Removal of Heavy Metals from Contaminated Soils by Washing with Citric Acid and Subsequent Treatment of Soil-Washing Solutions

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<ul><li><p>Removal of Heavy Metals from Contaminated Soils by Washing with Citric Acid and Subsequent Treatment of Soil-washing Solutions </p><p>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 </p><p>2Tianjin Technology &amp; Engineering Center of Contaminated Sites Remediation, 300468, Tianjin, China </p><p>3School of Environment science and Technology, Tianjin University, 300072, Tianjin, China </p><p>* </p><p>Keywords: heavy metals; contaminated soil; washing; adsorption </p><p>Abstract: Soil washing experiments were carried out with citric acid as washing reagent for the </p><p>remediation of soils highly contaminated with heavy metals, then activated carbon was used in </p><p>absorption processing for leaching solution. In this study, the effects of the main operating variables </p><p>for removal of metals from soils were first discussed. The results showed that 36.% Pb, 47.74% Cu </p><p>and 61.88% Cd were removed from the contaminated soils by optimizing the washing parameters at </p><p>citric acid concentration 0.2 mol/l, mixing time 2 h, liquid-soil ratio 20 and solution pH=4, </p><p>respectively. In the adsorption experiments of leachates, the optimum conditions were found as </p><p>follows: solution pH=7, mixing time of 2 h, standing time of 60 min and activated carbon dosage of </p><p>1g/100ml. </p><p>Introduction </p><p>With the rapid development of industrialization and urbanization, soils contaminated by heavy </p><p>metals have become an environmental problem that cannot be ignored. According to statistics, soils </p><p>contaminated by heavy metals are about 20 million km2 which are about one-sixth of total arable </p><p>land in China, and the amount of grain polluted by heavy metals is as high as millions of tons every </p><p>year. Due to refractory, high accumulation and concealment of heavy metals in contaminated soils, </p><p>the pollutions are more and more serious coupling with increase of human activities [1]. There are </p><p>generally two fundamental technologies to remediate soils. The first technology is to immobilize </p><p>heavy metals into tightly bound solid matrix to minimize their migration. However, this technology </p><p>is not a permanent solution. Site reuse of soil is limited and long-term monitoring is generally </p><p>required. The second technology is to promote heavy metals mobility and migration to the liquid by </p><p>desorption and solubilization. This technology is a permanent solution, providing recycle of </p><p>remediated soils and improving land-use option in the future [2]. The soil washing as a kind of </p><p>chemical remediation method has some advantages of simple operation, low cost and little </p><p>secondary environmental pollution, so it has already been researched widely [3]. </p><p>The choice of washing reagent is essential to the remediation technology. Washing reagents of </p><p>the heavy metals mainly include surfactants and acid washing reagents. Surfactant can be divided </p><p>into synthetic complexing agent such as EDTA, biosurfactant including rhamnolipids, sophorolipid, </p><p>surfactin and saponin. EDTA becomes generally available in the remediation of soils contaminated </p><p>with heavy metals because of its combination with most of the metal ions into the stability of </p><p>chelate and higher removal efficiency of heavy metals in the soil [4]. Ze [5] had studied the </p><p>operating variables in soil washing with EDTA. Nonetheless, EDTA also exist some problems in </p><p>expensive price, poor biodegradability and difficult recovery. In recent years, EDTA has been </p><p>replaced by biosurfactant in soil washing, but the biosurfactant production is dwindling, the </p><p>production cost is high [6]. Therefore, biosurfactant does not have an advantage in the </p><p>industrial-scale applications. Acid washing reagents mainly include inorganic acid solution (HCl </p><p>and HNO3), natural organic acid solution (citric acid, malic acid, oxalic acid, malonic acid, humic </p><p>acid and fulvic acid). Shuzo [7] had reported an acid-washing process on a laboratory scale to </p><p>Advanced Materials Research Vol. 937 (2014) pp 646-651Online available since 2014/May/07 at (2014) Trans Tech Publications, Switzerlanddoi:10.4028/</p><p>All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID:, Georgia Tech Library, Atlanta, USA-14/11/14,09:03:40)</p><p>http://www.scientific.net</p></li><li><p>extract arsenic from contaminated soil. Inorganic acid can remove heavy metals from soils largely, </p><p>and its price is low, but it changes the soil physical and chemical nature easily to be acid after soil </p><p>washing, so it is more suitable for the severe pollution of the soil industrial wasteland. While natural </p><p>organic acid has a good removal capacity for heavy metals, a mild acid, good biodegradability and </p><p>no pollution to the environment, so its application prospect is very environmentally friendly. Elliott </p><p>[8] had proved that oxalic acid that belongs to natural organic acid solution, was more effective than </p><p>EDTA to remove the metals associated with Fe-Mn oxides of soil (attacks the hydrous oxides). </p><p>Recent report has shown that citric acid could remove Cd and Cu from contaminated soils </p><p>effectively, but low removal rate of Pb relatively. </p><p>Currently, achievements have been gained from research on single type metal contaminated soil </p><p>remediation with organic acid, but treatments of washing solution are reported rarely. Commonly </p><p>used treatment methods of washing solution include setting, chemical precipitation and activated </p><p>carbon absorption [9]. The purpose of this paper is to remove heavy metals from contaminated soils </p><p>by washing with citric acid and subsequent treatment of soil-washing solutions, and to determine </p><p>the optimal washing conditions including citric acid concentration, mixing time, liquid-soil ratio, </p><p>solution pH. The best adsorption conditions of activated carbon in washing solution treatment are </p><p>also discussed. </p><p>Materials and Methods </p><p>Soil Sample Preparation </p><p>Soil samples were collected from the greenbelt in Tianjin Chengjian University, which were </p><p>measured as weak acid by pH meter. And the purity of citric acid was greater than 99.5%. All </p><p>samples were air-dried, crushed and were then passed through a sieve of 200 meshes. After </p><p>screening, the fine soil samples were weighed 4kg respectively. According to the scale of the </p><p>moisture content of 30%, the solutions including 200 mg/L Cu2+ solution, 500 mg/L Pb2+ and 50 </p><p>mg/L Cd2+ solution which were prepared by using copper sulfate, lead nitrate and nitric acid </p><p>cadmium drugs (analysis),were added into soil samples, respectively. After about 30 minutes, heavy </p><p>metals in soil were stirred to equilibrium, then put to the outdoor to air-dry for 96 h, placed in the </p><p>labeled bags for further use. </p><p>Washing </p><p>In the soil washing, a certain amount for contaminated soil was added in a 250 ml conical flask, </p><p>then stirred on the magnetic stirrer for a period of time after adding citric acid and in standing for </p><p>analysis. The experiments were carried out by varying different operating parameters such as citric </p><p>acid concentration, mixing time, liquid/soil ratio and solution pH, which were set as range of </p><p>0.01~0.4 mol/L, 1~12 h, 5~30, 3~9, respectively. One variable at a time was altered while keeping </p><p>other variables at constant. After washing, about 4 ml supernatant were taken out with pipette and </p><p>put into clean centrifugal tube, then centrifuged for 10 minutes to separate solid from liquid </p><p>completely by using a high speed centrifuge (RT-TGL-16G-2) which was set as the speed of 5000 </p><p>r/min. The concentrations of heavy metals in the solution were measured by AFS9700 atomic </p><p>absorption spectrometer. Mixed soil in conical flask was for suction filtration by a vacuum pump, </p><p>dried and then weighed. The samples were treated by MDS - 6 microwave digestions, then the </p><p>concentrations of heavy metal were measured by using AAS and the pH of the soil needed to be </p><p>measured. The experiments were repeated 3 times to achieve precise data for analysis and </p><p>comparison to choose the best washing reagent and leaching concentration. </p><p>Treatment of Soil-washing Solutions </p><p>In the activated carbon adsorption experiment, soil-washing solutions were divided into four on </p><p>average in the conical flask, where a certain amount of granular activated carbon was put (about </p><p>4mm in diameter, length of 5mm), then stirred by the magnetic stirrer. The experiments were </p><p>carried out under different operating conditions such as activated carbon dosage, mixing time, </p><p>Advanced Materials Research Vol. 937 647</p></li><li><p>incubation time and solution pH value, which were set as range of 0.2 ~ 1.2 g, 1 ~ 12 h, 30 ~ 120 </p><p>min, pH value of 3 ~ 10, respectively. When a parameter changed, the other parameters remained </p><p>the same. Other operating methods and equipment were same as washing experiment. </p><p>Results and Discussion </p><p>Soil Washing </p><p>Influence of citric acid concentration </p><p>Different concentrations of citric acid were used and the washing results are shown in Fig.1. It </p><p>can be seen that with increase of concentrations of citric acid, the removal of Pb increased few </p><p>when the citric acid concentration was lower than 0.1 mol/L. With increasing of citric acid </p><p>concentration to 0.2 mol/L, the removal of Pb indicated a significant increasing, and its maximum </p><p>was 36.15%. The reason why the removal of Pb decreased with increase of concentrations of citric </p><p>acid was that the complexing reaction between citric acid and Pb was a reversible reaction and </p><p>when the concentration of complex increases to a certain degree, then the reaction reversed, causing </p><p>the efficiency of removal decreased. The removal of Cu reached to the maximum of 51.87% when </p><p>citric acid concentration was 0.1 mol/L. However, with the increase of citric acid concentration, the </p><p>removal of Cu decreased slightly and the removal of Cu reduced to 47.74% when citric acid </p><p>increased to 0.2mol/L. The change tendency of removal of Cd was similitude with that of Pb with </p><p>increase of concentrations of citric acid. The removal of Cd increased to 61.88% when citric acid </p><p>concentration was 0.2 mol/L. Therefore, the best citric acid concentration for soil washing was 0.2 </p><p>mol/L. </p><p>Fig. 1. Effect of citric acid concentration on </p><p>removal of heavy metals </p><p>Fig. 2. Effect of mixing time on removal of </p><p>heavy metals </p><p>Influence of Mixing Time </p><p>Fig.2 showed the removal of heavy metals for various mixing time. It was noted that with the </p><p>increase of mixing time, the removal of Pb did not change much. With the increase of mixing time, </p><p>the removal of Cu increased firstly and then decreased. When the mixing time was 6 h, the removal </p><p>of Cu reached the maximum of 68.52%. The results indicated as growth of the mixing time, the </p><p>trend of the removal of Cd was similar to the Cu. The reason is that there was too less mixing time </p><p>to make agents fully contacted with contaminated soil and to make heavy metals separate from </p><p>contaminated soil completely. While too much mixing time did not affect the removal of heavy </p><p>metals on one hand and increased energy consumption on the other hand, thus the optimum mixing </p><p>time is 2 h. </p><p>648 Material Science and Environmental Engineering</p></li><li><p>Influence of Liquid-soil Ratio </p><p>The results were illustrated in Fig. 3 and it showed that the removal of Pb with citric acid was </p><p>36.15% in liquid-soil ratio of 20, while 42.23% in liquid-soil ratio of 30. This was because the soil </p><p>and agents had a proper liquid-soil ratio to make good contact completely. The change in liquid-soil </p><p>ratio was equivalent to increase or decrease the concentration of the washing reagent that caused </p><p>different removal of heavy metals. So when liquid-soil ratio was small, the soil did not react with </p><p>agents fully and the amount of agents that dissolved in the solution was relatively small, while that </p><p>of soil was more, making hardly separate heavy metals from the soil. The trend of Cu increased </p><p>firstly and then decreased. With the increase of liquid-soil ratio, citric acid removal of Cd was </p><p>reduced after increased first and its best citric acid/soil than of 20. </p><p>Fig. 3. Effect of liquid-soil ratio on removal </p><p>of heavy metals </p><p>Fig. 4. Effect of solution pH on removal of </p><p>heavy metals </p><p>Influence of Solution pH on Metal Extraction </p><p>The effect of solution pH value on soil washing was shown in Fig. 4. It can be seen that as pH </p><p>increased, the removal of Pb was increased after decreased first and the efficiency of removal </p><p>reached the maximum of 55.45%, when pH was 7 or so, then decreased significantly. This is </p><p>primarily because under acid condition, solid state of lead in soil mainly existed in the ionic state. </p><p>H+ in the acid exchanged with heavy metal ions on the surface of the soil, so that the heavy metal </p><p>ions separated from the soil. While in alkaline conditions, Pb in the soil exist in the form of </p><p>insoluble oxide, making it difficult to separate. Under the condition of low acid, Cu precipitated </p><p>quickly at the best flushing pH value of 3. With the increase of pH, Cd leaching efficiency was </p><p>reduced gradually. This experiment was selected for optimum pH 4, because acidic pH value </p><p>affected the physical and chemical properties of the soil that was not conducive to the growth of the </p><p>crops and the soil cannot be used for planting after remediation. </p><p>Treatment of Soil-washing Solutions </p><p>Influence of Solution pH </p><p>Fig. 5 showed the impact of pH value on the absorption of heavy metals in the soil-washing </p><p>solutions. Activated carbon adsorption of citric acid-Pb complex and citric acid-Cd complex was </p><p>decreased first and then increased, finally decreased with the increase of pH value. In the pH=3, </p><p>activated carbon adsorption of two kinds of heavy metal complex efficiency were 87.37% and </p><p>51.75%, respectively, while in the pH=7, were 85.24% and 62.77%, respectively. The tendency for </p><p>citric acid-Cu complex was down. Activated carbon adsorption of its complex reached to the </p><p>maximum of 75.25% in the pH=3. The reasons for this situation might be that Pb and Cd were </p><p>amphiprotic heavy metals and easy to precipitate in a certain range of acid or alkaline, while Cu was </p><p>different, which was dissolved only in acid condition and gave a precipitate in alkaline condition. </p><p>So the optimal pH is 7 for activated carbon adsorption of heavy metal complex. </p><p>Advanced Materials Research Vol. 937 649</p></li><li><p>Influence of Activated Carbon Dose </p><p>Fig. 6 was the effect of dosage of activated carbon. It showed that adsorption of three kinds of </p><p>heavy metal complex was increased with the increase of dosage of activated carbon. And the gentle </p><p>curves changed slowly when the activated carbon dose was beyond 1g/100ml. It showed that the </p><p>absorption reached the maximum which was 90.23%, 82.05% and 74.59% of citric acid - Pb, citric </p><p>acid-Cu and citric acid-Cd complex respectively when the dosage was 1g/100ml. Due to the certain </p><p>upper limit of a...</p></li></ul>


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