removal of heavy metals from contaminated soils by washing with citric acid and subsequent treatment...
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Removal of Heavy Metals from Contaminated Soils by Washing with Citric Acid and Subsequent Treatment of Soil-washing Solutions
Lei SUN1, Han-qiao LIU1,2,3*, Guo-xia WEI1, Zhen-hua WU1, Wei YANG2 1School of Energy and safety Engineering, Tianjin Chengjian University, 300384, Tianjin, China
2Tianjin Technology & Engineering Center of Contaminated Sites Remediation, 300468, Tianjin, China
3School of Environment science and Technology, Tianjin University, 300072, Tianjin, China
Keywords: heavy metals; contaminated soil; washing; adsorption
Abstract: Soil washing experiments were carried out with citric acid as washing reagent for the
remediation of soils highly contaminated with heavy metals, then activated carbon was used in
absorption processing for leaching solution. In this study, the effects of the main operating variables
for removal of metals from soils were first discussed. The results showed that 36.% Pb, 47.74% Cu
and 61.88% Cd were removed from the contaminated soils by optimizing the washing parameters at
citric acid concentration 0.2 mol/l, mixing time 2 h, liquid-soil ratio 20 and solution pH=4,
respectively. In the adsorption experiments of leachates, the optimum conditions were found as
follows: solution pH=7, mixing time of 2 h, standing time of 60 min and activated carbon dosage of
1g/100ml.
Introduction
With the rapid development of industrialization and urbanization, soils contaminated by heavy
metals have become an environmental problem that cannot be ignored. According to statistics, soils
contaminated by heavy metals are about 20 million km2 which are about one-sixth of total arable
land in China, and the amount of grain polluted by heavy metals is as high as millions of tons every
year. Due to refractory, high accumulation and concealment of heavy metals in contaminated soils,
the pollutions are more and more serious coupling with increase of human activities [1]. There are
generally two fundamental technologies to remediate soils. The first technology is to immobilize
heavy metals into tightly bound solid matrix to minimize their migration. However, this technology
is not a permanent solution. Site reuse of soil is limited and long-term monitoring is generally
required. The second technology is to promote heavy metals mobility and migration to the liquid by
desorption and solubilization. This technology is a permanent solution, providing recycle of
remediated soils and improving land-use option in the future [2]. The soil washing as a kind of
chemical remediation method has some advantages of simple operation, low cost and little
secondary environmental pollution, so it has already been researched widely [3].
The choice of washing reagent is essential to the remediation technology. Washing reagents of
the heavy metals mainly include surfactants and acid washing reagents. Surfactant can be divided
into synthetic complexing agent such as EDTA, biosurfactant including rhamnolipids, sophorolipid,
surfactin and saponin. EDTA becomes generally available in the remediation of soils contaminated
with heavy metals because of its combination with most of the metal ions into the stability of
chelate and higher removal efficiency of heavy metals in the soil [4]. Ze [5] had studied the
operating variables in soil washing with EDTA. Nonetheless, EDTA also exist some problems in
expensive price, poor biodegradability and difficult recovery. In recent years, EDTA has been
replaced by biosurfactant in soil washing, but the biosurfactant production is dwindling, the
production cost is high [6]. Therefore, biosurfactant does not have an advantage in the
industrial-scale applications. Acid washing reagents mainly include inorganic acid solution (HCl
and HNO3), natural organic acid solution (citric acid, malic acid, oxalic acid, malonic acid, humic
acid and fulvic acid). Shuzo [7] had reported an acid-washing process on a laboratory scale to
Advanced Materials Research Vol. 937 (2014) pp 646-651Online available since 2014/May/07 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.937.646
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extract arsenic from contaminated soil. Inorganic acid can remove heavy metals from soils largely,
and its price is low, but it changes the soil physical and chemical nature easily to be acid after soil
washing, so it is more suitable for the severe pollution of the soil industrial wasteland. While natural
organic acid has a good removal capacity for heavy metals, a mild acid, good biodegradability and
no pollution to the environment, so its application prospect is very environmentally friendly. Elliott
[8] had proved that oxalic acid that belongs to natural organic acid solution, was more effective than
EDTA to remove the metals associated with Fe-Mn oxides of soil (attacks the hydrous oxides).
Recent report has shown that citric acid could remove Cd and Cu from contaminated soils
effectively, but low removal rate of Pb relatively.
Currently, achievements have been gained from research on single type metal contaminated soil
remediation with organic acid, but treatments of washing solution are reported rarely. Commonly
used treatment methods of washing solution include setting, chemical precipitation and activated
carbon absorption [9]. The purpose of this paper is to remove heavy metals from contaminated soils
by washing with citric acid and subsequent treatment of soil-washing solutions, and to determine
the optimal washing conditions including citric acid concentration, mixing time, liquid-soil ratio,
solution pH. The best adsorption conditions of activated carbon in washing solution treatment are
also discussed.
Materials and Methods
Soil Sample Preparation
Soil samples were collected from the greenbelt in Tianjin Chengjian University, which were
measured as weak acid by pH meter. And the purity of citric acid was greater than 99.5%. All
samples were air-dried, crushed and were then passed through a sieve of 200 meshes. After
screening, the fine soil samples were weighed 4kg respectively. According to the scale of the
moisture content of 30%, the solutions including 200 mg/L Cu2+ solution, 500 mg/L Pb2+ and 50
mg/L Cd2+ solution which were prepared by using copper sulfate, lead nitrate and nitric acid
cadmium drugs (analysis),were added into soil samples, respectively. After about 30 minutes, heavy
metals in soil were stirred to equilibrium, then put to the outdoor to air-dry for 96 h, placed in the
labeled bags for further use.
Washing
In the soil washing, a certain amount for contaminated soil was added in a 250 ml conical flask,
then stirred on the magnetic stirrer for a period of time after adding citric acid and in standing for
analysis. The experiments were carried out by varying different operating parameters such as citric
acid concentration, mixing time, liquid/soil ratio and solution pH, which were set as range of
0.01~0.4 mol/L, 1~12 h, 5~30, 3~9, respectively. One variable at a time was altered while keeping
other variables at constant. After washing, about 4 ml supernatant were taken out with pipette and
put into clean centrifugal tube, then centrifuged for 10 minutes to separate solid from liquid
completely by using a high speed centrifuge (RT-TGL-16G-2) which was set as the speed of 5000
r/min. The concentrations of heavy metals in the solution were measured by AFS-9700 atomic
absorption spectrometer. Mixed soil in conical flask was for suction filtration by a vacuum pump,
dried and then weighed. The samples were treated by MDS - 6 microwave digestions, then the
concentrations of heavy metal were measured by using AAS and the pH of the soil needed to be
measured. The experiments were repeated 3 times to achieve precise data for analysis and
comparison to choose the best washing reagent and leaching concentration.
Treatment of Soil-washing Solutions
In the activated carbon adsorption experiment, soil-washing solutions were divided into four on
average in the conical flask, where a certain amount of granular activated carbon was put (about
4mm in diameter, length of 5mm), then stirred by the magnetic stirrer. The experiments were
carried out under different operating conditions such as activated carbon dosage, mixing time,
Advanced Materials Research Vol. 937 647
incubation time and solution pH value, which were set as range of 0.2 ~ 1.2 g, 1 ~ 12 h, 30 ~ 120
min, pH value of 3 ~ 10, respectively. When a parameter changed, the other parameters remained
the same. Other operating methods and equipment were same as washing experiment.
Results and Discussion
Soil Washing
Influence of citric acid concentration
Different concentrations of citric acid were used and the washing results are shown in Fig.1. It
can be seen that with increase of concentrations of citric acid, the removal of Pb increased few
when the citric acid concentration was lower than 0.1 mol/L. With increasing of citric acid
concentration to 0.2 mol/L, the removal of Pb indicated a significant increasing, and its maximum
was 36.15%. The reason why the removal of Pb decreased with increase of concentrations of citric
acid was that the complexing reaction between citric acid and Pb was a reversible reaction and
when the concentration of complex increases to a certain degree, then the reaction reversed, causing
the efficiency of removal decreased. The removal of Cu reached to the maximum of 51.87% when
citric acid concentration was 0.1 mol/L. However, with the increase of citric acid concentration, the
removal of Cu decreased slightly and the removal of Cu reduced to 47.74% when citric acid
increased to 0.2mol/L. The change tendency of removal of Cd was similitude with that of Pb with
increase of concentrations of citric acid. The removal of Cd increased to 61.88% when citric acid
concentration was 0.2 mol/L. Therefore, the best citric acid concentration for soil washing was 0.2
mol/L.
Fig. 1. Effect of citric acid concentration on
removal of heavy metals
Fig. 2. Effect of mixing time on removal of
heavy metals
Influence of Mixing Time
Fig.2 showed the removal of heavy metals for various mixing time. It was noted that with the
increase of mixing time, the removal of Pb did not change much. With the increase of mixing time,
the removal of Cu increased firstly and then decreased. When the mixing time was 6 h, the removal
of Cu reached the maximum of 68.52%. The results indicated as growth of the mixing time, the
trend of the removal of Cd was similar to the Cu. The reason is that there was too less mixing time
to make agents fully contacted with contaminated soil and to make heavy metals separate from
contaminated soil completely. While too much mixing time did not affect the removal of heavy
metals on one hand and increased energy consumption on the other hand, thus the optimum mixing
time is 2 h.
648 Material Science and Environmental Engineering
Influence of Liquid-soil Ratio
The results were illustrated in Fig. 3 and it showed that the removal of Pb with citric acid was
36.15% in liquid-soil ratio of 20, while 42.23% in liquid-soil ratio of 30. This was because the soil
and agents had a proper liquid-soil ratio to make good contact completely. The change in liquid-soil
ratio was equivalent to increase or decrease the concentration of the washing reagent that caused
different removal of heavy metals. So when liquid-soil ratio was small, the soil did not react with
agents fully and the amount of agents that dissolved in the solution was relatively small, while that
of soil was more, making hardly separate heavy metals from the soil. The trend of Cu increased
firstly and then decreased. With the increase of liquid-soil ratio, citric acid removal of Cd was
reduced after increased first and its best citric acid/soil than of 20.
Fig. 3. Effect of liquid-soil ratio on removal
of heavy metals
Fig. 4. Effect of solution pH on removal of
heavy metals
Influence of Solution pH on Metal Extraction
The effect of solution pH value on soil washing was shown in Fig. 4. It can be seen that as pH
increased, the removal of Pb was increased after decreased first and the efficiency of removal
reached the maximum of 55.45%, when pH was 7 or so, then decreased significantly. This is
primarily because under acid condition, solid state of lead in soil mainly existed in the ionic state.
H+ in the acid exchanged with heavy metal ions on the surface of the soil, so that the heavy metal
ions separated from the soil. While in alkaline conditions, Pb in the soil exist in the form of
insoluble oxide, making it difficult to separate. Under the condition of low acid, Cu precipitated
quickly at the best flushing pH value of 3. With the increase of pH, Cd leaching efficiency was
reduced gradually. This experiment was selected for optimum pH 4, because acidic pH value
affected the physical and chemical properties of the soil that was not conducive to the growth of the
crops and the soil cannot be used for planting after remediation.
Treatment of Soil-washing Solutions
Influence of Solution pH
Fig. 5 showed the impact of pH value on the absorption of heavy metals in the soil-washing
solutions. Activated carbon adsorption of citric acid-Pb complex and citric acid-Cd complex was
decreased first and then increased, finally decreased with the increase of pH value. In the pH=3,
activated carbon adsorption of two kinds of heavy metal complex efficiency were 87.37% and
51.75%, respectively, while in the pH=7, were 85.24% and 62.77%, respectively. The tendency for
citric acid-Cu complex was down. Activated carbon adsorption of its complex reached to the
maximum of 75.25% in the pH=3. The reasons for this situation might be that Pb and Cd were
amphiprotic heavy metals and easy to precipitate in a certain range of acid or alkaline, while Cu was
different, which was dissolved only in acid condition and gave a precipitate in alkaline condition.
So the optimal pH is 7 for activated carbon adsorption of heavy metal complex.
Advanced Materials Research Vol. 937 649
Influence of Activated Carbon Dose
Fig. 6 was the effect of dosage of activated carbon. It showed that adsorption of three kinds of
heavy metal complex was increased with the increase of dosage of activated carbon. And the gentle
curves changed slowly when the activated carbon dose was beyond 1g/100ml. It showed that the
absorption reached the maximum which was 90.23%, 82.05% and 74.59% of citric acid - Pb, citric
acid-Cu and citric acid-Cd complex respectively when the dosage was 1g/100ml. Due to the certain
upper limit of activated carbon for the adsorption of heavy metals, increasing the amount of
activated carbon had no effect on absorption on heavy metal, when the concentration of heavy
metals in the solution kept a certain amount. Activated carbon adsorption mechanism mainly
included: (1) Heavy metal ions deposit on the surface of activated carbon that was physical
adsorption. (2) Heavy metal ions started ion exchange adsorption on the surface of activated carbon.
(3) Chemical adsorption took place between heavy metal ions and the oxygen-containing functional
groups on the surface of the activated carbon.
Fig. 5. Effect of pH on the removal of metal
complex
Fig. 6. Effect of activated carbon dose on the
removal of metal complex
Influence of Mixing Time
The mixing time affected efficiency of activated carbon on adsorption of heavy metals as showed
in Fig. 7. It was obvious that activated carbon had a good adsorption for citric acid-Pb complex and
citric acid-Cu complex along with the increase of mixing time and the maximum of efficiency were
84.27% and 79.65% respectively when the mixing time was 2 h. While there was little effect on
citric acid-Cd complex that the efficiency approached a plateau and remained almost constant with
increase of mixing time. Adsorption efficiency for 2 h was 62.97%, while for 6 h was 64.26%.
Furthermore, the increase of mixing time inevitably led to the increase of energy consumption,
therefore mixing time of 2 h was chosen for the experiment.
Influence of Standing Time
The influence of standing time was shown in Fig. 8. It was found that the standing time did not
have too much of an impact on the adsorption of these three citric acid - heavy metal complex. But
it was clear that the effect of the activated carbon adsorption under different standing time for citric
acid - Pb complex and citric acid - Cd complex was better than that for citric acid - Cu complex.
The efficiency of activated carbon adsorption for Pb, Cu and Cd attained their maximum of 57.39%,
52.05% and 65.39% respectively with standing time for 60 minutes.
650 Material Science and Environmental Engineering
Fig. 7. Effect of mixing time on the removal
of metal complex
Fig. 8. Effect of standing time on the removal
of metal complex
Conclusion
Because of a certain removal for heavy metals, good biodegradability and no pollution to the
environment, citric acid is very suitable for soil washing. Soil washing with citric acid combining
with subsequent treatment for the solution with activated carbon is one of the most promising
alternatives and has wide application perspective. As concentrations of citric acid increased, the
removal of Pb, Cu and Cd increased little firstly and then largely, decreased at last. With the
increase of mixing time, the removal of Pb, Cu and Cd did not change largely as well as liquid-soil
ratio. While the tendency of the removal of Pb, Cu and Cd was different from above with solution
pH increasing, this decreased largely. For Pb, Cu and Cd, the washing efficiency were 44.99%,
47.74% and 60.03% respectively under the best conditions such as citric acid concentration 0.2
mol/l, mixing time 2 h, liquid-soil ratio 20 and solution pH=4. After washing, the leachate was dealt
with activated carbon under various operating variables. The best adsorption conditions of activated
carbon were pH=7, mixing time was 2 h, standing time was 60 min, its dosage is 1g/100ml. Under
these conditions, the vast majority of heavy metals from contaminated soil can be removed.
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Material Science and Environmental Engineering 10.4028/www.scientific.net/AMR.937 Removal of Heavy Metals from Contaminated Soils by Washing with Citric Acid and Subsequent
Treatment of Soil-Washing Solutions 10.4028/www.scientific.net/AMR.937.646
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