi th long-

term s up erphospha t e additions . . . 2 1 9 7 . 3 Comparison o f measured and predicted increases

in total Zn and Cd contents of soils due to

long-term s uperphosphat e fertiliser addit ions . . . 2 2 1

Adsorption of zinc and cadmium by soils and synthetic hydrous metal oxides : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University

HYDROUS METAL OXIDES

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

CHAPTER TWO

CHAPTER FIVE

CHAPTER S IX

SUMMARY . . . BIBLIOGRAPHY APPENDICES