An Investigation Into The Ultrasonic Treatment Of Polluted Solids

gg/aaum SONOCHEMISTRY

ELSEVIER

Ultrasonics Sonochemistry 4 (1997) 153 156

An investigation into the ultrasonic treatment of polluted solids A.P. Newman,

J.P. Lorimer, T.J. Mason

*, K . R . H u t t

School of Natural and Environmental Sciences, Coventry University, Coventry, CV1 5FB, UK Received 15 October 1996

Abstract Granular pieces of brick impregnated with copper oxide were used as a model for contaminated soil. Washing this model substrate by passing water across the substrate on an ultrasonically shaken tray irradiation afforded a 40% reduction in copper content. This was compared with only a 6% reduction when the sample was treated under otherwise identical conditions but using a tray shaken conventionally. The majority of the copper was removed as a result of the removal of surface materials which were more heavily contaminated with the copper oxide. © 1997 Elsevier Science B.V. Keywords: Ultrasound; Soil washing; Decontamination

1. Introduction The utilisation of ultrasound for the treatment of waste materials is a growing area of sonochemical research. The majority of this work has been confined to small scale studies and although some work has been carried out in areas such as sludge dewatering and filtration [1-5], the most widely researched area of 'environmental sonochemistry' has been the destruction of organics in aqueous solution. However, the development and availability of ultrasonic apparatus capable of sonicating relatively large volumes of slurry has allowed for more extensive investigations to be performed in the area of solid waste treatment. As yet, no significant work involving the ultrasonic treatment of waste solids has been carried out in the UK, but some trial work has been performed in the USA by Fairbanks et al. [6]. It was found that ultrasound was able to enhance a precious metal recovery process in several ways. Firstly the cleaning action of ultrasound removed an unwanted clay coating from raw ore, and this gave rise to the secondary affects of accelerated leaching of minerals from the ore and improved filtration rates. Conventional soil washing processes are based on the principle that m a n y pollutants adsorb onto the fine fractions of soil such as silt, clay and humic matter

* Corresponding author. E-mail: [email protected]; fax: + 4 4 1203 838173.

1350-4177/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S 1350-4177 (97)00020-5

which tend to be attached to coarser sand and gravel particles which make up the majority of the soil content. Therefore the primary aim in soil washing is to separate these fine components from the bulk soil. Isolation of the fine materials will result in a 'concentrated' volume of polluted soil which may be treated or disposed of, and a large volume of residual soil which requires relatively little treatment and can be returned to the site as back fill. In this study we examine the use of ultrasound as a means of soil washing wherein the abrasive action of ultrasound in a heterogeneous environment is harnessed to separate the different fractions of soil. The efficiency of a soil washing process can be expressed in terms of both the volume reduction attained and to what degree the clean fraction meets specified clean-up standards.

2. Experimental Soil washing is itself an innovative land treatment technique enjoying success in the Netherlands, Germany and the U.S.A. [7-10]. As this was a relatively new area of research all experimental parameters were kept as uniform as possible in our experiments. The bulk soil was represented by crushed house brick sieved to 2-5 m m and the fine particulate parts of the soil which are normally attached to the coarse fraction were represented by the surface of the brick pieces. The 'pollutant'

154

A.P. Newman et aL / Ultrasonics Sonochemistry 4 (1997) 153-156

introduced to the crushed brick was copper oxide produced by impregnation with aqueous copper sulphate, treatment with sodium hydroxide followed by washing and drying. Analysis of copper content in the detached sample was relatively simple using Atomic Absorption (AA) and Inductively Coupled Plasma (ICP-AES). Sonication of the brick pieces was achieved using an ultrasonic vibrating tray. This piece of apparatus consists of a stainless steel tray (surface area 21.5 cm x 58 cm) the base of which is fitted with a 20 kHz magnetostrictive ultrasonic transducer such that sound nodes and anti-nodes are generated in a standing wave pattern along the surface of the tray. The most intense area of ultrasound will be produced directly above the transducer where it is welded to the tray with the ultrasonic field weakening towards the extreme ends. The 'standard' conditions used as a comparison with conventional technology comprised of a steel tray of equal dimensions to that used under ultrasonic conditions which was attached to a sieve shaker as the source of agitation. The experimental set-up is as shown in Fig. 1, de-ionised water was passed over the tray containing 750 g of copper contaminated brick. Gentle mechanical mixing during sonication ensured that all the brick pieces had some exposure to the most intense area of ultrasound in the centre of the tray. The resulting particle rich wash water was collected and underwent centrifugation to separate out the brick fines. The brick remaining in the tray was wet sieved at 20 mesh to collect particles under this size which was considered as detached particulates. The remaining brick pieces, the majority of the sample, were retained for analysis. All of the different brick fractions were dried, ground (where appropriate) and subjected to microwave digestion. The resulting solutions along with the wash water were analysed for copper content by AA.

1._~.__constanthead

3. Results and discussion

3.1. Copper removal Analysis of the brick pieces before and after 30 min sonication revealed an average reduction in copper content of some 40% (Table la and lb). Under conventional shaking for 30 min the corresponding figure was only 6% (Table 2a and 2b).

3.2. Mass balance Having established the extent of copper loss from the brick pieces it was important to be able to demonstrate a mass balance relating to the removal of copper from the bulk brick pieces and the quantities identified in the > 20 micron particulate material, fines and water given by: Cu(on brick before treatmentl--CU(on brick after treatment) = Cu(on fines) Jr- Cu(on < 20 mesh) q- Cu(in water)"

The results show that good agreements between the two totals can be achieved for both the ultrasonic and conventional shaking trays (Table 3). Interestingly, the distribution of copper between the fines, < 20 mesh and water fractions differs according to which form of washing was used. In the case of ultrasonic washing 96% of the total copper removed was found in the fine particulates carried in the wash water, only 1% was 'solubilised' and appeared in the wash water and the remaining 3% resided in the particles < 20 mesh wet sieved from the remaining brick (Table 3a). In contrast (Table 3b) when the non-ultrasonic shaking tray was used the majority of copper was again removed in the fines (68%) but in this case a larger proportion was in the particles <20 mesh (31%) with a similar amount (1%) 'solubilised' in the wash water.

3.3. The action of ultrasound

device

---1

shakingtray ing device

/ pump . ~ reservoir ofwater.

[ .

~...wash

"~-- water

Fig. 1. Experimental apparatus for soil washing.

Sonication of an heterogenous system can have a number of physical and/or chemical effects which have been well reported, particularly in areas such as sonocatalysis. The collapse of cavitation bubbles near a solid can produce microjets which on impact cause the solid surface to pit and erode [11 ]. Furthermore effects such as acoustic streaming can result in inter-particle collisions which can also contribute to particle size reduction. The extreme agitation caused by sonication is likely to account for the improved copper removal levels observed in this study. Sonication resulted in higher masses of copper in the fine fraction of brick particulates (Table 3), which indicates that more of the brick surface was dislodged than with conventional shaking. Indeed, a study carried out by Coles et al. [12] on the removal

A.P. Newman et al. / Ultrasonics Sonochemistry 4 (1997) 153-156

155

Table 1 Results of ultrasonic 'soil washing' Run No.

Original mass of brick (g)

Mass of brick < 2 0 mesh (g)

Mass of brick fines (g)

Volume of water collected (1)

(a) Massesofthe various brickfragments obtained and water used 1 750.44 2 750.37 3 750.26 4 750.30 5 750.40 Average 750.35

3.27 3.19 3.19 3.81 3.73 3.44

1.95 1.70 2.02 1.88 1.92 1.89

14.01 13.92 13.44 12.95 13.16 13.51

Run No.

Copper conc. on < 20 mesh

Copper conc. on fines

Copper conc. in water

2772.09 5135.40 5253.29 5308.36 6330.88 4690.00

0.50 0.54 0.54 0.43 0.44 0.49

Mass of brick < 2 0 mesh (g)

Mass of brick fines (g)

Volume of water collected (1)

(a) Masses of the various brick.fragments obtained and water used 1 750.57 2 750.43 3 750.56 4 750.10 5 750.34 Average 750.40

4.38 4.47 3.98 0.98 0.88 2.94

0.56 0.44 0.95 0.57 0.61 0.63

13.14 12.72 11.60 12.74 12.70 12.58

Run No.

Copper conc. on < 20 mesh

Copper conc. on fines

Copper conc. in water

3351.16 5731.99 1754.90 2999.13 2153.85 3198.21

0.25 0.29 0.23 0.17 0.14 0.22

Residual copper conc. on treated brick i.e. > 20 mesh

(b) Concentration ofcopper(ppm) in the var&usfract&ns obtained 1 31.79 104.50 2 28.69 108.51 3 27.23 90.61 4 30.47 87.58 5 35.72 87.30 Average 30.78 95.70 Copper concentration on doped brick prior to washing under ultrasound = 51.41 ppm. Average reduction in copper concentration on treated brick =40%. Table 2 Results of conventional vibrating tray 'soil washing' Run No.

Original mass of brick (g)

Residual copper conc. on treated brick i.e. > 20 mesh

(b) Concentration of copper (ppm) in the various fractions obtained 1 50.34 304.09 2 50.82 374.78 3 47.67 189.96 4 46.79 193.29 5 45.84 505.15 Average 48.29 313.45 Copper concentration on doped brick prior to ultrasound = 51.41 ppm. Average reduction in copper concentration on treated brick = 6%.

of organic pollutants from sandy soil revealed that 5 min of sonication in conjunction with 0.75% aqueous surfactant was equivalent to stirring for 60 min with the same solution. This promising result was attributed to the improved interaction of the surfactant brought about by the mechanical effects of ultrasound expressed in terms of: (a) Ultrasonically induced high fluid-solid shear

stresses, which promote mechanical detachment and removal of contaminants and (b) Ultrasonically promoted mass transfer of surfactant monomer between the bulk fluid phase and the soil/substrate interface. These improvements might be expected to occur in all ultrasonically agitated extractive soil washing and would thus offset the problems identified in conventional

156

A.P. Newman et al. / Ultrasonics Sonochemistr) 4 (1997) 153 156

Table 3 Mass balance of copper from washing experiments Run No.

Mass of copper (rng) On brick before u (s)

(a) Ultrasonic washing 1 38.58 2 38.58 3 38.57 4 38.58 5 38.58 Average 38.58

Run No.

On brick after u (s)

Total removed

On < 20 mesh

In water

On fines

Total removed

23.69 21.39 20.29 22.69 24.71 22.55

14.98 17.19 18.28 15.88 13.88 16.04

0.342 0.346 0.289 0.334 0.325 0.327

7.078 7.531 7.204 5.543 5.830 6.637

5.397 8.718 10.636 9.957 10.430 9.028

12.82 16.59 18.13 15.83 16.59 15.99

On brick after shaking

Total removed

On < 20 mesh

In water

On fines

Total removed

37.54 37.89 35.54 35.02 34.33 36.06

2.80 2.45 4.80 5.30 5.90 4.25

1.33 1.68 0.76 0.19 0.44 0.88

3.26 3.70 2.68 2.18 1.83 2.09

1.88 2.49 1.66 1.71 1.32 1.81

6.47 7.87 5.10 4.08 3.59 5.42

Mass of copper (rag) On brick before shaking

( b ) Con yen tional shaking

1 2 3 4 5 Average

40.34 40.34 40.34 40.32 40.23 40.31

i n v e s t i g a t i o n s w h i c h s e e m to be a s s o c i a t e d w i t h p o o r p o l l u t a n t r e m o v a l rates a n d p r e v e n t such m e t h o d s f r o m b e i n g c o m m e r c i a l l y v i a b l e [ 13 17]. In this study, u l t r a s o n i c i r r a d i a t i o n c a u s e d o v e r twice the m a s s o f c o p p e r to e n t e r t h e a q u e o u s p h a s e ( T a b l e 3) w h i c h suggests t h a t u l t r a s o u n d was a b l e to p r o m o t e t h e l e a c h i n g o f c o p p e r f r o m t h e b r i c k to the w a s h w a t e r . H o w e v e r as w i t h all u l t r a s o n i c a p p a r a t u s s o m e d e g r e e o f h e a t i n g w a s o b s e r v e d a n d this is also likely to influence c o p p e r solubilities.

4. Conclusions Ultrasound improves the removal of copper oxide c o n t a m i n a t i o n f r o m b r i c k pieces via a p r o c e s s i n v o l v i n g the d e t a c h m e n t o f s u r f a c e m a t e r i a l u p o n w h i c h the m a j o r i t y o f p o l l u t i o n resides. T h i s suggests t h a t the m e t h o d o l o g y m a y be s u i t a b l e for the r e m e d i a t i o n o f soils c o n t a m i n a t e d w i t h i n o r g a n i c p o l l u t a n t s . H o w e v e r in o r d e r to m a k e a p r o p e r c o m p a r i s o n b e t w e e n the u l t r a s o n i c v i b r a t i n g t r a y a n d the n o n u l t r a s o n i c s h a k i n g e q u i v a l e n t t h e a m o u n t o f e n e r g y t h a t e a c h s y s t e m uses a n d t h e i r r e l a t i v e efficiencies m u s t be t a k e n i n t o a c c o u n t . S t u d i e s i n v o l v i n g electrical p o w e r i n p u t , c a l o r i m e t r i c a n d c h e m i c a l d o s i m e t r y are c u r r e n t l y in h a n d .

References [1] H.S. Muralidhara, N. Senapati, D. Ensminger, S.P. Chauhan, Filtration and Separation 23 (1986) 361. [2] E. Kowalska, K. Chmura, J. Bien, Ultrasonics 17 (1978) 184. [3] L. Bjorno, S. Gram, P. Steenstrup, Ultrasonics 17 (1978) 103. [4] J. Bien, Filtration and Separation 25 (1988) 425. [5] J.C. Lozier, R.A. Sierka, J. Am. Water Works Assoc. 77 (1985) 60. [6] H. Fairbanks, W. Morton, J. Wallis, Precious Met. Proc. Met. Inst. Conf. 9th, Int. Precious Met. Inst. 305 (1986). [7] M.F. Pruijn, 5th Forum on Innovative Hazardous Waste Treatment Technologies: Domestic and International, Chicago, IL (1994) p. 57. [8] Warren Spring Laboratory Review of Innovative Contaminated Soil Clean-up Process, 60. [9] M.E. Fristad, C. Jones, 5th Forum on Innovative Hazardous Waste Treatment Technologies: Domestic and International, Chicago, IL (1994) p. 62. [10] J.W. Assik, 1st Int. TNO Conf. on Contaminated Soil (1986) p. 655. [11] T.J. Mason, J.P. Lorimer, Sonochemistry: Theory Applications and Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1988. [12] E.T. Coles, B. Rubin, R. Gaire, Haztech Int. 90, Houston Waste Conf. and Exhib., Houston (May 1990). [13] B.J.W. Tuin, M. Tels, Environ. Technol. 11 (1990) 541. [14] B.J.W. Tuin, M. Tels, Environ. Technol. ll (1990) 935. [15] B.J.W. Tuin, M. Tels, Environ. Technol. 12 (1990) 178. [16] B.J.W. Tuin, M. Tels, Environ. Technol. 11 (1990) 1039. [17] J.E. Van Benschoten, B.E. Reed, M.R. Matsumoto, P.J. McGarvey, Water Environ. Res. 66 (1994) 168.