SPECIAL FEATURE: ORIGINAL ARTICLE The 7th International Conference on Waste Managementand Technology (ICWMT) 2012

Novel pilot-scale washing process and equipment for removingCr(VI) from contaminated soil

Xiao Lin • Ming Gao • Hongbin Cao

Received: 3 December 2012 / Accepted: 16 June 2013

� Springer Japan 2013

Abstract In this study, a novel horizontal rotating soil

washing process and equipment were developed and tested

for pilot-scale remediation of soils from a site polluted by

chromium ore process residue. Operating parameters,

including cylinder rotational velocity, cylinder tilt angle,

heating temperature and liquid/soil ratio, were investigated.

The Taguchi method was used for the experiment design,

and the standard L16 orthogonal array with four parameters

and four levels was selected for optimising the operating

parameters. Optimal removal efficiency was achieved at

cylinder rotational velocity of 2.5 rpm, cylinder tilt angle

of 2.6�, heating temperature of 200 �C and liquid/soil ratio

of 8. The efficiency of citric acid as an extractant in the

novel process was compared with that of water. The

analysis of the residual Cr(VI) concentration of the soil

shows that citric acid could efficiently remove 22.89 %

more Cr(VI) than water in one-stage washing. The residual

Cr(VI) concentration in the soil after the three-stage

washing is as low as 26.16 mg/kg, which meets the

screening levels for soil environmental risk assessment of

sites in Beijing City (30 mg/kg). Further study is currently

underway to optimise the novel process and equipment for

commercial-scale use.

Keywords Soil washing � Chromium removal �Pilot scale � Citric acid

Introduction

Hexavalent chromium [Cr(VI)] is toxic and carcinogenic to

mammals [1]; it is widely used in many industrial pro-

cesses such as those involved in the chromium salt industry

and electroplating. In China, Cr(VI)-contaminated soils,

which result from the improper disposal of chromium ore

process residues (COPR), are an environmental threat in

many regions. Solidification/stabilisation methods are

mainly used to reduce the mobility of contaminants in soils

[2]. Given that the metals are not removed from the soils,

the extractable fraction of the pollutants can leach from the

treated soils over a long period of time. Thus, ensuring the

long-term stability of immobilised metals is challenging,

and continuous monitoring of their potential release is

required [3]. Furthermore, effective soil treatment tech-

nologies should be developed to remove metals from soils.

Soil washing is the most commonly used treatment

technology for the remediation of metal-contaminated

soils. This technique is an ex situ treatment method and a

relatively inexpensive alternative for soil remediation [4].

In this method, the contaminated soils are excavated and

mixed with an extractant. The extractant could be water,

acid, an oxidising agent, a chelating agent or surfactant,

depending on the type of the contaminant. Compared with

other remediation technologies, soil washing via chemical

extraction methods presents several advantages, such as

rapid cleanup of a contaminated site, significant volume

X. Lin � M. Gao � H. Cao

National Engineering Laboratory for Hydrometallurgical

Cleaner Production Technology, Beijing, China

X. Lin � M. Gao � H. Cao (&)

Research Centre for Process Pollution Control, Institute

of Process Engineering, Chinese Academy of Sciences,

P.O. Box 353, Beijing 100190, China

e-mail: hbcao@home.ipe.ac.cn

X. Lin � M. Gao � H. Cao

Key Laboratory of Green Process and Engineering, Institute

of Process Engineering, Chinese Academy of Sciences,

Beijing, China

123

J Mater Cycles Waste Manag

DOI 10.1007/s10163-013-0161-6

reduction of contaminated soils, and the possibility of

metal recovery by extracting the dissolved metals from the

washing solution [3]. This method originated from the

mineral processing industry, and several treatment units are

combined for a complete and efficient soil washing process

[5]. The main treatment unit of the soil washing system is

the extraction unit, where the contaminated soil is reacted

with an extractant solution to ensure the efficient transport

of contaminants from the soil phase into the liquid phase.

Soil washing in the extraction reactors based on a batch

process involves sufficient contact time and stringent

physical treatments. This process is harsh for soil flora and

can deteriorate the physical quality of the soil [6].

Rotary kilns have been widely used in the inorganic

chemistry and metallurgy industry [7]. This device is a key

element in the ore roasting processes during production of

nonferrous metal, cement and lime. Rotary kilns are well

suited for the pyrolysis of solid waste [8]. In this study, a

novel process based on rotary kiln equipment was developed

and used in the soil washing process. The equipment was

designed to work with a low liquid/soil ratio, continuous

processing and mild conditions. Compared with the batch

equipment, the novel equipment is more adaptable to differ-

ent conditions and more tolerant to soil debris. While rotating,

the rotary kiln induces interaction forces such as friction or

shear via impact of the contaminated soil on the inner wall or

collisions among the contaminated soils. Furthermore, sev-

eral blades were installed inside the kiln to augment the forces

by dropping the contaminated soil. The treatment rate was

controlled based on the kiln angle and length, whereas the

other forces were controlled based on the kiln revolution.

Based on preliminary experiments, four experimental

parameters (cylinder rotational velocity, cylinder tilt angle,

heating temperature and liquid/soil ratio) were selected as

major factors affecting treatment results. The Taguchi

method was used to determine the optimal levels of these

parameters to increase the removal of Cr(VI) from Cr-

contaminated soils using water and citric acid solution as

extractant for a pilot-scale soil washing process. Multiple-

stage washing was also tested for enhancing the removal of

Cr(VI) from contaminated soils. The extracted Cr(VI) was

removed effectively from wastewater. Fe(II) salt was

consumed for Cr(VI) reduced to the less toxic, immobile

Cr(III) form. The Cr(III) can then be precipitated as

Cr(OH)3 when the pH is between 8 and 10.

Materials and methods

Soil

Contaminated soil samples were collected from 1.2 to

2.2 m below the ground surface around a COPR pile in

Henan, China. The soil samples were air-dried at room

temperature (20–30 �C). The soils used for laboratory

testing were ground and passed through a 2 mm mesh sieve

to remove stones and large particles. The soils were then

stored in a moisture-proof container at room temperature.

Soil pH was determined with a pH meter using a soil to

water ratio of 1:2.5. Organic compound percentage was

determined by roasting the samples at 350 �C for 5 h [9].

The particle size distribution of the samples was mea-

sured with a laser particle size analyser (LS13320, Beck-

man Coulter, USA). Cr(VI) content of the soil was

determined by alkaline digestion pre-treatment and color-

imetric determination with diphenylcarbazide at 540 nm

following Environmental Protection Agency (EPA)

Method 3060A and EPA Method 7196A, respectively [10,

11]. Total Cr, Ca, Mg, Fe and Al were determined using

inductively coupled plasma optical emission spectrometry

(Optima 7000DV, Perkin Elmer, USA) after acid digestion

(HF/HClO4/HNO3). The quality of the digestion process

was also controlled by the analysis of GBW07439 certified

standard reference material of soil. The properties of the

soil particles are listed in Table 1. Cr in the contaminated

soils was mainly of the hexavalent form.

Washing equipment

As shown in Figs. 1 and 2, the soil washing equipment

comprised a stainless steel rotating cylinder, an external

heating system, a soil feed system and an extractant feed

system. The rotating cylinder had a length of 3.8 m and an

inner diameter of 0.35 m. Twelve flights were interiorly

arranged. The angle of the rotating cylinder can be man-

ually adjusted between 2.5� and 6� so that the discharge

end is at a lower level than the feed end. The outer wall of

the rotating cylinder was heated by a furnace equipped with

heating resistors. The wall temperature can be approxi-

mately measured by thermocouples. The rotating cylinder

was fed with the contaminated soils through a loading

chute and feed screw located beneath the hopper. The feed

screw and cylinder were rotated by two variable frequency

electric motors. A separate control box situated next to the

equipment was used for regulating operational parameters

such as heating temperatures and rotating speed of the

cylinder and feed screw.

Washing procedure

A preheating stage is required for the rotating cylinder

before soil washing. After the cylinder was heated, the soils

were fed by the feed system, and the extractant was

simultaneously added into the rotating cylinder by the

distributor. The ratio between the extractant flow rate and

soil feed speed could be adjusted to form a designed liquid/

J Mater Cycles Waste Manag

123

soil ratio for soil washing. As the cylinder rotated, the inner

flights picked up the mixture and dropped it again to mix

the soil and extractant. Equilibrium conditions were

achieved after 10 min of mixing. Then, the resulting soil

slurry was collected for determining Cr(VI) removal effi-

ciency. The soil was separated by centrifugation at 800 rpm

for 1 min and then filtered through a 0.45 mm membrane.

The Cr(VI) concentration of the supernatant was analysed

using a colorimetric method according to EPA Method

7196A [11]. All tests were performed in duplicate to ensure

the reproducibility of the test results. Finally, the removal

of Cr(VI) was calculated using the following equation:

Cr VIð Þ removal %ð Þ ¼ CrðVIÞ mass in supernatant ðCLVLÞInitial CrðVIÞ mass in soil ðCSMSÞ� 100;

ð1Þ

where CL and CS are the concentrations of Cr(VI) in the

supernatant (in mg/L) and soil (in mg/kg), respectively; VL

is the volume of supernatant (in L); and MS is the dry mass

of the soil (in kg).

Experimental methods

The Taguchi method was used to optimise the operating

conditions of the soil washing equipment, achieve max-

imum Cr(VI) removal and obtain insights into the relative

importance of each factor. This method is very precise

and reliable and can be used in different applications in

science and engineering. This method provides a simple,

efficient and systematic approach for optimising designs

for performance, quality and cost. The parameter design

is the key step in the Taguchi method to achieve high

quality without an increase in cost. Four operating

parameters, namely, cylinder rotational velocity, cylinder

tilt angle, heating temperature and liquid/soil ratio, are

the most effective factors affecting the final Cr(VI)

removal efficiency. These parameters were varied at four

levels, as shown in Table 2. The ranges of the factors

used were determined by reviewing relevant studies such

as Ref. [12] and considering the operation and equipment

limitations.

Results and discussion

The orthogonal array selected was L16 (45), which had 16

rows corresponding to the number of experiments. The

array had 15� of freedom and can cover five design

parameters. Only four parameters were used, and the fifth

column (E column) of the array was left empty (Table 3).

In all the Taguchi tests, the soil feed speed was fixed at

70 kg/h, and the liquid/soil ratio was adjusted by changing

the extractant flow rate at different levels. Tap water was

Table 1 Physical and chemical properties of the soil used in this experiment

Organic matter

(%)

Sand

(%)

Silt

(%)

Clay

(%)

Organic matter

(%)

pH

(H2O)

Cr(VI) (mg/

kg)

Total Cr

(mg/kg)

Ca

(mg/kg)

Mg

(mg/kg)

Fe

(mg/kg)

Al

(mg/kg)

5.03 15.09 83.2 1.71 5.03 9.05 2081 2227 9619 8442 3580 1512

Fig. 1 Schematic of the

designed soil washing

equipment: 1, 2 electric motors

for the screw and cylinder,

3 feed screw, 4 pump, 5 liquid

distributor, 6 rotating cylinder,

7 electric heating device and

8 thermocouple

Fig. 2 Photograph of the equipment

J Mater Cycles Waste Manag

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used as the extractant in the experiment. The results of the

Taguchi tests are shown in Table 3.

As shown in Table 3, Cr(VI) removal varied from

46.01 % (Run No. 14) to 84.83 % (Run No. 6). The

influences of the factors on Cr(VI) removal were investi-

gated using difference analysis (Fig. 3). The effect of

liquid/soil ratio on Cr(VI) removal was the most signifi-

cant, and the heating temperature had the least influence.

Higher removal efficiency can be achieved with a larger

liquid/soil ratio and smaller cylinder tilt angle. Based on

the obtained results and previous studies [13], removal

efficiency increases with the liquid/soil ratio, which is

attributed to the good water solubility of most Cr(VI)

compounds. A decrease in the cylinder tilt angle contrib-

utes to a higher Cr(VI) removal because a smaller cylinder

tilt angle leads to a longer residence time for the soil–

extractant mixture, which enhances the interaction between

the soil and extractant and promotes the dissolution

process. A higher temperature in soil washing increases

heavy metal removal [14], since many Cr(VI) salts are

soluble in hot water. However, in this study, a distinct

increase in Cr(VI) removal rate was not observed when the

heating temperature increased from 80 and

200 �C. Heat transfer may have been hampered by the

contact resistance between the soil–extractant mixture and

the wall of the cylinder. This assumption was verified by

measuring the temperature of the soil slurry in the dis-

charge end, and the results showed that the maximum

temperature was 50 �C in all the Taguchi tests.

To determine the optimal parameters and their confi-

dence levels for Cr(VI) removal, a statistical ANOVA was

performed. The F-value for each parameter is simply a ratio

of the mean of the squared deviations to the mean of the

squared error. Based on the ANOVA data presented in

Table 4, the F-value of the liquid/soil ratio (9.321) indicates

that it is the most influential control factor. The percentage

contribution of liquid/soil ratio (49.08 %) is the highest, and

the cylinder tilt angle (22.53 %) is the effective parameter.

Cylinder rotational velocity and heating temperature only

have slight influences (13.62 and 9.49 %, respectively). The

effects of these four factors on Cr(VI) removal are ranked as

follows: liquid/soil ratio [ cylinder tilt angle [ cylinder

rotational velocity [ heating temperature.

Based on the results of the Taguchi method and ANOVA,

the optimal parameters for Cr(VI) removal using the hori-

zontal rotating soil washing equipment are level 2 of cyl-

inder rotational velocity (2.5 rpm), level 4 of heating

temperature (200 �C), level 1 of cylinder tilt angle (2.6�) and

level 4 of liquid/soil ratio. Maximum Cr(VI) removal can be

achieved with these optimal conditions. The results of the

Taguchi method can be evaluated through experiment. A

confirmation experiment was performed with the obtained

optimal conditions using tap water as the extractant. The

experiment results show a maximum Cr(VI) removal effi-

ciency of 85.32 %, which indicates that the optimal factor

level combination in this experiment is valid.

To investigate the effect of citric acid which is widely

used in studies on heavy metal extraction from contami-

nated soils [15–18], soil washing experiment using 0.1 M

citric acid as extractant was performed with the optimal

conditions. The results show that the Cr(VI) removal rate is

88.68 %. The residual Cr(VI) concentration in the soil is

235.56 mg/kg, which is 22.89 % lower than that obtained

with water as the extractant (305.49 mg/kg). Based on the

literature [3], the mechanism of heavy metal extraction

using citric acid includes acid dissolution and metal com-

plexation. Citric acid is a chelating agent and a weak acid.

Therefore, heavy metal removal depends on dissolution

due to lowering of pH and formation of metal complexes.

The residual Cr(VI) concentration in the soil after one-

stage washing using 0.1 M citric acid as the extractant is

Table 2 Factors and their levels studied by the Taguchi method

Parameter Levels

1 2 3 4

A. Cylinder rotational velocity (r/min) 1.2 2.5 3.8 5.1

B. Heating temperature (�C) 80 120 160 200

C. Cylinder tilt angle (�) 2.6 3.7 4.8 5.9

D. Liquid/soil ratio (L/kg) 2 4 6 8

Table 3 Experimental design using the L16 orthogonal array and

experimental results for Cr(VI) removal rate

No. A B C D E Cr(VI) removal

rate (%)

1 1 1 1 1 1 71.75

2 1 2 2 2 2 66.80

3 1 3 3 3 3 69.80

4 1 4 4 4 4 83.06

5 2 1 2 3 4 78.47

6 2 2 1 4 3 84.83

7 2 3 4 1 2 69.45

8 2 4 3 2 1 68.22

9 3 1 3 4 2 69.80

10 3 2 4 3 1 74.98

11 3 3 1 2 4 67.72

12 3 4 2 1 3 66.86

13 4 1 4 2 3 49.60

14 4 2 3 1 4 46.01

15 4 3 2 4 1 78.97

16 4 4 1 3 2 84.52

For description of the levels, Letters A, B, C and D are the operating

parameters

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greater than the Beijing screening level of 30 mg/kg for

residential areas (DB11/T 811-2011) [19]. Therefore, a

multiple-stage washing process was designed and evalu-

ated. Multiple-stage washing was first conducted by

washing the soils with 0.1 M citric acid using the hori-

zontal rotating equipment. The soils were then dried and

washed again. The soil samples were collected after each

wash, and the residual Cr(VI) concentrations in the soil

samples were determined following EPA Method 3060A

[10]. The results of the multiple-stage washing test are

shown in Fig. 4. Although Cr(VI) removal increases with

liquid/soil ratio, the high water consumption and post-

treatment cost may not be practical for large-scale

application. Moreover, heating temperature has the least

influence on Cr(VI) removal. Therefore, to reduce water

and power consumption, multiple-stage washing was per-

formed under the following conditions: 2.5 rpm cylinder

rotational velocity, 30 �C heating temperature, 2.6� cylin-

der tilt angle, and liquid/soil ratio of 4. Figure 4 shows the

three series of washing tests conducted with the same

conditions. After the first washing, 82.30 % of Cr(VI) is

extracted. The second and third runs solubilised 76.77 and

69.41 % from the remaining amount of Cr(VI) after the

first run, leading to a cumulative removal rate of 98.74 %

after the third washing. The final residual Cr(VI) concen-

tration in the soil was 26.16 mg/kg after the third wash,

which is less than the screening level of 30 mg/kg. These

results indicate that the multiple-stage procedure is effec-

tive in removing Cr(VI) from the contaminated soils.

Moreover, the estimated power consumption for reducing

the volume of the contaminated soils to meet the screening

level using the horizontal rotating equipment is 20 kWh

per ton of processed soils.

Conclusions

The results of the pilot-scale study indicate that this novel

horizontal rotating soil washing equipment effectively

removes Cr(VI) from Cr-contaminated soils. The optimal

operating conditions were obtained using the Taguchi

method. The ANOVA results show that the liquid/soil ratio

and the cylinder tilt angle have more important roles in

Cr(VI) removal compared with other factors. The residual

Cr(VI) concentration in the soil obtained using citric acid

in one-stage washing (235.56 mg/kg) is lower than that

acquired using tap water (305.49 mg/kg). Three-stage

washing effectively enhances Cr(VI) removal rate. The

pilot-scale result of 26.16 mg/kg meets the Beijing

screening level of 30 mg/kg for residential areas, and the

operation cost is relatively lower than that of the traditional

process because of the higher washing efficiency and lower

water and power consumption.

Fig. 3 Effects of the factors on

Cr(VI) removal efficiency. For a

description of the levels, refer to

Table 2. Letters A, B, C and

D are the operating parameters

Table 4 Analysis of variance on Cr(VI) removal rate

Factor Sum of

sqrs. (S)

DOF

(f)

Variance

(V)

F test

(F)

Contribution

(%)

A 244.443 3 81.49 2.587 13.62

B 170.398 3 56.79 1.803 9.49

C 404.441 3 134.82 4.280 22.53

D 880.798 3 293.60 9.321 49.08

E (error) 94.493 3 31.49 1.000 5.26

Total 1794.606 15

Fig. 4 Multiple-stage washing of Cr-contaminated soils

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Acknowledgments This work was financially supported by the

National Natural Science Foundation of China (Grant No. 51178446).

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