Release of Trace Elements From Wood Ash by Nitric Acid

8/6/2019 Release of Trace Elements From Wood Ash by Nitric Acid

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R E L E A S E O F T R A CE E L E M E N T S F R O M W O O D A S H B Y N I T ~ C

A C I D

GUOGEN ZHAN , M. SUSAN ERICH and TSUTOMU OHNODept. of Ap plied Eco logy and Environmental Sciences, Univ. of Maine, Orono, M E 04469.

Contribution f ro m the Ma ine Agric. For. Exp. Stn., No. 1852

(Received 23 February, 1994; accepted 8 October, 1994)

Abst ract . Currently wood ash is being used as a soil amendment. Its use is regulated based on traceelement content. However, no published information exists on solubilities of trace elements in woodash. We investigated the release of environmentally-s ignificant race elements (Cd, Cr, Cu, Pb and

Zn) from wood ash as a funct ion ofpH and of particle size. Wood ash was sampled from three sourcesin Maine and sieved into


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298 GUOGEN ZHAN ET AL.

In the Northeastern U.S., approximately 15% of industrially-generated wo odash is landfilled, 80% is applied on land, and 5% is disposed of in some other way(Cam pbell, 1990). Currently, wo od ash is being used an agricultural land amen d-ment in Maine, New Hampshire, Vermont, New York, Michigan, and Oregon.The Maine Department of Environmental Protect ion is regulat ing trace elementinputs to soi ls from wood ash with s tandards that were developed from studieson the usage of sewage sludge (Maine Dep artment of Environme ntal Protect ion,1989). H owev er, the high ly organic sludge matrix is very different from the largelyinorganic ash m atrix. In general, trace elem ent availability to terrestrial plants an daquatic life is strongly influenced by w hethe r the dissolved trace el em ent exists inorganic or inorganic comp lexes. Because o f this difference, t race elements ad dedin ash may have very different environmental consequ ences than those added insewage sludge. No published information exists on solubilit ies of trace elementsin woo d ash.

Th e solubilit ies, distributions, and po tential solubility-controlling solid pha sesof t race elements contained in ash from coal-fueled pow er plants have been studied(Ainsworth and Rai, 1987; Earyet a l . , 1990). Research on co al flyash has s how nthat so me trace elem ents are not dis tr ibuted uniformly through out ashes but areenriched in the smaller particle sizes and on the particle surfaces (Natusche tal . , 1974). Enrichment occurs because the elements that are volatil ized during

com bustio n conden se onto sol id particles at different rates and in varying am ountsas comb ustion gases cool (Natuschet a l . , 1974; Smith, 1980). The most volatileelements, which are the last to condense, are preferentially partit ioned onto thesma ller particles. A variety of particle fractionation and depth profiling studies hassho wn that As, Cd, Se, Sb, Mo, and Zn are preferentially conc entrated on particlesurfaces (Davidson et a l . , 1974, Linton et a l . , 1977; Haynes et a l . , 1982). Otherelem ents su ch as Co, Cr, Cu, Ni, U, and V are mo destly enriched on particle surfaces(Klein and Andrew , 1975; Hansen an d Fisher, 1980). H ulettet a l . (1980) reportedthat the least volatile elements such as A1, Ca, Fe, Mg, Mn , an d Ti occurred almo stexclus ively within the matrix of glass and m ullite crystals of fly ash particles. N osimilar s tudies have been condu cted for woo d ash.

W ood ash typical ly consists of mineral com ponen ts (nutr ients and non-nutr ients)from the t rees, un burned carbon, and soil which entered the fuel s t ream with thewood . The amo unt of ash generated is variable dep ending on the species bume d,the propo rt ion of bark to w ood burned, the burning technology, and the degree o fsoil contaminat ion. For most t ree species the ash c ontent of woo d is probably


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RELEASE OF TRACE ELEMENTS FROM WOOD ASH BY NITRIC ACID 2 99

r e p r e se n t th e c o m m o n t y p e s o f i n d u s tr i al l y -p r o d u c e d w o o d a s h . T h e s a m p l e s e tinc ludes two w oo d ashes f rom d i ff e ren t wood-b urn ing u ti li ti e s a s we l l a s an a shf r o m a p a p e r c o m p a n y w h i c h b u r n e d b o th w o o d a n d p a p e r m i l l sl u d g e. O f t h e t w oashes f rom wo od-b urn ing u t il it ie s , one con ta ins a h ighe r l eve l o f s i l ica te m ine ra l sthan the o the r. The d i ff e rences in chem ica l comp os i t ion o f the th ree sam ples r e fl ec td i ff e rences in fue l type and l eve l o f con tamina t ion by so il . A secondary ob jec t iveof the s tudy was to inves t iga te wh e the r d i ff erences in chem ica l com pos i t ion o f theashes co u ld exp la in any obse rved d i ff e rences in r e l ease o f t race e l emen t s .

Th ese ob jec t ives were m e t by r eac t ing the a sh sam ples wi th n i tr ic ac id -con ta in ingso lu t ions . N i t ra t e w as chosen as the ac id an ion fo r these s tud ies because i t gene ra l lyfo rm s re la t ive ly weak co m plexes wi th a lum inum as we l l as wi th the t r ace e l em en t s

u n d e r s t u dy. A n i o n s w h i c h c o m p l e x m o r e s t ro n g l y w i th a l u m i n u m t e n d t o i n c re a s ethe r a te o f d i s so lu t ion o f a lum inum -con ta in in g mine ra l s (B loom and Er i ch , 1987 ;S t u m m e t a l . , 1983, 1985) due to an ion a t t ack o f the su r faces . Reac t ion o f the a shwi th n i t r ic ac id was m ean t to s imula te r eac tion o f a sh w i th so i l ac id i ty ove r theshor t t e rm, in pa r t icu la r r el ease o f t race e l em en t s f rom the non-a lum ino-s i l i ca tef r ac t io n o f t h e a s h. T h i s s i m p l e m o d e l s y s t e m d o e s n o t r e p r o d u c e th e c o m p l e x i t yof so i l and so i l so lu tions which con ta in a va r i e ty o f o rgan ic and ino rgan ic ionscapab le o f in f luenc ing d i s so lu t ion r eact ions . Ho w ever i t does p rov ide bas i c da taon h ow ac id i ty in f luences the r e lease o f t r ace e l em en t s f rom w oo d ash . To our

kn ow ledg e the re a re no o the r pub l i shed s tud ies which r epor t t h is in fo rmat ion .

M a t e ri a ls a n d M e t h o d s

W o o d a s h w a s s a m p l e d f ro m o u t d o o r st o ra g e p i le s a t t h re e w o o d b u m i n g p o w e rp lan t s in M aine . Th ese samp les rep resen t the ma te r i a l 's chem ica l comp os i t ion jus tp r i o r to a s h l a n d s p re a d i n g . O n e s a m p l e w a s f r o m a p a p e r c o m p a n y b u m i n g a

m ix tu re o f wo od and paperm i l l s ludge (A , Jay, ME) . Th e o the r samples were f romw oo d-b urn ing u ti l it ies (B, Enf ie ld , M E; C , Fo r t Fa i r fie ld , M E) . A l l ashes we re am ix tu re o f f ly a sh and b o t tom ash . P la st ic sam ple r s and con ta ine rs were used inco l l ec t ion and s to rage o f wo od ash to avo id t r ace e l em en t con tamina t ion . Sam pleswere dr ied in the labora tory a t 105 C in a forced a i r draf t oven. Samples werethen f r ac tiona ted by s i ze in to


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300 GUOGENZHANETAL.

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t o b e a p p l i e d t o l a n d i n M a i n e ( M a i n e D e p a r t m e n t o f E n v i r o n m e n t a l P r o t e ct i o n ,1989). T he r e s idue r emain ing a f te r d iges t ion was d r i ed and w e ighed . The sam pleso f w oo d ash w ere a l so ana lyzed af t er d iges t ion by H F (L im and Jackson , 1982).Th i s m e th od q uan t i t a tive ly d i s so lves a lumino-s i l ica t e s, r e su lt ing in n o r e s idue a f t e rd iges t ion .

S o l u b l e h e x a v a l e n t c h r o m i u m w a s d e t e r m i n e d u s i n g d i p h e n y l c a r b az i d e r e a g e n t(Reisenau er, 1982) af te r ex t rac t ing wi th de ioniz ed water (1 :10 , ash:water ) andf i lt e ring the ex t ract s th roug h 0 .45 # m f il te rs . Ca lc ium ca rbona te equ iva lence w as

de te rm ined ti t rime t r ica l ly (Er i ch and O hno , 1992) . Th i s m e tho d invo lves bo i l ing1 g o f a sh and 50 m L of 0 .5 M HC1 fo r 5 minu tes Excess ac id i ty was d e te rm inedby t i t r at ing wi th 0 .25 M NaO H to a pH 7 endpo in t .

Samples f rom each o f the th ree s i ze f r ac t ions and sources were we ighed (1g) and p laced in 60 m L p las t ic bo tt le s A 50 m L a l iquo t o f s t andard ized HNO 3(0 .01-0 .25 M ) w as added . There w ere 14 to 16 d i ffe ren t concen t ra t ions o f HNO3used for the A wood ash , 9 to 12 for the B ash , and 12 to 13 for the C ash . Thed i ff e ren t am oun t s o f ac id w ere added to ach ieve a r ange o f f inal pH va lues (Greenand M anahan , 1978) . Each o f the tr ea tmen t combina t ions was r ep l i ca ted twice . The

samp les were equ i l ib ra ted fo r a week w i th 1 h r shak ing each day on a wr i s t ac tionshaker. The f ina l pH o f each o f the su spens ions was de te rm ined in equ i l ib r iumwith a i r us ing a g lass e lec t rode . The equi l ibra ted samples were f i l te red throughm em bran e fi lt er s (M i l lipo re HA, 0 .45 #m ) .

Th e concen t ra t ions o f Cr, Cu , Zn in so lu t ion were de te rm ined us ing induc t ive lyc o u p l e d p l a s m a a t o m i c e m i s s i o n s p ec tr o sc o p y T h e c o n c e n t r at i o n s o f C d a n d P b


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RELEASE OF TRACE ELEMENTS FROM WOOD ASH BY NITRIC ACID

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w e r e d e t e r m i n e d b y f l a m e a n d g r a p h i te f u r n a c e a t o m i c a b s o r p ti o n s p e c t ro s c o p y,r e s p e c ti v e ly, T h e x - i n te r c e p ts f o r e a c h o f t h r e e g r a p h s ( e a c h r e p r e s e n t in g a d i f f e r e n tw o o d a s h ) i n F i g u r e 1 a n d in F i g u r e 2 w e r e d e t e r m i n e d b y p o o l in g t h e d a t a f r o m t h eth ree s i ze f r ac t ions and ca l cu l a t i ng t he l inea r r eg re s s ion o f Y on X u s ing pH va lues


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b e t w e e n 3 .8 a n d 7 .0 ( c o n s i d e r ed t o b e th e e n v i r o n m e n t a l l y - r e l e v a n t r a n g e f o r s o ilp H v a l u e s ) . T h e 9 5 % c o n f i d e n c e l im i t s fo r t he x - i n te r c e p ts w e r e d e t e r m i n e d a sd e s c r i b e d b y S o k a l a n d R o h l f (1 9 8 1 ).


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R e s u l t s a n d D i s c u s s i o n

P H Y S I C A L A N D C H E M I C A L C H A R A C T E R I Z AT IO N O F W O O D A S H

The sma l l e s t pa r t i c l e s i ze f r ac t ion (2 .0 mm mate r i a l was exc luded . Am ajo r ob jec t ive o f t he s tudy was to com pare de f ined s ize f r ac t ions bo th wi th ina n d b e t w e e n w o o d a s h es . T h e s m a l l e r si ze f r a ct io n s w e r e j u d g e d l i k e l y t o b e t h e

m os t r eac t ive , and the re fo re , t he s tudy concen t r a t ed on these . The >2 .0 m m s izef r a c ti o n s p r o b a b l y c o n t a i n e d u n b u m e d w o o d c h u n k s , s m a l l st o n es , a n d a g g r e g a t e so f c o m b u s t e d m a t e ri al .

T h e c o m p o s i t i o n o f th e t h r e e w o o d a s h e s v a r i ed w i d e l y ( Ta b le I I) . T h e d o m -i n a n t e l e m e n t s a r e C a ( 5 . 8 - 2 1 % ) , K ( 0 . 8 - 5 .7 % ) , A 1 ( 0 .8 - 4 .9 % ) , a n d M g ( 0 . 5 - 3 % ) .B e c a u s e t h e A a s h w a s g e n e r a t e d f r o m t h e b u m i n g o f w o o d a n d w a s t e p a p er m i lls ludge toge the r, i t had a r e l a t i ve ly h igh l eve l o f A1 . The e l eva t ed A1 l eve l s i npape rm i l l s ludge a re l ike ly to be due to bo th a lumino- s i l i ca t e c l ays and a lu m in iums u l fa t e w h i c h w e r e a d d e d d u r i n g p a p e r m a n u f a c t u r i n g (E r i c h a n d O h n o , 1 9 92 ).

Th e A ash had the lowes t l eve ls o f Ca an d M g (Tab le II ) , cons i s t en t w i th it s l owc a l c i u m c a r b o n a t e e q u i v a l e n t v a l u e ( Tab le I) . M o s t o f t h e C a a n d M g i n w o o d a s hsam ples i s p roba b ly p re sen t a s ca rbona te s (E r i ch and Ohn o , 1992) .

Tr a c e e l e m e n t c o n c e n t r a t io n s a l so d i s p l a y e d h i g h v a r ia b i li ty a m o n g t h e t h r e ew oo d a shes , bu t w ere wi th in the r ange o f va lues i n o the r pub l i shed r epor t s (Cam pe l l ,1990) . In t hese w oo d a shes Cd , Cu , Cr, and Zn t en ded to inc rease in con cen t r a t ion


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3 0 4 G U O G E N Z H A N E TA L .

TA B L E I

W eigh t (g kg -~) and ca lc ium carbona te equ iva lence a va lues (%, in

paren thes is ) o f d i ffe ren t f rac t ions o f wood ash

Wo o d A s h < 0 .5 m m 0 . 5 - 1 . 0 m m 1 . 0 -2 . 0 m m > 2 .0 m m

A 503 (19.2 ) 100(14.7) 161(13.1) 236B 765 (60.6 ) 143(47.0) 31(52.5) 61C 654 (58.4 ) 111(40.1) 130(37.8) 105

aNeut ra l iz ing ab i l i ty o f mate r ia l compa red to neu t ra l i z ing ab i l i ty o fan equa l w e igh t o f ca lc ium carbona te ; me thodo logy o f de te rmina t iong iven in M ate r ia l s and M ethods sec t ion .

TA B L E I I

E lem enta l com pos i t ion o f wo od ash f rac tions (HNOa/H202 d iges tion) . S i l i ca va lues p resen ted a rethe resu l t o f an HF d iges t ion . Cr (VI) va lues a re the resu l t o f an ex t rac t ion wi th wa te r and subsequen tana lys i s wi th d ipheny lca rbaz ide

A a l A 2 A 3 B 1 B 2 B 3 C 1 C 2 C 3

A1, g kg -1 49 33 32 8.3 10 13 12 11 8.2

Ca, g kg -1 80 65 58 210 140 160 200 140 120M g, g kg - l 7 .7 5 .2 4 .9 30 17 24 17 13 11K, g kg -1 12 8.1 8.0 22 28 27 51 55 57Na, g kg - l 3 .8 2 .7 2 .7 2 .2 2 .3 2.1 2 .6 2 .8 2 .1Fe , g k g - l 14 9.4 10 12 11 15 13 15 10M n , g k g - 1 2 . 7 2.0 1.8 2.9 2.6 2.8 3.1 3.1 3.2P, g kg -1 3 .2 2 .3 2 .0 4 .7 4 .2 4 .6 8 .4 6 .5 5 .6bSi(H F),g k g - l 330 260 270 280 340 330 140 140 97C d, m g kg -1 3.3 2.0 1.9 6.2 3.8 5.8 12 8.4 7.9Cr, mg k g - 1 45 27 25 27 74 92 34 29 24Cu, m g kg - l 75 53 50 33 38 34 75 61 57Zn, m g kg -1 290 160 130 380 430 420 1400 930 930Residu e, g k g -~ 450 510 450 420 560 620 260 190 160Cr( VI ), m g kg -1 0.15 0.12 0.03 2.7 1.4 2.0 0.77 0.92 0.89

a 1 , 2 , and 3 are


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R E L E A S E O F T R A C E E L E M E N T S F R O M W O O D A S H B Y N I T R IC A C I D 3 0 5

TABLE IIIEstimate and con fidence limits (95% ) of pH values where elemental release

begins. Calculated from Figures 1 and 2Zn Cd

Woodash aLl X-intercept L2 L1 X-intercept L2

A 5.9 0 6.9 4 8.11 6.1 6 7.71 9.65B 6.13 6.65 7.21 6.4 6 6.8 6 7.29C 6.2 9 6.7 6 7.26 6.03 6.6 7 7.35

a L l represents the pH value of the low er 95% con fidence limit and L2 repre-sents the pH value o f the higher 95% confidence limit.

o f so i l con t am ina t i on , t he s i li con con t en t o f e ach o f t he a shes wa s de t e rm ineda f t e r HF d ig es t i on o f t he s am p le s (Tab l e II ). The C a sh appea r s t o be l ow es t i ns o i l c o n t a m i n a t i o n b a s e d o n b o t h S i v a l u e s a n d t h e w e i g h t o f t h e re s i d u e a f t e rd iges t i on . T he B a sh i s r e l a t i ve ly h igh i n S i and l o w in A1 . The po w er p l an t f romwh ich t he B a sh w as ob t a ined i s l oca t ed in an a r ea o f sandy, qua r tz - r ich so i ls . Theso i l con t am inan t s a s soc i a t ed w i th t h is a sh may b e la rge ly SIO2. The com pos i t i o nso f t h e A , B , a n d C a s h e s a re s im i l a r to t h o s e o f w o o d a s h e s p r e v i o u s l y c o l l e c t e df r o m o t h e r g e n e r a to r s i n M a i n e a n d N e w H a m p s h i r e ( O h n o a n d E r ic h , 1 9 9 0 ).

R E L E A S E O F T R A C E E L E M E N T S B Y W O O D A S H

T h e d i s so l u ti o n o f C d f r o m w o o d a s h w a s h ig h l y d e p e n d e n t o n th e a m o u n t o fadd ed ac id and hence on t he f ina l pH o f t he so lu t i on (F igu re 1 ). The re w e re nod i f f e r ences a m ong the t h r ee s i ze f rac t ions . A t pH va lues h ighe r t han 6 .5 , li tt le C dw a s d i s s o l v e d . T h e r e w a s a s h a r p i n c r e a se i n th e p e r c e n t ag e o f d i s s o l v e d C d a t p H s

b e l o w 6 .0 . T h e n e a r l y c o m p l e t e r el e a se o f C d a s p H d e c r e a s e d s u g g e s t s t h at C dw as a s so c i a t ed p r e f e r en ti a l ly w i th acces s ib l e , so lub l e , su r f ace phas e s o f wo od a shpar t ic les .

T h e d i s s o l u ti o n o f Z n w a s a l s o d e p e n d e n t o n p H ( F i g u re 2 ) . A t f i na l s o l u ti o n p Hv a l u e s b e l o w 6 .5 , m o r e Z n d i s s o l v e d . T h e A w o o d a s h te n d e d t o r el e a s e a s m a l le rp e r c e n t a g e o f Z n a t e a c h p H v a l u e b e l o w a b o u t 5 . 0 t h an t h e o t h er tw o a s h e s.A l though Zn was concen t r a t ed i n t he sma l l e r s i z e f r ac t i ons , a l l t h r ee f r ac t i onsexh ib i t ed s imi l a r d i s so lu t i on pa t te rn s .

A n e n v i r o n m e n t a l l y - u s e f u l p a r a m e t e r i s t h e p H b e l o w w h i c h t r a c e e l e m e n t s

b e g i n d i s s o l v i n g f r o m t h e a s h a n d a b o v e w h i c h s o l u b il i ty i s g e n e ra l ly b e l o w d e t e c -t i on l im i t s . Th i s pa r ame te r, wh ich i s e s t ima ted f rom the X- in t e r cep t s i n t he g r aphsin F igu re s 1 and 2 , i s s im i l a r f o r bo th Z n and Cd r e l ea se fo r a ll t h r ee a shes (Tab l eI II ). Th e 9 5% con f idence l im it s on t he pH va lues sug ges t tha t a ll t h r ee a shes beg inr e l e a s in g b o t h Z n a n d C d i n m e a s u r a b l e a m o u n t s a s a m b i e n t p H f a ll s b e l o w t h erange o f 6 .5 t o 7 .0 .


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R E L E A S E O F T R A C E E L E M E N T S F R O M W O O D A S H B Y N I TR I C A C ID

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307

about 30% of total Cr was released from the B ash. As pH decreased from 10 to3, the percentage of total Cr released from the B ash increased from about 30 to50%. The behavior of the C ash in releasing Cr was intermediate to the other twoashes.

Chromium commonly exists in either of two oxidation states under the pH andEh condit ions possible in soils. Hexavalent chromium is both more toxic to humansand more soluble than trivalent chromium (James, 1994). The ash fractions werescreened for Cr(VI) using extraction by water. The B ash was relatively high in

Cr(VI), while the C ash had moderate amounts of Cr(VI). The A ash contained onlyvery small amounts of Cr(VI) (Table II). No attempt was made to partition totalCr values into Cr(VI) or Cr(III) for any of the ashes. The acid digestion necessaryto determine total Cr would be likely to influence Cr oxidation and reductionreactions. However the B ash, which had the highests levels of Cr(VI) in the waterextract, also showed the highest solubility of Cr, and the A ash was very low bothin water-soluble Cr(VI) and in percentage of total Cr solublized throughout thepH range investigated. Thus Cr solubility in the ashes seems strong influenced byCr(VI) content. This study did not address what factors would control Cr oxidation

and reduction in the wood ash samples. Such factors as 02, pH, readily-oxidizableorganic matter, manganese oxides, Fe 2+, and S 2- may influence the oxidation o fCr(lll) or reduction of Cr(VI) (James and Bartlett, 1983).

Solubility of Cu was extremely low at pH values higher than 4 for all theashes (Figure 4). At pH values below 4 Cu was slightly soluble from the A andC ashes and relatively soluble from the B ash. Lead in all three wood ashes was


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308 GUOGENZHANETAL.

1 .2

1 .0@

0 . 8

o 0 .6i t - -

o

co 0 .4- -

O A1 A 2

A 3

u _ 0 . 2

0 . 02 4 6 8 10 12

pH

Fig. 4a. Re leaseof Cu from wood ash.

1o2

1 .0"(7@cO

-~ 0 ,8@r Y

06

t-O 0 . 4

2

u_ 0,2

O

O

o B1 B 2

B 3

V

O0,0 , b i , ~ . ~ 7 . ~ , 0

2 4 6 8 1 0 1 2p H

Fig. 4b. Re leaseof Cu from wood ash.

h igh ly i n so lub l e , w i th con cen t r a t i ons b e low the de t ec t i on l im i t ( 0 . 007 ng m L -1 ,c o r r e s p o n d i n g t o < 0 . 0 1 % o f t h e t o ta l P b ) f r o m p H 1 0 t o b e l o w p H 4 ( d a t a n o ts h o w n ) . U n l i k e C d , Z n , a n d C r, th e m a j o r i ty o f C u a n d P b i n t h e w o o d a s h e ss t ud i e d wa s l i ke ly a s soc i a t ed w i th t he i n t ema l g l a s sy ma t r i x o f t he a sh pa r ti c l e s o r


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R E L E A S E O F T R A C E E L E M E N T S F R O M W O O D A S H B Y N I TR IC A C I D

1o2 . . . .

309

I D

:.2e

N . -o

t -

O, _

O

2I ,

1 .0

0 .8

0 .6

0 .4

0 .2

o C1 C 2v C 3

0 . 0 ~ - "2 4 6 8 10

pH

F i g . 4 c . R e l e a s e o f C u f r o m w o o d a s h .

wi th a lum ino-s i l ica t e con tam inan t s and hence was n o t eas ily d i s so lved . It ma y betha t Cd , Zn , an d Cr a re m ore r ead i ly t aken up by t rees than Cu an d Pb . I f t he Cd ,Zn , and C r we re s ign i fi can t ly p resen t in the woo d , t hey wo u ld be a s soc ia t ed wi thac tua l a sh f r ac t ion ( ca rbona te s and o x ides ) a ft e r com bus t ion and s to rage . On theo the r han d i f Cu and Pb were no t taken up by t r ees appreciab ly, t hey cou ld on lye n t e r t h e s a m p l e s t h r o u g h c o n t a m i n a t i o n o f a s h b y s oi l.

S t u d i e s w i th c o a l a s h h a v e s h o w n t h at C d a n d Z n a re p r e s e n t a l m o s t e x c lu s i v e lyin the ou te r l aye r o f coa l f ly a sh pa r t ic l e s and a re r e l eased wi thou t t he necess i tyo f d i s so lv ing the r e l a t ive ly ine r t a luminos i l i ca te co re o f t he pa r t ic l e s (Han sen and

Fisher, 1980). In coa l ashes , Pb and C u typica l ly exhib i t low solubi l i ty ov er a widepH range (H ansen an d F ishe r, 1980; The i s and W irth , 1977 ; Ph un ge t a l . , 1979).Th ese e l e m en t s a re l i ke ly a s soc ia ted wi th the l e s s so lub le in t e rna l g l a s sy ma t r ix o fash pa r t i c l e s o r w i th con tam inan t so i l pa r ti c le s .

C o n c l u s i o n s

T h e r e le a s e o f tr a ce e l e m e n t s f r o m t h e w o o d a s h e s in v e s t ig a t e d w a s h i g h l y p H -

d e p e n d e n t . W i th d e c r e a s in g p H v a l ue s t h e a m o u n t o f d i s s o lv e d C d , C r, C u , a n dZn gene ra l ly inc reased . The a shes behave d s imi la r ly wi th r e spec t t o t race e l e m en tre l ease desp i t e the i r d i f fe rences in concen t ra t ions o f ma jo r e l em en t s , deg ree o f so i lcon tam ina t ion , and fue l com pos i t ion (p resence o r absence o f pape rm i l l sludge ) . Inpa r ti cu la r, t he th ree w ood ashes behav ed s imi l a r ly wi th r e spec t t o the r e l ease o fCd and Z n . Th e l a t te r a r e more so lub le than the o the r t r ace e l emen t s . How ever, t he


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310 GUOGENZHANETAL.

r e le a s e o f C r a n d C u w a s s o m e w h a t i n f lu e n c e d b y t h e a s h c o m p o s i t io n ; B h a d ah i g h e r p e r c e n t a g e o f d i s s o lv e d C r a n d C u t h a n C a n d A . B w a s s im i l ar to C b o t hi n c a l c i u m c a r b o n a t e e q u i v a l e n c e v a l u e a n d i n c h e m i c a l c o m p o s i t i o n a lt h o u g h i thad h ighe r l eve ls o f S i than the C ash and the h igh es t l eve l o f hexava len t Cr o f a llth ree ashes . Hex ava len t Cr l eve l in a wa te r ex t rac t o f the a sh sam ples was a g oodpred ic to r o f C r so lub il i ty fo r a l l th ree a shes . We obse rved l i tt le in f luence o f w oo dash pa r ti c le s i ze on the r e l ease o f t race e l eme n t s excep t fo r the d i s so lu t ion o f Cuin A ash wh ere the sm a l l e s t pa r t ic l e f r ac tion had the h ighe s t so lub i l i ty.

Du e to the h igh e r concen t ra t ions o f t race e l eme n t s in w oo d ash than so il , un reg-u la t ed app l i ca tion o f w oo d ash on ag r icu l tu ra l l and ma y l ead to the accum ula t ion o ft r ace e l em en t s in the so il and , consequen t ly, cons t i tu te a pe rm anen t r i sk fo r c rops .

In gene ra l t o t al Cr l eve l s a re ve ry low in w oo d ash in comp ar i son to sew age s ludge .Al th oug h to tal Cr co n ten t is t he pa ramete r w h ich i s r egu la t ed , ch rom ium so lub i l i tyin w oo d ash se ems re l a t ed to C r (VI ) con ten t ra the r than to to ta l Cr con ten t . There la tive ly h igh C d co ncen t ra t ion o f w oo d ash co m pare d wi th so i l, and i t s r e l at iveso lub i li ty in w oo d ash , sh ou ld be co ns ide red in eva lua t ing the po ten t i a l env i ron-m en ta l imp ac t o f sp read ing w oo d ash on l and . The r esu l t s o f th i s s tudy sug ges t tha tC d w i l l b e r el e a se d f r o m w o o d a s h a t p H v a l u es b e l o w a p p r o x i m a t e l y 6 .5 .

Acknowledgemen t s

The au tho rs thank Drs. Ivan J . Fe rnan dez and S tep hen A. Nor ton o f the Unive r s i tyo f M aine fo r ve ry h e lp fu l and c r i ti ca l r ead ings o f the sen io r au thor ' s M .S . thes i son wh ich th i s pub l i ca t ion i s based . T he r e sea rch r epor t ed he re was f inanced in pa r tby the De par tm en t o f the In te r io r, U .S . G eo log ica l Survey, th roug h the Unive r s i tyo f M a i n e W a te r R e s o u r c e s P r o g r a m . T h e c o n t e n t s o f t h is p u b l i c a ti o n d o n o t n e c -essa ri ly r e fl ec t t he v iew s and po l i c i e s o f the D epar tm en t o f the In te rio r, no r doesm e n t i o n o f t ra d e n a m e s o r c o m m e r c i a l p ro d u c t s c o n s t it u te t h ei r e n d o r s e m e n t b yt h e U n i t e d S t a te s G o v e r n m e n t .

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Release of Trace Elements From Wood Ash by Nitric Acid