speciation of as(iii) and as(v) in some ghanaian gold tailings by a simple distillation method

SPECIATION OF As(III) AND As(V) IN SOME GHANAIAN GOLDTAILINGS BY A SIMPLE DISTILLATION METHOD

KHALID AHMAD 1 and DERICK CARBOO21 Chemistry Department, Ghana Atomic Energy Commission, P.O. Box 80, Legon, Ghana

2 Department of Chemistry, University of Ghana, P.O. Box 56, Legon, Ghana

(Received 17 February 1998; accepted 12 October 1999)

Abstract. As(III) and As(V) in goldtailings and river-bed sediments from Obuasi were determinedby distillation of arsenic as AsCl3. Results yielded 3750±426 mg kg−1 (45.2%) for As(V) and3050±66 mg kg−1 (36.7%) for As(III) in the tailings. In the river-bed sediments, one spot yielded:As(III) 0 mg kg−1 (0%) and As(V) 1447±51 mg kg−1 (100%), whilst a second spot yielded: As(III)0 mg kg−1 (0%) and As(V) 2976±51 mg kg−1 (100%). Using arsenic oxide standards, the recoveryof As(III) and As(V) in the trioxide were 94.8 and 0.6% respectively. In a mixture of the two oxides,the recovery of As(III) was 87.6% with practically no interference from As(V). Total As content ofthe tailings was determined by neutron activation analysis (NAA) to be 8305±75 mg kg−1.

Keywords: arsenic, distillation, gold tailing, neutron activation analysis, sediment, speciation

1. Introduction

Ghana has been associated with production of gold which is increasingly becominga major foreign exchange earner for the economy. The result is that arsenic, occur-ring in ores as the pyrites and arsenopyrites (Kesses, 1985), has become a majorenvironmental pollutant in gold-mining areas, through gold processing. Vegetationaround the Obuasi gold mine, the richest in the country, has virtually perishedfrom aerial arsenic pollution in addition to the huge gold tailings that continue tobe washed into streams and rivers.

Recent reports also indicate that the arsenic content of food stuffs from ma-jor towns as Kumasi (Ghana’s second capital), about 50 miles from Obuasi, hasreached appreciable levels to raise concern (Amasa, 1975). Apart from a recentstudy conducted by Bowellet al. (1994), the few studies carried out on Ghanaiangold mines have focused on total arsenic content of the pollutant.

In this work we describe the speciation of As(III) and As(V) in mine tailingsand river sediment using a simple simple distillation method based on volatility ofAsCl3 (Kingsley and Schaffert, 1951). As(III) is converted into AsCl3 by reactingthe tailings with conc HCl and distilling it followed by its quantification usingiodine titration. After that the same amount of tailings is reacted with KClO3 toconvert all As(III) into As(V). The latter is then reduced back to As(III) usinghydrazine sulphate and distilled as AsCl3. After quantification by iodine titration,

Water, Air, and Soil Pollution122: 317–326, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

318 K. AHMAD AND D. CARBOO

Figure 1.Distillation set-up for determining As(III) and As(V).

As(V) is obtained by difference. For comparison total As in the tailings was also de-termined by neutron activation analysis. The validity of the method was establishedby analyzing authentic mixtures of As2O3 and As2O5.

2. Materials and Methods

All chemicals were of analytical grade. High purity As2O3 and As2O5 used asstandards, were obtained from Fluka, Switzerland.

2.1. ANALYTICAL PROCEDURE

2.1.1. Determination of As(III) in Gold TailingThe goldtailing samples obtained from Obuasi goldmines in Ghana were weighed(0.5078, 0.5025 and 0.5986 g) and put into a three-necked 250 mL round bottomedflask. 30 mL of concentrated HCl was added and stoppered to prevent loss of theAsCl3 formed. After 2 hr, the flask was then fitted with a distillation condenser, athermometer and an N2-inlet. The receiver was made to stand in a trough filled withice and NaCl (Figure 1). AsCl3 was distilled under N2 atmosphere and the fractionboiling in the range 105–110◦C was collected. A strict control of the temperaturewithin this range was necessary to ensure that no other chlorides, especially SbCl3

and SnCl2 interfered.

SPECIATION OF As(III) AND As(V) IN SOME GHANAIAN GOLD TAILINGS 319

Figure 2.Gamma spectrum of irradiated goldtailing.

C was first opened followed by F. After that K was opened till the nitrogen gasbegan to flow at a steady and gentle rate to bubble into the mixture of acid andtailings. The aspirator was turned on to circulate water around the condenser ata temperature of 20◦C. The flow of heat in the heater was carefully controlledusing knob J to keep temperature in the range of 105 to 110◦C within which theAsCl3 distilled into the receiver at 108◦C. The receiver contained 15 mL of conc.HCl plus 5 mL of water. After collecting all the AsCl3 in the specified temperaturerange, the heat was turned off. The flask and its contents were allowed to coolsufficiently after which the flow of nitrogen gas was stopped. 30 mL of deionisedwater was used to wash the condenser and the washings added to the AsCl3 inthe receiver. A few drops of phenolphthalein indicator were added and sufficientamount of 6 M NaOH was added to neutralize the strong acid solution (colourchange from colourless to pink). Few drops of 1:4 HNO3 were added to dischargethe colour. 10 g NaHCO3 was added with shaking followed by 1 mL of starchindicator. The sample solution was titrated using 0.005 M iodine solution (Furham,1966). The end-point of the reaction was marked by the characteristic dark-bluecolour of starch-indicator.

A blank determination was performed by titrating the iodine solution against amixture of HCl, 5 mL HNO3, 15 mL of deionised water plus 15 mL of HCl, 30 mLof water, 10 g of NaHCO3 and 1 mL of starch indicator all contained in a 400 mL


conical flask. 0.1 mL of the iodine solution from the burette was used to reach theend-point. This volume was subtracted from the titre of all samples.

2.1.2. Determination of As(III) and As(V) in the Tailings0.5147, 0.5087, and 0.5009 g of the tailings were first weighed into 100 mL roundbottomed flasks. All As(III) in the tailings were converted into As(V) by the fol-lowing chemical treatment: 5 mL of conc HNO3 and a pinch of KClO3 were addedto the contents of each flask and heated to dryness. After that 5 mL of conc HClwas added and heated to dryness. Then 20 mL of 1:1 HCl was added and boiled for30 min. After cooling it was filtered, 30 mL of conc HCl added and transferred intoa three-necked flask. 0.5 g of hydrazine sulphate and 0.5 g of NaBr were added anddistilled within 105–110◦C as described above. Addition of hydrazine sulphateensured the reduction of all As(V) present back to As(III) to be distilled as AsCl3.The condenser was again disconnected and washed with 30 mL of deionised waterinto the distillate. 0.005 M iodine solution was titrated against the solution usingstarch as indicator. Determination was carried out in triplicate for each sample.

2.1.3. Determination of Total As in the Gold Tailings and River Sediment byNeutron Activation Analysis

Neutron activation analytical determination of the total arsenic content of the samp-les was carried out by irradiating 100 mg each of the samples for an hour in theMiniature Neutron Source Reactor (MNSR) facility at the Ghana Atomic EnergyCommission (GAEC) at a neutron flux of 1×1014 n cm−2 s−1 (Ahmad et al.,1996). It was subsequently measured in the gamma spectrometer consisting of anN-type high purity germanium detector Model GR 2518 (Canberra Industries Inc)with resolutions of 0.85 Kev at 122 Kev, 1.8 Kev at 1.33 Kev of60Co gammaenergy. The detector had a relative efficiency of 25%, whilst the multichannelanalyzer is computer-based (Silena-Emcaplus). SPAN 5.0 gamma spectra softwarewas used for the spectra evaluation. Delay time was 24 hr and counting time was1000 s. Figure 2 shows the spectrum of a sample after an hour irradiation.

Other elements detected included: Sm-153, 41.5 Kev; Au-198, 4118 Kev, La-140, 487.03 Kev; Sb-124, 602.71 Kev; Sb-122, 593.93 Kev: Sc-46, 493.3 Kev;Fe-59, 1090.22 Kev; Sc-46, 1120.5 Kev; Fe-56, 1291.56 Kev: La-140, 1596.6 Kev.

2.1.4. Determination of As in Standard Chemicals2.1.4.1. Determination of As(III) in As2O3 and As2O5 High purity As2O3 andAs2O5 were obtained from Fluka, Switzerland. 0.500 g of each of As2O3 andAs2O5 were accurately weighed and treated with conc HCl as described above.After the usual distillation and quantification by iodine titration using 0.05 Miodine solution As(III) was determined in each case.

2.1.4.2. Determination of As(III) in a mixture of As2O3 and As2O5 Similarly,0.500 g of As2O3 and 0.500 g of As2O5 were weighed and mixed in a round


bottomed flask and treated with conc HCl as described earlier. AsCl3 was distilledand quantified with 0.05 M iodine solution to determine the amount of As(III) inthe mixture.

2.1.4.3. Determination of Total As in a mixture of As2O3 and As2O5 0.500 gof As2O3 and 0.500 g of As2O5 were again weighed into a round bottomed-flaskand treated chemically to convert all As(III) into As(V) using KClO3. It was thenreduced to As(III) using hydrazine sulphate and then distilled as AsCl3. Quantific-ation using 0.05 M iodine solution yielded total As.

2.2. ANALYSIS OF ENVIRONMENTAL SAMPLE

Two river-bed sediment samples (sample D and sample L) obtained from the Kw-abrafo stream in the catchment area of Obuasi goldmines were analysed for theirAs(III) and As(V) contents.

2.2.1. Determination of As(III) in River-Bed Sediment0.5053, 0.5045 and 0.5091 g of sample L and 0.5043, 0.5016 and 0.5034 g ofsample D were taken and put into separate flasks. Each was treated with concHCl and the ensuing AsCl3 distilled as described above. The AsCl3 distillateswere titrated in the usual manner against standard iodine solution using starch asindicator.

2.2.2. Determination of As(III) and As(V) in River-Bed SedimentSimilarly, 0.5068, 0.5054 and 0.5078 g of sample L and 0.5034, 0.5331 and 0.5075 gof sample D were weighed and put into separate flasks. Each sample was reactedwith 5 mL HNO3, followed by a pinch of KClO3 to convert all As(III) to As(V).So then all As(V) was reduced back to As(III) by hydrazine sulphate as describedabove. After that As(III) is converted to AsCl3 and quantified by I2 titration. As(V)is obtained by difference.

3. Results and Discussion

3.1. CALCULATIONS

The reactions underlying the arsenic determination via iodine titration can be sum-marised as follows:

(1) As(III) is converted to AsCl3 by conc HCl. A similar reaction does not occurwith As2O5

As2O3 + 6 HCl→ 2 AsCl3 + 3 H2O(2) In aqueous solution (dilute HCl) AsCl3 hydrolyses to arsenous acid.

AsCl3 + 3 H2O→ H3AsO3 + 3 HCl


(3) Arsenous acid is easily oxidised to arsenic acid by iodineH3AsO3 + KI3 + H2O→ H3AsO4 + KI + 2 HI

(4) Sodium bicarbonate was added to bind the acids formed in these reactions.2 NaHCO3 + HI + HCl→ 2 H2O + 2 CO2 + NaI + NaCl

(5) Addition of all the equations yields:As2O3 + 2 KI3 + 2 NaHCO3 + HCl + 3H2O→2 H3AsO4 + 2KI + 3HI + 2 CO2 + NaCl + NaI

from which 2 KI3 ≡ As2O3 or 2I2 ≡ As2O3; 1 mL of 0.005 M iodine solution≡ 0.000375 g As; 4.11 mL of 0.005 M iodine solution≡ 0.00154125 g As; In0.5086 g of tailings we have 0.00145125 g of As.Therefore the amount of As in 1000 mg of tailings =

1000× 0.00154125 mg

0.5086= 3030 mg kg−1

Tables I and II give the results of the arsenic determinations in the gold tailings.The data on the river-bed sediments are in Tables III and IV while those for thestandard chemicals are in Tables V, VI and VII.

TABLE I

Determination of As(III) in gold tailings

Weight of Total vol of Weight of As Concentration

sample 0.005 M I2 solution in sample

(g) (mL) (g) (mg kg−1)

0.5057 4.23 0.0016 3139

0.5026 3.99 0.0015 2978

0.5086 4.11 0.00154 3030

Average As(III) concentration = 3050±66.8 mg kg−1.

TABLE II

Determination of As(III) and As(V) in gold tailings



(g) (mL) (g) (mg kg−1)

0.5147 9.98 0.0037 7271

0.5087 9.34 0.0034 6882

0.5009 8.35 0.0031 6248

As(III) + As(V) = 6800±421 mg kg−1.


TABLE III

Determination of As(III) in river-bed sediments



(g) (mL) (g) (mg kg−1)

Sample L

0.5053 NDa ND ND

0.5045 ND ND ND

0.5091 ND ND ND

Sample D

0.5043 ND ND ND

0.5016 ND ND ND

0.5034 ND ND ND

a ND = not detected.As(III) in sample L = 0 mg kg−1.As(III) in sample D = 0 mg kg−1.

TABLE IV

Determination of As(III) and As(V) in river-bed sediments



(g) (mL) (g) (mg kg−1)

Sample L

0.5068 2.03 0.00076 1499

0.5054 2.06 0.00077 1526

0.5078 1.90 0.00071 1407

Sample D

0.5034 4.05 0.0015 3019

0.5331 4.27 0.0017 3005

0.5075 3.93 0.00147 2905

Average concentration of As(V) in sample L = 1477±51 mg kg−1.Average concentration of As(V) in sample D = 2976±51 mg kg−1.


TABLE V

Determination of As(III) in As2O3

Weight of Vol of 0.05 M Weight of As in As2O3 Recovery

As2O3 I2 solution Calculated Determined

g (mL) g mg kg−1 g mg kg−1 (%)

0.5038 98.06 0.3817 757 642 0.3676 730 100 96.3

0.5021 96.30 0.3804 757 618 0.3514 701 977 95.0

0.5029 94.48 0.3810 757 606 0.3543 704 514 93.0

Determination of As(III) in As2O5

Weight of Vol of 0.05 M Weight of As in As2O5 Recovery

As2O5 I2 solution Calculated Determined


0.5071 0.435 0.3366 652 336 0.0016 3210 0.5

0.5091 0.662 0.3325 653 113 0.0023 4572 0.7

0.5053 0.442 0.3395 652 086 0.0020 3917 0.6

Mean As(III) recovery from As2O3 = 94.8±1.36%.Mean As(III) recovery from As2O5 = 0.6±0.08%.

TABLE VI

Determination of As(III) in a mixture of As2O3 and As2O5

Weight of As2O3 Vol of 0.05 M Weight of As(III) in the mixture Recovery

and As2O5 I2 solution Calculated Determined


0.3052 (As2O3) 55.40 0.2312 456 039.7 0.2078 409 958 89.9

0.2018 (As2O5)

0.2062 (As2O5) 35.78 0.1562 303 831.9 0.1344 261 428 86.0

0.3079 (As2O5)

0.2512 (As2O3) 44.15 0.1903 379 461.6 0.1656 330 138 87.0

0.2503 (As2O5)

Mean recovery As(III) in the mixture (As2O3 + As2O5) = 87.6%.


TABLE VII

Determination of As(V) in a mixture of As2O3 and As2O5

Weight of As2O3 Vol of 0.05 M Weight of As(III) in the mixture Recovery

and As2O5 I2 solution Calculated Determined


0.3066 (As2O3) 88.90 0.1335 261 009.2 0.1296 253 472 97.0

0.2047 (As2O5)

0.2036 (As2O3) 90.60 0.2087 411 312.6 0.1981 387 444 105.0

0.3038 (As2O5)

0.2512 (As2O3) 90.1 0.1634 325 882.5 0.1552 309 472 95.0

0.2503 (As2O5)

Mean recovery As(V) in a mixture (As2O3 + As2O5) = 99%.

TABLE VIII

Concentrations of As(III), As(V) and total As in the goldtailings,sediment samples and the arsenic oxides

As species Tailings Sediments Sediment Standard

sample L sample D chemical

mg kg−1 mg kg−1 mg kg−1 % recovery

As(III) 3050±66 ND ND 87.6

As(V) 3750±426 1477±51 2976±51 99.0

Total As 8305±75 1521±35 2743±41 100

4. Discussion

The accuracy and great versatility of this method of speciating As(III) and As(V)in gold tailings is based on the fact that in strongly acidic solutions only AsCl3 isformed and that AsCl5 is never formed (Kingsley and Schaffert, 1951). The data inTable V clearly confirm this. From the data it is clear that the 94.8% recovery ofAs(III) in As2O3 is far in preponderance compared to the 0.6% in As2O5. As expec-ted, in a mixture of the two oxides (Tables VI and VII), the method allows a highpercentage recovery of As(III) (87.6%) and As(V) (99%). Table VIII summarisesthe concentrations of As(III), As(V) and total As (from the NAA determination) inthe goldtailings, sediment samples and the arsenic oxides.

From Table II the sum of the As(V) and As(III) concentrations in the tailingsdiffered by 1495 mg kg−1 from the total As concentration of 8305±75 mg kg−1


obtained by NAA. This possibly could be attributed to the presence of some or-ganoarsenicals. Similarly, the virtual non-existence of the As(III) in the river-bedsediments may be attributed to the fact that all As(III) has been oxidised to As(V).The values of the As(V) in the river-bed sediments determined in this work agreeswith that of total As in sediments determined by Carbooet al. (1997). It appearsalso that there is not much of organoarsenicals in the sediment as the As(V) valuesare about same for total As.

Acknowledgement

We wish to express our gratefulness and thanks to the International Atomic EnergyAgency (IAEA) for providing the financial support for this project. We particu-laraly thank Dr Irene Lewkowicz and Dr Theresa Benson for their help and support.We wish also to thank Mr S. Kumi (Glassblowing Unit, University of Ghana,Legon) who constructed the distillation apparatus.

References

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Technology14 (No. 1), Kumasi, Ghana.Bowell, R. J., Morley, N. H. and Din, V. K.: 1994,Applied Geochemistry9, 15.Carboo, D. and Serfor-Armah, Yaw.: 1997Proceedings, National Symposium on The mining In-

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Furham, N. H.: 1966,Standard Methods of Chemical Analysis, Vol. 1, 6th ed., D. Van NostrandCompany Inc., New Jersey, U.S.A., pp. 114–115.

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Kingsley, J. E. and Schaffert, R. R.: 1951,Anal. Chem.23, 509.

speciation of as(iii) and as(v) in some ghanaian gold tailings by a simple distillation method

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