MP Y Deposicion Acida

8/18/2019 MP Y Deposicion Acida

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Soil heavy metal contamination and acid deposition:experimental approach on two forest soils in Hunan, Southern

China

Abstract

In 19!, a tailin" dam collapsed in Hunan province #southern China$ leadin" to soil contamination by heavy metals from

the tailin"s waste% &oreover, acid deposition becomes more and more serious in this area% In this context, two forest soils #a

red soil and a yellow red soil, typically and commonly found in southern China$ were collected from Hunan% 'he ob(ectives

are #i$ to determine releases and chan"es in speciation fractions of heavy metals #especially Cd, Cu, and )n$ when the soils

are contaminated with heavy metals and affected by simulated acid deposition, and #ii$ to study effects of soil heavy metals

and acid deposition on releases of soil Ca*+, &"*+, and Al+% 'he soil samples were soa-ed in the solutions of CdCl*, CuCl*,

and )nCl* for 1! days to ma-e contaminated soils containin" *.. m" -"1 of Cd, Cu, and )n% 'hen the contaminated soils and

the ori"inal soils were extracted with five simulated acid deposition solutions #pH ran"ed from !%/ to %. and total dissolvedsalts increased$% 'he experimental results indicate that acid deposition leads to "reat releases of soil heavy metals due to

complicated soil chemical processes, mostly cation exchan"e and partly dissolution of minerals at pH lower than 0%*% 'hese

released heavy metals come mainly from soil exchan"eable pools and other labile fractions% eleases of heavy metals are

closely controlled by pH values or, in some cases, total cation contents in acid deposition2 meanwhile, concentrations of

heavy metals are ne"atively related to the relevant pH values in soil e3uilibrium solutions when pH values are in a ran"e of

0%*4!%1% 5rom the point of view of heavy metal releases, )n is the most sensitive to acid deposition, followed by Cd and Cu%

Compared with the ori"inal soils, the contaminated soils could probably release more base cations Ca *+ and &"*+ and less

Al+% 6reater amounts of Cd, Cu, )n, and Al released from Soil 7 show that this soil is more sensitive to acid deposition, and

we could expect serious environmental contamination in Soil 7 area if minin" activities and acid deposition are not under

control in the future% D *..0 8lsevier 7%% All ri"hts reserved%

eywords: Heavy metal2 Acid deposition2 Complex contamination2 5orest soil2 Southern China

; Correspondin" author% 'el%: + !/1*/.2 fax: + /1*!/.% 8


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and neutral soils #u et al%, 199?$% &artin andaplan #199$ conducted a field study of . monthsand su""ested that metallic #Cu, >b, )n$ andal-aline earth #Ca and &"$ cations reduced Cdadsorption by competin" for available specific

adsorption and cation exchan"e sites, and presenceof li"ands such as Cl and S0

* mi"ht decrease Cdadsorption by formin" poorer


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*%*% 8xperimental conditions

*%*%1% Simulated acid deposition

5ive solutions of simulated acid deposition used inthe followin" experiment were made of Ca# $*,#H0$*S0, &"S0, CaC, *S0, a*S0, andH*S0, and the compositions were "iven in 'able 1%

'able 1

A dilute solution of HCl was used to ad(ust pHvalues of the solutions% 'he composition of Awas similar to that of present


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*%.$, bound to or"anic matter #&


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heavymetals

(mgkg1)

Cd Cu Zn

B.Liaoetal./Geodema1!"(!##$)%1&1#'

'able *

>hysico


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%$

8x


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fraction was residue #?94!O$, and the other fractions accounted for only a small part%

0% esults

0%1% eleases of heavy metals and ma(orcations

0%1%1% eleases of heavy metalsfrom the contaminated soils after1. days

elease of a certain element represented the sumof the amount of this element in ! extracted soile3uilibrium solutions durin" a period of 1. days%5rom A1 to A! #increasin" the acidity and 'JS$,the releases of heavy metals from the contaminatedsoils after 1. days increased "enerally, especially for Soil A #5i"% 1$% Amon" the three elements, )n was themost easily released #1.0419. m" -"1 for Soil A,1!?41?9 m" -"1 for Soil 7$ and showed the hi"hestsensitivity to acid deposition, probably due to hi"her )n contents in the two soils% 'he releases of Cd werein ran"es of !941*? m" -"1 for Soil A and 1.1411!m" -"1 for Soil 7% 'he releases of Cu #14?? m" -" 1

for Soil A, *4. m" -"1

for Soil 7$ were much lower than those of )n and Cd, implyin" that soil Cu wasless sensitive to acid deposition compared with theother two elements% ith increasin" acidity and 'JSof the simulated solutions, increasin" ma"nitudes of the releases of heavy metals from Soil A were muchobvious than from Soil 7, showin" that the releasesof heavy metals from Soil A were stron"ly controlled

by the acidity and 'JS in deposition% 5or Soil 7, thedependence on the acidity was less clear, and evenwea- acid deposition could lead to "reat releases of heavy metals, accordin" with hi"her releases of Cd,

Cu, and )n from Soil 7 than from Soil A in mostcases%

0%1%*% 'ime evolution of heavymetal concentrations in the soile3uilibrium solutions

In order to simulate the leachin" process of aciddeposition in the field, . " contaminated soil samplewas continuously extracted with 1!. ml of freshsimulated acid deposition solutions ! times for 1.days, so that concentrations of heavy metals in thesoil e3uilibrium solutions declined rapidly in "eneralwith increasin" extractin" time #5i"% *$% 'his


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indicated that heavy metals in these two contaminatedsoils could be easily removed by acid deposition fromsoil phase to solution phase% In most cases, the

differences were clearly observed amon" various

treatments of simulated acid deposition, especially for Soil A and for Cu in Soil 7% 'he treatment of A!resulted in hi"her


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oil oil B

#

$#

1##

1$#

!##

*1 *! *' *+ *$ *1 *! *' *+ *$

*eleases o, heavy metals , om

the -ontaminated soils. mg kg

/1

Cd

CuZn

7% Giao et al% @ 6eoderma 1*? #*..!$ 9141. 99

Simulated acid deposition solutions

5i"% 1% 8ffects of simulated acid deposition on releases of heavy metals from the contaminated soils% elease of a certain element was the

sum of this element in ! soil e3uilibrium solutions extracted with different simulated acid deposition solutions durin" a period of 1. days%

A1 to A! were different simulated acid deposition solutions #pH ran"in" from !%/ to %. and total dissolved salts increasin"$%

8xtractin" time, d 8xtractin" time, d

5i"% *% Concentration trends of heavy metals in soil e3uilibrium solutions% 8ach step represented the heavy metal concentration in a soile3uilibrium solution extracted from . " contaminated soil sample by addin" 1!. ml of fresh simulated acid deposition solution% 'he total

extractin" process was conducted ! times durin" a period of 1. days% A1 to A! were different simulated acid deposition solutions # pH

ran"in" from !%/ to %. and total dissolved salts increasin"$%

concentrations of heavy metals than the treatments

of the other four solutions% Comprehensive

analysis of compositions of simulated acid

deposition solutions and concentration trends of

heavy metals in 5i"% * demonstrated that effects of

various simulated solutions on releases of soil

heavy metals were A!NA0QANA*NA1,accordin" with the

potentials of these solutions to acidify soil showed

by the ratio values of MCa*++&"*+N1@*@MH+N in 'able

1% Hi"her concentrations of )n and Cd in soil

e3uilibrium solutions were related to hi"her )n

contents and hi"her Cd exchan"eable fractions in

the two contaminated soils, respectively #see 'able

$, and showed hi"her sensitivity of soil )n andCd to acid deposition than that

'able

Speciation fractions of heavy metals in the contaminated soils treated with various simulated acid deposition solutions #m" -" 1$

Heavy metal 5ractiona Soil A Soil 7

A1 A* A A0 A! A1 A* A A0 A!

Cd 8x< /*%!/ /.%9/ 0%00 0%/. 9%. **% *.% 1.%?* 1*%.0 %1

&n< **%? 10%! 10%*0 1*%?? 11%99 *1%90 *1%? *1%00 *1%0* *1%


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1.. 7% Giao et al% @ 6eoderma 1*? #*..!$ 9141.

!

&< 9%9 /%0* 0%./ %!0 1%19 0*%!* 0*%0/ 0.%9? !%! 0%

/*

A5e< .%* .%01 .%? .% .%*/ .%*0 .%*. .%1 .%1 .%**C5e< .%01 .%1? .%* .%! .%*? .%*/ .%0 .%*! .%*. .%*

es< .%0 .%*9 .%0. .%/ .%! .%/* .%?* .%// .%!0 .%!!

Cu 8x< 19% 19%/! *0%!! *?%0? 1%9. 1.% 11%*/ 9%/ 9%!/ %0/

&n< ?1%? ?.%91 /%?0 /!%* 0*%1* !1%*1 !.%/? 0?%.* 0*%11 %

*?&< /%0! //%0 /%9. !9% 0%1* /9%!? ?.%0. /0% /%1* !0%

?A5e< 1?%/ 1/%!9 1/%0. 10%/ 1/%?? *0%*0 *?%0 *!%?. */%?9 *1%

?C5e< 1%0! .%?* */%1/ *!%/* *9%9 %?* /%* !%9. /%/ *%

1?es< */%9 *!%9 *!%?. */%0 *%!1 1%!1 10%0* 1%!1 1%/9 1%

1!)n 8x< 0%11 01%. *%.* *?%9 %.! 10%99 1%* %01 %0. 0%/9

&n< 1?%1. 1!%1 11%9 ?%09 0%0 9%/9 9%9 1%1. ?%/1 !%.!

&< *0%* *%/ 1?%0* 1%?. /%09 %9? .%. *0%1* *0%?1 *%

/A5e< 0%. %?! %?! 0%1 0%*? /%9* /%09 /%!1 /%?/ /%*?

C5e< *.%?/ *%// **%0* */%.* *!%* 0%00 .%/. *?%!/ *%? .%

90es< 19%/. 1!%. 199%*. 19/%/ 1?% 19%91 19.%*/ 191%/9 *..%9 *.!%

./a

8x


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considerably: Cd from 1!41O to about /.O #1194 1*. m" -"1$, Cu from *%!4%?O to 1*41?O #*?49m" -"1$, and )n from 1%!O to 194*O #9!41.! m"

-"1$% 'his showed that after 1! days of soa-in"cultivation with metal chlorides, most part of addedCd and some part of added )n and Cu becameexchan"eable fractions in the soils, and activity andmobility of heavy metals were enhanced "reatly%However, because absolute exchan"eable Cu wasmuch lower than exchan"eable Cd and )n, the releaseof Cu from the contaminated soils and the Cuconcentrations in the soil e3uilibrium solutions weremuch lower than those of )n and Cd #5i"s% 1 and *$%


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oil

#

!#

+#

0#

#

1##

1!#

1+#

10#

1#

!##

!!#

l.oig l.-ont Ca.oig Ca.-ont 2g.oig 2g

Cations and teatments

Cation eleases , om the o iginal

and -ontaminated soils. mg kg

/1

Cation eleases , om the o ig

and -ontaminated soils. mg kg

/1*1

*!

*'

*+

*$

*1

*!

*'

*+

*$

oil B

#

!#

+#

0#

#

1##

1!#

1+#

10#

7% Giao et al% @ 6eoderma 1*? #*..!$ 9141. 1.9

'reated with various simulated acid deposition

solutions, the ma(or fractions ofCd were

exchan"eable,

bound to man"anese oxides, and bound to or"anicmatter in the both soils #'able $% 5or Cu, the mostimportant fractions were bound to man"anese oxides,

bound to or"anic matter, and bound to crystalline ironoxides, and the other three fractions accounted alsofor a certain amount% 'he most important fraction of )n was in the residue #1!%1419/%/ m" -"1 in Soil Aand 19.%4*.!%1 m" -"1 in Soil 7$, but it was lower than that in the ori"inal soils #**/%1 m" -"1 in Soil A,**9%. m" -"1 in Soil 7$% 'his indicated that mineraldissolution, caused by acid deposition, resulted insome soil )n in the lattices to be transferred to other fractions, i%e% some soil heavy metals could berelocated from non


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3aametes oil oilB

11. 7% Giao et al% @ 6eoderma 1*? #*..!$ 9141.

elements were "enerally decreased, implyin" thatthese three fractions were labile and controlled by theacidity and 'JS of acid deposition% In the non<

extracted contaminated soils, the total contents of Cdand Cu in Soil A were almost the same as in Soil 7,and that of )n was hi"her in Soil 7 than in Soil A% Inmost cases, however, the total contents of Cd, Cu,and )n were hi"her in Soil A after extractin",showin" that heavy metals in Soil 7 were more easilyremoved out by acid deposition%

!% Jiscussion

!%1% 8ffects of acid deposition on releases of soilheavy metals

Jue to cation exchan"e processes, acid depositionleads to releases of heavy metals from soils

'able 0

#>robst et al%, *..$, and transforms heavy metals tothe forms ta-en up more easily by plants #Fie et al%,19912 6ao et al%, 1990$% 'he linear re"ression

analysis between the releases of soil heavy metalsand the parameters of simulated acid depositionsolutions indicated that pH and total contents of cations in the solutions were important factors

influencin" the releases of soil heavy metals #'able0$% It was obvious that the release of Cu in the twocontaminated soils increased very si"nificantly with

increasin" H+ concentrations or total contents of cations in deposition solutions, and those of Cd and)n increased with decreasin" solution pH valuesvery si"nificantly in Soil A and si"nificantly in Soil7% Hi"her total contents of cations in the solutionscould also si"nificantly enhance Cd release in SoilA% In this experiment, however, no special patternswere observed between the releases of soil heavymetals and the ratios of MCa*++&"*+N1@*@MH+N insimulated acid deposition solutions%

!%*% elationships between heavy metalconcentrations and pH values in soile3uilibrium solutions

After various treatments with different simulatedacid deposition solutions, pH values in the soile3uilibrium solutions #%!4!%1* for Soil A and%9/40%/9 for Soil 7$ were around pH values of theori"inal soils because the soils had a certain

bufferin" capacity #5i"% 0$% 5or Soil A, when pHB0%**, the concentrations of Cd # *R.%/$, Cu# *R.%//.$, and )n # *R.%9.*$ very si"nificantlyincreased with decrease in pH values in soile3uilibrium solutions #nR*1, 3.%.1R.%.1$% 5or Soil7, when pHB0%0, the concentrations very si"nifi<

elationships between releases of soil heavy metals and parameters of simulated acid deposition solutions #correlation coefficients *, nR!$a

Cd Cu )n Cd Cu )n

pH .%9/0;; .%! .%9;; .%9.0; .%/0. .%??

?;H+ #Amol l1$ .%?00 .%9;; .%/00 .%0 1%...;; .%*1

*'otal cations #Ae3 l1$ .%01; .%91;; .%?/. .%!0/ .%9!!;; .%1

.MCa*++&"*+N1@*@MH+N .%/09 .%*.9 .%?! .%/9 .%*? .%/9

.a 1

elease #m" -" $ of a certain element was the sum of this element in ! soil e3uilibrium solutions extracted with different simulated acid

deposition solutions durin" an period of 1. days%

; 3.%.!R.%??1%

;; 3.%.1R.%9*.%


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pH in soil e3uilibrium solutions pH in soil e3uilibrium solutions

5i"% 0% elationships between heavy metal concentrations and pH values in soil e3uilibrium solutions% 5or Soil A, when pHB0%** #nR*1 ,

3.%.1R.%.1, 3.%.!R.%1?$, CdR.%/!%?9;pH # *R.%/$, CuR11%/*%*;pH # *R.%//.$, )nR9%?%1!;pH # *R.%9.*$% 5or Soil 7, when

pHB0%0 #nR1/, 3.%.1R.%, 3.%.!R.%*0?$, CdR1%*/%10;pH # *R.%*!!$, CuR1!%/%*9;pH # *R.%??/$, )nR01%?%!;pH # *R.%*$%

cantly increased for Cu # *R.%??/$, andsi"nificantly for Cd # *R.%*!!$ and )n # *R.%*$,with decrease in pH values in soil e3uilibriumsolutions #nR1/, 3.%.1R.%, 3.%.!R.%*0?$% 'hese datademonstrated that heavy metal concentrations werestron"ly controlled by pH values in a ran"e of about0%4 !%1, and that the mobility of soil heavy metalsincreased at relatively lower pH, resultin" in hi"her concentrations and "reater releases of heavy metals%'hese results were similar to those of il-ins et al%#199$ and Fia #199?$% hen pH values in soile3uilibrium solutions declined to about 0%* for SoilA and 0% for Soil 7, however, no special patternwas observed between the concentrations of heavymetals and pH values for both contaminated soils%'his implied the concentrations of heavy metals to

be controlled by other soil processes when pH

values were lower than about 0%* in our caseexperiment% Jissolution of soil minerals andreadsorption of heavy metals by soil surface are

probably -ey processes involved #an" et al%,

*..0$%

!%% elationships between chan"es in soilexchan"eable heavy metals and releases of heavymetals

Some researches indicated that mostly the heavy

metals released were ori"inally in water soluble and@or exchan"eable fractions in the soils #6ao et al%,1990$% However, we should -eep in mind that heavymetals released to soil solutions due to aciddeposition are not only from the exchan"eable poolsin the soils% Jurin" the leachin" processes, somesoil heavy metals would transform from other fractions to exchan"eable fractions, especially when

pH of the solution was low #an" and u, 199!$%Chan"es in exchan"eable heavy metal were thedifferences of this heavy metal in exchan"eable

fractions in the contaminated soils before and after extraction with different simulated acid depositionsolutions% 6enerally, the release of Cd or )nincreased with the lo"arithmic increase of thechan"e of exchan"eable Cd # *R.%?9.2 nR1. ,3.%.1R.%!!$ or )n #

*R.%?!0$, but no si"nificant pattern was observed between the release of Cu andthe chan"e of exchan"eable Cu #5i"% !$% >robably,the first release of soil Cd and )n were very

proportional to the exchan"eable capacity depletion2 but after probably due to re


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11* 7% Giao et al% @ 6eoderma 1*? #*..!$ 9141.

!%0% Competition between heavy metals and ma(or

cations

7ecause acid deposition carries a lar"e amount of acidic anions #S0

*, $, acidic cation #H+$, and

base cations #Ca*+, &"*+, H0+$ to the soils,

complicated soil chemical processes will happen% ecan mention exchan"e of soil cations with externalcations #Giao et al%, 199?$, adsorption of external ionsand desorption of soil ions #Giao et al%, 1990$,chemical weatherin" of primary ma(or and accessoryminerals or secondary minerals #>robst et al%, *...$,transformation of soil cations from one pool toanother #Giao et al%, 199$, and leachin" of soil base

cations #Giao et al%, 199?$% 'herefore, acid depositionwill not only result in releases of soil heavy metals,

but also lead to leachin" of base cations Ca*+ and&"*+ and release@retention of toxic Al+, so that soilacidification and mobiliBation of heavy metals will

probably happen simultaneously% Comparison of theori"inal soils and the contaminated soilsdemonstrated that relatively more Al+ was releasedfrom the ori"inal soils, and more Ca*+ and &"*+ wasreleased from the contaminated soils under effects of the same acid deposition #5i"% $% In the soil systems,there was a competition between soil Al+ and heavy

metals #8scri" and &orell, 199$, but addition of heavy metals enhanced exchan"e with soil basecations, which demonstrated the mobility of soilelements to be base cationsNheavy metalsNAl%

!%!% Comparison of the two tested soils

'o evaluate effects of acid deposition on soilscontaminated with heavy metals, we could comparethe two soils from two aspects: releases of soil

5i"% !% elationships between heavy metal releases from the contaminated soils and chan"es in exchan"eable heavy metals #nR1.,

3.%.1R.%!!$% elease of a certain element was the sum of this element in ! soil e3uilibrium solutions extracted with different simulated

acid deposition solutions durin" a period of 1. days% Chan"es in exchan"eable heavy metal were the differences of this heavy metal in

exchan"eable fractions in the contaminated soils before and after extraction with different simulated acid deposition solutions%


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heavy metals, and releases of soil base cations Ca *+

and &"*+, and toxic Al+% After addition of metalchlorides but before extractin", the total contents of

heavy metals in the two contaminated soils werealmost the same2 after extractin" processes,however, Soil 7 released more heavy metals thanSoil A in most cases #5i"% 1$% Simultaneously, moreCa*+ and &"*+ and less Al+ was released from Soil Athan those from Soil 7 #5i"% $, althou"h these twosoils had the same amount of exchan"eable basecations #exchan"eable base cationsRC8C7S, see'able *$% 'he physico


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7olt, 6%H%, 7ru""enwert, &%6%&%, 19?/% Soil Chemistry: A% 7asic

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MP Y Deposicion Acida