Protection of groundwater from migration of infiltrates from a chromic waste storage site and methods of treating these infiltrates

doi:10.4236/health.2010.23026

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Vol.2, No.3, 177-187 (2010)

doi:10.4236/health.2010.23026

Health

Protection of groundwater from migration of infiltrates

from a chromic waste storage site and methods of

treating these infiltrates

Zygmunt Kowalski, Adam Kozak, Marcin Banach, Agnieszka Makara

Institute of Chemistry and Inorganic Technology, Cracow University of Technology, Cracow, Poland; zkow@chemia.pk.edu.pl

Received 14 October 2009; revised 10 December 2009; accepted 14 December 2009.

ABSTRACT

This work presents the results of investigations

to develop and implement methods to effectiv ely

collect and purify infiltrates from hea ps, situated

in the region of Alwernia near Cracow, where

more than 3 million tonnes of waste material

resulting from the production of chromium com-

pounds have been stored. It describes a system

for the protection of groundwater from these

infiltrates which contain 50-400 g m-3 Cr6+, as

well as the effectiveness of cheap and simple

chemical methods to purify these chromic was-

tewaters. The infiltrate collection system and the

most effective method to decrease the concen-

tration of Cr6+ to a level below 0.1 ppm, as re-

quired by Polish and European Union regula-

tions, were implemented in the Alwernia Che-

mical Works S. A. in the years 1998-1999.

Keywords: Was t e H eaps; Chromi c Infiltrates;

Collect; Purify

1. INTRODUCTION

The quantities of chromic waste accumulated in Poland

are considerable. In Alwernia, in the course o f 50 years of

production of chrome compounds, more than 3 million

tonnes of waste have been stored. This waste contains

more than 1% of carcinogenic Cr6+ compounds. The

waste heap of a former pr oducer of chr omium compounds

in Rudniki store a further 0.5 million t. Other 1.5 million t

of waste from the production of ferro-chromium can be

found in Siechnice. The amount of stored waste resulting

from tanning and galvanic processes is estimated to be at

least 50,000 tons per year [1-3].

Because of the position and the quantity of accumu-

lated waste, the heap in Alwernia represents a serious

hazard to the natural environment (Figure 1). The waste

heap consists of two parts. The old one that was in use

until 1967, and the n ew one, which has been in use since

1968. It is situated in the macroregion of the Cracow-

Wi e l uń Upland (being a layer of u pper-Jurass i c l imestone)

and the mesoregion of the Tenczyński Ridge [4,5]. The

heap is located 2.5-3.0 km north of the Vistula River, in

the valley of the meandering stream Regulanka. The dis-

tance between this stream and the old heap is 75 to 150

metres and that between t he stream and the new heap is 75

to 200 m. The old heap at present forms a partly reculti-

vated block, whi ch t owers 5 meters above the level of the

Chemical Works plant. The new heap forms an irregular

cone with a cut top. On its slopes special terraces have

been prepared to improve the stability of the slope.

In recent years ef forts aiming to limit the environmental

impact of chromic waste accumulated in the heaps have

mostly focused on the protection of groundwater

Figure 1 . The “A lwernia” Chemical Works— a general view. An

active heap of ch romic w astes ca n be seen in the f oreground. An

old reclaimed he ap occupies the area b etween the factory and the

heap.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

178

against infiltration from leachates containing Cr6+ and the

development of an efficient and economical method to

remove chromium from these wastewaters [6,7].

The present work describes the results of investigations

which led to the implementation of a collection and

treatment system for infiltrates from the chromic waste

dumps, containing 50 to 400 g m-3 of Cr6+. The results of

experiments to devise and implement a cheap and simple

method of removing chromium from wastewaters, thus

reducing its concentrations below 0.1 ppm, to order to

comply with Polish and European Union regulations [8],

also are presented. These methods were implemented in

the Chemical Works “Alwernia” in 1998/1999.

2. RESEARCH ON THE SPREAD OF

POLLUTION IN THE GROUNDWATER

ENVIRONMENT IN THE REGION OF

CHROMIC W ASTE MATE RIAL ST ORAGE

The measurement of the electrical conductivity of the

ground near the places of storage was conducted with the

device 34-3XL produced by Geonics Ltd., Canada, based

on the electromagnetic method (EM) utilising low fre-

quencies [9]. Measurements were conducted every 20 m

(old heap every 10 m ), with perpendicular arrangem ent of

coils. The results of the specific conductivity of the

ground t o a de pth of 15 m are given by Guli ński et al. [6].

The graphic illustration of the results indicates the p laces

of potential groundwater pollution (Figure 2). Maps of

groundwater pollution with ions of Cr6+ have been pre-

pared on the basis of the analysis of samples with pie-

zometers (archival and actual), and in some cases based

on EM measurements [6,7], conducted simultaneously.

On the basis of these investigations, both distribution and

directi on of movem ent of poll ution have be en det erm ined.

Based on the analysis of the data, a model of the dis-

tribution of under ground pollution between the Chemical

Works and the Regulanka River was identified. All pol-

lutants affecting the soil in this region permeate to the

“quaternary” water-bearing layer and are then carried

further by t he un derground w aters. B etwee n the hea ps the

water-bearing layers are connected by water-bearing

gravel. Ho wever , the syst em of hydrois ohypses (lines on a

map joining pl aces with the e qual groundwater elevation),

together with the electrical conductivity distribution,

indicates a disturbance in the flow of the waters from

north to south. A small hill in the neighbourhood of the

southern border of the Chemical Works which is charac-

terized by minimal thickness of “quaternary” redundant

contains impenetrable formations which causes the

groundwater to flow in eastern direction. It is not ex-

cluded that a part of the infiltrating wastewater flows in a

south-easterly direction (Figure 2). It can be also stated

Figure 2. Distribution of chromium pollution in the vicinity of

the chromic heaps at the “Alwernia” Chemical Works (accord-

ing to Guliński et al. [6]).

that at the moment, in the region of Skowronek and

Młyńczysko, the polluted waters from the Chemical

Works are drained by the river Regulanka and carried

away to the river Vistula. The hydrogeological conditions

also indicate that, in the case of an attempt at regulating

the riverbank and tightening its bed, the water may mi-

grate to the Vistula through permeable sediments at the

bottom of the Regulanka River valley.

The results of the investigations are presented in Figure

2. The occurrence of three streams of polluted ground-

water has been identified. These are the following:

1) From the grounds of the production facilities and the

northern part of old hea p;

2) From the grounds of the production facilities and the

southern pa rt of ol d heap;

3) From the grounds of the new heap.

The main direction of the flow of pollution in the

groundwater is from north to south, with some local de-

viations. These flow directions a re indicated in Figure 2 [6].

Considering the fact that all the infiltrates might po-

tentially enter the V istula, the possibility of pumping these

polluted waters and cleaning them, or returning them to

the technological processes has been considered to be an

improvement to the natural environment.

Decisions were taken to build the girding tren ches “A”

and “C” and the two drainage wells “B” and S-25 in 1998

and at the beginni ng of 1999 (Figure 2). The original plan

of changing the Regulanka riverbed over a distance of 1.5

km has been given up. Initiall y, the quantity o f infiltrates

from these sources (intakes) was estimated to be between

600 to 2000 m3 per day.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

179

3. INVESTIGATIONS INTO THE

REMOVAL OF Cr6+ FROM

INFILTRATES ORIGINATING FROM

CHROMIC WASTE HEAPS

Taking into consideration the fact that it is possible to

reuse o nly 20 0 m3 of chromic wastewater per 24 hours in

the chromium compounds production, it was decided that

the only feasible possibility to so lve the problem of infil-

trates was to clean them from Cr6+ compounds. It was

necessary, however, to develop a new method of cleaning

chromic wastewaters. Such a method should be particu-

larly adapted to low and variable chromium concentra-

tions in wastewater and it should be cost effective to be

used in practice.

The laboratory investigations were carried out on

model solutions, prepared on the basis of the composition

of water t aken from the girding t rench “A” in the region o f

the old chromic heap. The quantitative composition of the

wastewater in the trench was taken from the analyses

presented by Kania [7]. According to these analyses, the

concentration of Cr6+ i n the wastew ater varie d between 1 1

to 112.5 g m-3 in th e first quarter of 1998 . The SO42- ani-

ons in concentrations of 240 to 555 g m-3 can also be

found in wastewater, and their presence has to be taken

into consideration for the successful removal of chromate

anions from the solution.

In previous years the “Alwernia” Chemical Works in-

vestigated ways of cleaning the wastewaters which con-

tained more than 1 kg m-3 of Cr6+, using a classical

chemical method, based on the reduction of Cr6+ to Cr3+

and the precipitation of Cr(OH)3. Sodium sulphite was

used as a reducing agent at a pH between 2 and 4. Pre-

cipitation of the reduced chromium as a chromium (III)

hydroxide was the n accom plished with the us e of cal cium

oxide [10].

The results of the laboratory investigations [11] on

model solutions containing Cr6+ ions in concentrations of

50 to 200 g m-3 show that with the use of both, sodium

sulphite and sodium pyrosulfite (Na2S2O5) as a reducing

agent, lowe ring of the concen tration o f Cr6+ below 1 g m-3

was possible. In alm ost all the experim ents between 1 an d

12.5 g m-3 of Cr6+ were left in the solution after the re-

duction, and in most of the analyses the concentration of

Cr6+ ranged from 1 to 3 g m-3, with a tendency towards

higher values.

Many authors suggest the use of anion exchangers to

remove Cr6+ [12-16]. The advantage of this method is that

the Cr6+ compounds in the solution u sed to reg en erate the

anion exchanger are more concentrated than in the ori-

ginal wastewater. In the case of diluted wastewaters con-

taining considerable quantities of suspended solids, this

method is not practical because it requires the use of very

large columns and the anion exchanger could be blocked

by the suspended solid matter present in the infiltrates

thus resulting in an increase in flow resistance.

What is suggested most often is the purification of

chromic wastewater by reducing hexavalent chromium to

trivalent chromium in an acid environment and then pre-

cipitating the latter as chro mium (III) hydroxide. In addi-

tion to the above-mentioned sodium sulphite and pyro-

sulfite, the use of the reducing agents sulphur dioxide

(SO2) and sodium acid sulphate (NaHSO3) at a lower pH

range (2-2.5) is recommended [17-19].

Reduction of Cr6+ to Cr3+ may also be achieved in a

basic environment using iron (II) sulphate as a reducing

agent [14]. This reaction follows the equation:

CrO42- + 3Fe2+ + 4OH- + 4H2O = Cr(OH)3 + 3Fe(OH)3 (1)

Another interesting possibility of chromium removal is

the precipitation of chromium from wastewater in the

form of chromic ettryngite. This method can be used to

purify chromic tanning wastewater containing Cr3+

[20-22]. The occurrence of ettr yngite was first observed in

the research on cement mortar. Its chemical formula is

2CaOAl2O33CaSO432H2O [23,24]. Ettryngite forms

during cement bonding and also after mixing of ashes

from fluidised hearths with water [25]. It was stated by

several authors that the aluminium in ettryngite can be

substituted by Fe3+, Cr3+ and other metal ions [26-29].

Furthermore, the SO42- ion can be exchanged by other

anions, i.e. chromate CrO42- [24-30]. In spite of some

qualitative changes, such a compound retains the same

structure. According to the above-mentioned authors the

requirement for the formation of this compound, generally

called ettryngite, is a high pH, and the maintenance of a

CaO:Me2O3:anion(II) ratio of 6:2:3. A practical example

of the use of the ettryngite precipitation to remove SO42-

ions from wastewater has be en presente d by Woroszyńska

et al. [31].

Among the chemical methods, the possibility of pre-

cipitating chromate after first converting it into com-

pounds with a very low solubilit y pro duct, i.e. BaCrO 4 or

PbCrO4 [32], should also be mentioned.

Four methods to purify wastewater containing Cr6+

compounds were tested under laboratory conditions in the

“Alwernia” Chemical Works. Two of them were base d on

the incorporation of the CrO42- ion into the ettryngite

structure, in the presence of aluminium or iron (III) salts

in a basic environment, and its subsequent precipitation.

Another method was based on the precipitation of the ion

of CrO42- by lead salts, utilising the low solubility of lead

chromate. The last method was carried out in two stages:

in the first stage the Cr6+ ion was reduced with iron (II)

salts, after which the chromium ion was precipitated by

adding CaO to the solution. The investigations were con-

ducted at temperatures between 291 and 295 K. Model-

ling solutions and wastewater from the band trench “A” in

the “Alwernia” Chemical Works were used in our in-

vestigations.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

180

3.1. Attempts at Removing Chromate Lons

by Incorporating Them into the

Ettryngite Structure

3.1.1. Attempts to Produce Chromic-Aluminium

Ettryngite

The first experiments were carried out with 250 cm3 of

model solutions prepared from potassium or sodium

chromate, with concentrations of Cr6+ equal to 10, 50 and

100 g m-3. These concentrations corresponded to the Cr6+

concentrations in the wastewater in the band trench “A”

during the period of January to Mar ch 1998 [7].

The followin g method was used:

Calcium oxide in the so lid state was add ed to solution s

containing Cr6+ in the above-mentioned concentrations.

The solutions were then mixed for two h ours, after which

Al(NO3)39H2O was introduced. In some investigations

solid NaOH was added in order to obtain higher basicity.

Theoretically , the m olar ratio of each component, i.e. CaO,

Al3+ and CrO42+ in ettryngite should be 6:2:3. In the initial

investigations an ettryngite structure was formed when

CaO and Al(NO3)39H2O were added in excess by about

20-30 % (Figure 3(a)). However, the concentration of

Cr6+ ions in the filtrate still exceeded the permitted con-

centrations (<0.1 ppm), although it was possible to re-

move up to about 99 % of CrO42- ions (Table 1).

(a)

(b)

(c)

(d)

(e)

Figure 3. Results of the X-ray analyses of precipitates from

the purification of the infiltrates from chromic heaps. (a) Pre-

cipitate consisting of chromic-aluminium ettryngite; (b) Pre-

cipitate consisting of lead chromate; (c) Sample of the solid

phase a fter the reduction of chromium (VI) t o chromium (III )

with simultaneous precipitation of the latter; (d) Sample is

desiccated at a temperature 378 K; (e) Sample of the solid

phase precipitated with the ash from fluidised bed instead of

calcium hydroxide.

Results of analyses reported by Kania [7] show that the

wastewater from the band trench “A” also contains sul-

phates (SO42-), and their molar ratio to CrO42- ions varies

from 5:1 to 10:1. No information was found in the lit-

erature about differences in properties between the et-

tryngite c ontai nin g SO42- or Cr O42- and from the results of

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

181

our initial investigations it can be concluded that their

properties are similar. Therefore, the quantities of added

CaO and aluminium salts were referred to the sum of

moles of CrO42- and SO42- in s ubseque nt experi ments. The

results of investigations carried out on model solutions

containing 104 mg of Cr6+ and 960 mg of SO42- in one litre

of solution have been summarised in Table 2 and the

results of investigations conducted on the wastewater

from trench “A” are presented in Ta ble 3. In none of the

above-mentioned investigations (with the exception of

two-Table 1, No. 3 and 4), results below 1 g m-3 Cr 6+ were

obtained. Using large excesses of added aluminium salts

and calcium oxide only resulted in concentrations of Cr6+

in the range of 1 g m -3 (Table 3).

When aluminium salts were added in excess of amounts

theoretically needed for ettryngite formation, aluminium

appeared in the filtrate. This was a result of the high pH of

the solution, at which aluminium exists in the AlO2- form .

3.1.2. Attempts to Produce Iron-Chromic

Ettryngite

Several experiments were conducted to synthesise a

compound composed of 3CaOFe2O32CaSO4nH2O, in

which aluminium was substituted by the less amphoteric

iron (III) (some authors are of the opinion that iron (III) is

not even amphoteric [17]). The influence of iron (III)-

added as ir on (III) chloride- was tested on a m odel solution

containing 104 mg of Cr6+ and 960 mg of SO42- in one litre.

The objective of the investigations was to determine the

most economic ratio of calcium oxide and iron chloride

added in relation to the sum of the ions of CrO42- and SO42-

for the effective removal of Cr6+ ions. The experiments

were also conducted on industrial wastewater containing

38.26 g m-3 of C r6+. All investigations were conducted in a

manner sim ilar to the previous ones: a we ighed am ount of

CaO was added to the Cr6+ solution and the solution was

mixed. After 1 to 2 hours FeCl36H2O was ad ded and th e

solution was mixed for a further 0.5 to 1.0 hours. The

results of the investigations obtained with the model so-

lutions are given in Table 4, and the results obtained with

wastewater are presented in Table 5. Some improvement

of filtrate quality can be observed. At a molar ratio of CaO

and FeCl36H2O to the sum of chromates and sulphate

equalling 20:1 and 3:1, respectively, the removal of the

Cr6+ ions from the solution is about 94 %.

3.2. Precipitation of Chromate Lons in Form

of Sparingly Soluble Compounds

According to Ufimciewa and Smietanic [19], lead chro-

mate is a sparingly soluble compound with a solubility

product of 1.8  10-14. Thus, in the presence of lead the

precipitation of C rO42- wi th subseque nt decre ase of Cr 6+ to

values below 1 g m-3 Cr6+ is expected. The tests with

model solutions rendered the expected results. The model

solution contained 0.194 g of K2CrO4 and 1.62 g of

Na2SO410H2O in 500 cm3 of water, which corresponded

to concentrations of Cr6+ ions equal to 104 g m -3 and SO42-

ions equal to 960 g m-3. Solid PbCl2 was added to the

solution. The molar ratios of Pb2+:Cr6+ were 1.5:1, 2.0:1

and 2.5:1. The sol ution was then mixed for two ho urs, after

which the coagulant was introduced. Separating the solid

phase from the solution did not present any difficulties.

Filtrate analysis showed Cr6+ concentrations of 0.99, 0.27

and 0.025g m-3 for the Pb2+: Cr6+ ratios 1.5:1, 2.0:1 and

2.5:1, respectively. Further experiments were conducted

with wastewater from the trench “A”, which had a Cr6+

concentration of 17 .28 g m-3. The molar ratios of Pb2+: Cr6+

were 1.5:1, 2.0: 1 and 2.5:1 . T he best resul ts were obt ained

when 2.5 moles of lead chloride were introduced per 1

mole of Cr6+ (about 97% of Cr6+ removal ).

The quality of the filtrate is also influenced by its pH.

The pH of the wastewater is weakly alkaline and under

these conditions the formation and precipitation of lead

hydroxide takes place (solubility product equal to 1.1 

10-20) thus reducing the participation of lead (II) in the

formation of lead chromate, which results in an incomplete

removal of CrO42- from the solution. Under acidic conditions

(pH < 4) Cr6+ ions are still present in the so lution. There-

fore, the most profitable precipitating conditions are ob-

tained at a pH between 4 and 6 (acidification of the

wastewater with 2-3 dro ps of conce ntrated muriatic acid).

However, the presence of SO42- in the wastewater causes

the precipitation of Pb2+ ions as hardly soluble lead sul-

phate (Ir =1.610-8), thus reducing the efficiency of this

method. The results of the experiments are presented in

Table 6. T he best treatment results (Cr6+ c ontent belo w0.1

ppm) are obtained when molar ratio Pb2+/Cr6+ was 2.5: 1

and pH was between 5.0 and 6.1. X-ray analysis shows

(Figure 3(b)) that the precipitate consists of lead chro

mate (PbCrO4) and of another, not precisely identified,

solid phase, probably Pb4O3SO4H2O.

3.3. The Reduction of Cr6+ to Cr3+ in Alkaline

Environment with Simultaneous

Precipitation of Cr(OH)3

The reduction of Cr6+ to Cr3+ with simultaneous precipi-

tation of the latter can be described by the following

equation:

Na2CrO4+2Ca(OH)2+3FeSO4+8H2O=Cr(OH)3+3Fe(OH)3

+2CaSO42H2O+Na2SO4 (2)

To determine the optimum quantities of iron (II) sul-

phates and calcium hydroxide, the experiments were

initially conducted with model solutions. The solution

simulating was t ewate r co ntai ned potassium chromate and

sodium sulphate in concentrations corresponding to 104

mg Cr 6+ and 960 mg of SO42- in one litre of the solution.

The added components are subsequently only referred to

as Cr6+. In a first at t empt calcium hydr oxide was added to

the solution in a molar ratio of Ca(OH)2 to Cr6+ of 25:1.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

182

Table 1. Results of exper im ents to optim ise th e q ua ntit ies of C aO and Al(NO3)39H2O necessary to remove chromate ions from model

solutions (initial volume 250 cm3) by formation of chromic-aluminium ettryngite.

No. Initial

concentration of Cr6+

[g m-3]

Quantity of

added compounds

[g]

Molar ratios Filtrate characteristics

CaO

Al(NO3)39H2OCaO/Cr6+ Al3+/Cr6+ Cr6+

[g m-3]Al3+

[g m-3]CaO

[g m-3] pH NaOH

addition

[g]

pH after

NaOH

addition

100

0.088

0.176

0.220

0.440

0.188

0.376

0.470

0.940

31 : 1

63 : 1

16 : 1

10 : 1

20 : 1

5 : 1

3.30

3.22

0.11

0.92

1.21

1.15

1.08

1.26

58.0

35.0

24.5

12.0

16.0

52.0

11.0

3.5

4.2

31.0

78.4

19.6

234.0

204.4

447.0

2.8

10.70

10.88

11.37

11.09

10.98

11.03

11.17

10.77

0.2

0 2

0.2

0.4

11.75

11.78

11.93

11.86

11.71

11.04

11.98

12.00

Table 2. Results of experiments to optimise the quantities of CaO and AlCl36H2O necessary to remove chroma te ions from a model

solution contain ing 104 g m-3 Cr6+ and 960 g m-3 SO42- (initial volum e 250 cm 3) by formation of chromic-aluminium ettryngite.

No. Quantity of

added compounds [g] Molar ratios Filtrate characteristics

CaO

AlCl36H2O CaO/

(Cr6+,SO42-)Al3+/

(Cr6+,SO42-)Cr6+

[g m-3] Al3+

[g m-3] pH NaOH

addition

[g]

pH after

NaOH

addition

1.01

2.02

2.52

3.03

2.17

6 : 1

12 : 1

15 : 1

18 : 1

3 : 1

70.99

2.86

2.73

5.80

2.87

3.70

3755

3174

4375

3280

566

10.33

12.16

11.91

11.80

12.13

11.66

1.6

12.29

12.06

Table 3. Results of exp eriments to optimise the quantities of CaO and AlCl 36H2O necessary to rem ove chromate ions from wastewater

collected in trench “A” containing 17.28 g m-3 Cr6+, 264.0 g m-3 SO42- and 33.0 g m-3 Cl- (pH: 8.36, initial volume 250 cm3) by for-

mation of chromic-aluminium ettryngite.

No. Quantity of

Added compounds [g] Molar ratios Filtrate characteristics

CaO

AlCl36H2O CaO/

(Cr6+,SO42-)Al3+/

(Cr6+,SO42-) Cr6+

[g m-3] CaO

[g m-3] pH Cl-

[g m-3]

0.20

0.22

0.40

1.29

1.94

1.29

1.94

0.47

0.94

2.82

0.94

2.82

0.56

0.93

1.86

1.08 : 1

1.2 : 1

2.2 : 1

3 : 1

5 : 1

10 : 1

11 : 1

22 : 1

65 : 1

22 : 1

65 : 1

30 : 1

45 : 1

30 : 1

45 : 1

10.0

7.6

4.8

5.8

3.4

2.79

2.70

2.74

1.18

1.17

1180

1160

1420

460

960

1614

1954

2183

3980

3855

10.20

10.90

11.30

11.93

12.00

12.53

12.86

12.71

10.26

12.07

360

440

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

183

Table 4 . Results of experiments to optimise the quantities of CaO and FeCl36H2O necessary to remove chromate ions from a model

solution containing 104 g m-3 Cr6+ and 960 g m-3 SO42- (initial volume 250 cm3) by formation of chromic-iron ettryngite.

No. Quantity of added

compounds [g] Molar ratios Filtrate characteristics

CaO

FeCl36H2O CaO/

(Cr6+,SO42-)Fe3+/

(Cr6+,SO42-) Cr6+

[g m-3] CaO[g m-3] Fe3+

[g m-3] pH

1.68

3.36

5.04

1.68

3.36

1.68

3.36

5.04

2.52

3.36

5.04

1.68

3.36

5.04

0.81

1.62

2.43

3.24

4.05

10 : 1

20 : 1

30 : 1

10 : 1

20 : 1

10 : 1

20 : 1

30 : 1

15 : 1

20 : 1

30 : 1

10 : 1

20 : 1

30 : 1

1 : 1

2 : 1

3 : 1

4 : 1

5 : 1

20.30

19.42

17.99

4.08

4.93

4.36

5.17

3.68

1.13

3.95

3.18

1.02

1.62

19.83

1.25

1.07

1.24

2182

2416

2468

2311

2423

2971

2782

3672

3073

3681

4105

3679

4979

5087

5045

5549

5434

< 0.01

0.05

0.17

0.28

0.02

0.18

< 0.01

0.18

< 0.01

0.30

0.08

< 0.01

12.05

12.22

12.31

12.27

12.39

11.73

12.27

12.25

12.42

12.35

12.43

12.48

12.25

11.62

12.33

12.30

12.28

Table 5. Results of expe riments to optimis e the quantities of CaO and FeCl36H2O necessary to r emove chromate ions from wastewater

collected in trench “A” containing 38.26 g m-3 Cr6+, 407.0 g m-3 SO42- and 43.0 g m-3 Cl- (initial pH: 7.97, initial volume 250 cm3) by

formation of chromic-iron ettryngite.

No. Quantity of added

compounds [g] Molar ratios Filtrate characteristics

CaO FeCl36H2O CaO/

(Cr6+,SO42-)Fe3+/

(Cr6+,SO42-) Cr6+

[g m-3] Ca2+

[g m-3] Fe3+

[g m-3] pH

0.697

1.046

1.395

1.01

10 : 1

15 : 1

20 : 1

3 : 1

3.66

3.77

2.38

2.34

2.08

2.23

1715

997

2464

2410

2492

2558

0.02

0.14

0.03

0.02

11.72

11.63

12.53

12.26

12.40

12.37

The molar ratio of FeSO47H2O to Cr6+was 6:1. After

addition of calcium hydroxide, the solution was mixed

for two hours, then solid iron (II) sulphate was also in-

troduced. After a few minutes the formation of the solid

phase was noticed, and after another two hours and addi-

tion of a coagulator a separation of the solid phase from

the solution was observed. The solution was clear and

colourless. The content of Cr6+ in th e solution was barely

detectable. Using model solutions, further experiments

were conducted to determine the smallest quantities of

iron sulphate and calcium hydroxide necessary to de-

crease the “solutions” Cr6+ concentrations below 0.1

ppm. The order in which iron (II) sulphate and calcium

hydroxide were added were also changed, but it did not

affect the results when the molar ratio iron (II) sulphate

to chromium (VI) was equal to or greater than 5.5. At

lower ratios it seems more effective to introduce

FeSO47H2O first. Also the quality of iron (II) sulphate

influences the quantity of iron (II) sulphate necessary to

remove Cr6+. When ash containing the required quantity

of CaO was used instead of calcium hydroxide, a very

good solid phase separation was achieved without the

use of a coagulator. The results of the investigations are

presented in Table 7. At molar ratio of Ca(OH)2 to Cr6+

and Fe3+ to Cr6+ equal to or higher than 20:1 and 5.4:1,

respectively, the concentration of the Cr6+ ions in the

filtrate is below 0.01 ppm, which is lower than required

by the Polish and European regulations.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

184

Table 6. Results of experiments to optimise pH and Pb2+/Cr6+

ratio in order to remove chromate ions from wastewater col-

lected in trench “A” containing 264.0 g m-3 Cr6+ and 33.0 g m-3

SO42- (initial pH: 7.28, initial volume 250 cm3) by precipitation

as lead chromate.

No. Molar ratio

Pb2+/Cr6+ pH after

correction Filtrate

pH Filtrate content

[g m-3]

Cr6+ Pb2+

1.5 : 1

2.0 : 1

2.5 : 1

4.0

5.4

6.0

4.0

5.0

6 0

4.3

5.0

6.1

8.72

8.68

8.34

4.800

0.470

1.440

2.110

1.220

1.010

0.066

2.710

0.910

0.092

0.074

2.700

0.258

< 0.2

4.57

Further investigations were conducted with wastewa-

ter from trench “A”. Calcium hydroxide was added to

the solution in molar ratios of Ca(OH)2 to Cr6+ of 15:1,

20:1,27:1 and 68:1. The molar ratios of Fe3+ to Cr6+ were

3.5:1, 4:1, 5.5:1, 6:1, 7.5:1, 13.3:1 and 15:1. After the

addition of calcium hydroxide, the solution was mixed

for two hours, then solid iron (II) sulphate was intro-

duced. After a few minutes, without addition of a co-

agulator, the formation of the solid phase was noticed

and after another two hours a separation of the solid

phase from the solution was observed. The solution was

clear and colourless. The results of these investigations

are presented in Table 8. The experiment conducted

with the natural wastewater proved that by adding cal-

cium hydroxide and iron (II) sulphate in molar ratios of

Ca(OH)2 to Cr6+ and Fe3+ to Cr6+ equal to or higher than

20:1 and 4:1, respectively, the Cr6+ concentration of the

wastewater can be decreased to bel o w 0. 1 pp m.

The results of the X-ray analysis of the precipitates

showed that the solid phase, which had formed had an

amorphous character with some ettryngite traces (Figure

3(c)). The same sample, having been desiccated at a

temperature of 378 K still retained its amorphous char-

acter (Figure 3(d)). According to the chemical reaction

presented above on e can expect the presence of iron (III)

and chromium (III) hydroxides and gypsum in the solid

phase. Calcium hydroxide is also expected to be present

due to the high quantities of Ca(OH)2 which were added

to the solution. The fact that the X-ray analysis could not

confirm the presence of calcium compounds, especially

of gypsum, which shows peaks even in small concentra-

tions, indicates either their absence (which is very

unlikely), or the creation of compounds of the spine type,

which can be amorphous. This problem will be the object

of further investigations. Introduction of the ash from

fluidised hearths instead of calcium hydroxide, led to

formation of the components of ash: SiO2 and Fe2O3, in

addition to the ettryngite and the amorphous phases

(Figur e 3(e)). Gypsum was not detected either, in spite of

the fact that the anhydrite is detectable in ash by X-ray

radiography. In this case CaO and SO42- had been incor-

porated in the e ttryngite structure. The calci um carbonate,

which appeared on the X-ray graphs, was a secondary

product, caused by carbonisation.

4. RECAPITULATION AND

CONCLUSIONS

The geophysical investigations show that the pollutants

from chromic heaps are drained by the Regulanka River

and carried to the Vistula River. The hydrogeological

conditions also ind icate, that in the case of an attempt to

regulate the river Regulanka and tighten its river-bed, the

polluted water is going to migrate to the Vistula through

the permeable sediments on the bo ttom of the Regu lanka

River valley.

Assuming that all the infiltrates could potentially mi-

grate to the Vistula River, the possibility of draining the

polluted wate rs and cleanin g them has b een recognized a s

an option to improve the environment. Decisions were

taken to build the two trenches “A” and “C” and two

draining well s “B” and S- 25 in 1 998 an d at t he be gi nni ng

of 1999. At the same time the initial plan to regulate the

Regulanka riverbed along the distance of 1.5 km, had

been given up.

The quantity of drained and purified infiltrates, coming

from these trenches and draining wells in 1999, was av-

erage 900 m3 per day.

Results of investigations into the purification of infil-

trates containing Cr6+ demonstrated t hat such wastewaters

could be efficiently purified to chromium concentrations

below 0.1 ppm using the following methods:

1) Precipitation of CrO42- as lead chromate (PbCrO4)

using lead (II) salts. The removal of CrO42- by precipita-

tion with lead salts should take place at a pH of 4 to 5.

At a lower pH, Pb2+ ions can be observed in solution

which at a higher pH, on the other hand, an increased

consumption of lead salts is observed, since lead pre-

cipitates as hydroxide (Pb(OH)2) as a result of hydroly sis.

The best results were obtained when the molar ratio of

Pb2+ to Cr6+ was to 2.5:1.

2) Reduction of Cr6+ with iron (II) salts and precipita-

tion as chromium hydroxide. The most economic com-

bination of reagents assuring the complete precipitation

of Cr6+ in this reaction is obtained when the molar ratio of

Fe2+ to Cr6+ is 4.5:1 and that of CaO to Cr6+ is 20:1. Such

ratios guarantee a complete removal of all Cr6+ ions from

the solution, even when part of the iron (II) salt is oxidised.

With the method based on the incorporation of CrO42- into

an ettryngite compound, it was possible to remove 94 % of

Cr6+, although not to concentrations below 0.1 ppm, when

iron (III) salt and calcium oxide had been added in large

quantities.

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

185

Table 7. Results of experimen ts to opti mise the co ndition fo r the r emova l of chromate ions from a model solu yion co ntaining104 g m-3

Cr6+ and 960 g m -3 SO42- (initial vo lume 250 cm 3) b y redu c tion of Cr6+ to Cr3+ and precipitation o f th e la tter. The lett er “P ” next to the

sequence number indicates that ash was used instead of calcium oxide. The quantities of ash and corresponding quantities of CaO are

given in the second column (No. 9, 10 and 15). The asterisk symbol “*” next to a compound indicates that the compound was intro-

duced to the solution in the first place.

No. Quantity of

added compounds [g] Molar ratios Filtrate characteristics

CaO FeSO47H2O CaO/Cr6+ Fe3+/Cr6+ Cr6+

[g m-3] CaO

[g m-3] SO42-

[g m-3] Fe3+

[g m-3] pH

10P

15P

0.42*

0.56*

0.42

0.56

0.56*

0.56

2.8/0.56*

0.56*

0.56

0.56*

0.56

2.8/0.56*

0.701*

0.487

0.626*

0.626

0.626*

0.626

0.750

0.750*

0.750

0.750*

0.750

0.834

15 : 1

20 : 1

15 : 1

20 : 1

25 : 1

3.5 : 1

4.5 : 1

4.5 :1

4.5 : 1

5.4 : 1

6 : 1

2.36

1.25

0.79

< 0.01

0.04

0.80

< 0.01

1.50

0.05

0.01

< 0.01

587.6

725.3

605.0

650.5

598.3

924.3

785.1

678.0

967.4

725.3

658.9

692.3

647.7

694.6

1739.4

1706.3

2078.9

1675.2

1982.7

1638.6

1878.7

1938.4

1884.7

1882.1

1920.1

1892.4

1789.7

0.04

0.01

0.05

0.07

0.02

0.04

0.02

11.82

11.94

12.00

11.68

11.82

11.92

12.13

11.85

11.82

11.92

12.05

12.02

11.84

11.82

12.02

12.03

Table 8. Results of experiments to optimise the conditions for the removal of chromate ions from wastewater collected in trench “A”

by reduction of Cr6+ to Cr3+ and precipitation of the latter (sample A: pH: 7.97, Cr6+: 38.26 g m-3, SO42-: 407.0 g m-3, Cl- : 43.0 g m-3,

initial volume 250 cm3; sample B: pH: 7.98, Cr6+: 12.99 g m-3, SO42-: 247.0 g m-3, Cl- : 43.0 g m-3, initial volume 250 cm3) . The letter “P” next

to the sequence n umber indicat es that as h was used inst ead of ca lciu m oxi de. T he qu antities of as h and t he corre spo ndin g quan ti ties of

CaO are given in the second column (No. A6 and B7). The asterisk symbol “*” next to a compound indicates that the compound was

introduced to the solution in the first place.

No. Quantity of

added compounds [g] Molar ratios Filtrate characteristics

CaO FeSO47H2O CaO/Cr6+ Fe3+/Cr6+ Cr6+

[g m-3] CaO

[g m-3] SO42-

[g m-3] Fe3+

[g m-3] pH

A6P

B7*P

B8*

B9*

B10*

B11*

0.697

0.280

0.156

0.206

1.03/0.206

0.35/0.07

0.052

0.070

0.052

0.070

0.765

0.680

0.384

0.307

0.281

0.077

0.069

0.070

0.061

68 : 1

27 : 1

15 : 1

20 : 1

15 : 1

20 : 1

15 : 1

20 : 1

15 : 1

13.3 : 1

7.5 : 1

6 : 1

5.5 : 1

4.4 : 1

4 : 1

3.5 : 1

< 0.010

0.031

0.080

< 0.010

0.020

0.049

1.140

0.037

1.620

0.310

1427.0

1109.0

803.0

793.2

679.0

692.7

130.0

174.7

75.0

65.0

110.0

1497.0

1573 3

1579.5

1612.4

0.03

0.07

0.10

0.08

0.03

0.05

0.03

0.04

0.02

0.10

0.03

12.50

11.82

9.61

10.92

11.47

11.65

7.96

7.67

8.09

7.79

8.83

Amongst the methods tested to remove Cr6+ from

wastewater the most effective is the method of reducing

Cr6+ with the use of iron (II) sulphate in an alkaline en-

vironment. It is now being used on an industrial scale.

The molar ratio of FeSO47H2O and CaO to Cr6+ should

be equal to 4.5:1 and 20:1 in order to render positive

results. After the addition of each reagent, the solution

should be mixed for a minimum of 2 hours. When the

reaction is almost finished, a coagulant should be added

for a faster and more complete separation of the solid

Z. Kowalski et al. / HEALTH 2 (2010) 177-187

186

Figure 4. Block dia gram sh owing the removal p rocedure of chro -

mium (VI) from wastewater in the “Alwernia” Chemical Works.

phases from the solution. When ash is used, no coagulant

is necessary. The decisive argument for selecting this

method was the fact that it is possible to use precipitates

from the infiltrates as raw material for the production of

sodium chromate [33].

The following quantities of chemicals are necessary for

the wastewater purification:

For each kmol Cr6+ it is necessary to add 4.5 kmols

FeSO47H2O and 20.0 kmols CaO. For each kg (tonne)

Cr6+ it is necessary t o ad d 24.06 kg (ton nes) FeSO47H2O

and 21.54 k g (tonnes) CaO. 10 00 m3 of waste f rom trench

“A” with concentration 12.99 g m-3 Cr6+ (see Table 8)

contained for example 0.25 k mol, i.e. 12.99 kg Cr6+.

The quantities of the main produc ts received from the

precipitat ion of 1 kmol Cr6+ are 1.0 kmol Cr(OH)3 and 4.5

kmols Fe(OH )3. The precipitation of 1 kg (tonnes) of Cr6+

results in the formation of 1.98 kg (tonnes) of Cr(OH)3

and 3.08 kg (tonnes) of Fe(OH)3. These quantities were

produced for example from 4000 m3 above-mentioned

waste from trench “A”.

A schematic diagram of this process is presented in

Figure 4.

The reduction and precipitation of Cr6+ from waste-

water can be conducted at ambient temperature even in

winter since the introduction of CaO causes a rise in

temperature. The addition of the chemicals should take

place under intensive mixing with the stirrers being in-

stalled in such a way that the reagents can be evenly

distributed throughout the container.

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