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

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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 (2014) Trans Tech Publications, Switzerlanddoi:10.4028/ 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)http://www.scientific.nethttp://www.ttp.netextract 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 AFS9700 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 647incubation 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 EngineeringInfluence 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 649Influence 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 EngineeringFig. 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. References [1] P.S. Yarlagadda, M.R. Matsumoto, J.E.V. Benschoten, A. Kathuria. J. Environ. Eng, 1995, 121 (4): 276-286. [2] Xin Ke, Peijun Li, Qixing Zhou, Yun Zhang, Tieheng Sun. J Environ Sci (China), 2006, 18(4): 727-33. [3] M. Isoyama, S.I. Wada. J. Fac. Agric. Kyushu Univ, 2006, 51: 33-36. [4] A.P. Davis, B.V. Hotha. J. Environ. Eng, 1998,124: 1066-1075. [5] L.Z. Ze, L.Q. Rong, H.Z. Wei, Y.D. Hang, H.Z. Zhi, Z. Tao, G.W. Xian, D.C. Xin. Environmental Pollution, 2009, 157: 229-236. [6] N.M. Catherine, N.Y. Raymond, F.G. Bernard. Journal of Hazardous Materials, 2001, 85: 111-125. [7] T. Shuzo, H. Toshikatsu. Chemosphere, 2002, 46: 31-38. [8] H.A. Elliott, N.L. Shastri. Water Air Soil Pollut, 1999, 110: 335-346. [9] C.K. Ahn, Y.M. Kim, S.H. Woo, J.M. Park. Journal of Hazardous Materials, 2008, 154: 153-160. Advanced Materials Research Vol. 937 651Material Science and Environmental Engineering 10.4028/ Removal of Heavy Metals from Contaminated Soils by Washing with Citric Acid and SubsequentTreatment of Soil-Washing Solutions 10.4028/ DOI References[1] P.S. Yarlagadda, M.R. Matsumoto, J.E.V. Benschoten, A. Kathuria. J. Environ. Eng, 1995, 121 (4): 276-286.


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