Remediation of Arsenic Contaminated Soil Using Phosphate and Colloidal Gas Aphron Suspensions Produced from Sapindus mukorossi
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Phosphate and colloidal gas aphrons (CGAs) generated from saponin extracted from Sapindus mukorossi fruit, were evaluated for washing low levels of arsenic from an iron rich soil. Phosphate is one of the most commonly dispersed chemicals that increases arsenic mobility in soil due to their structural similarities, making it an important factor in arsenic removal process. Column washing experiments were performed with CGAs in down flow and up flow modes on soil of pH 5 and 6. Soapnut CGAs, when paired with phosphate removed up to 95 % arsenic while soapnut CGAs alone could only remove up to 70 % arsenic. The presence of phosphate improved efficiency of soapnut solution by up to 35 %. SEM image of washed soil revealed minor corrosion of soil surface while using phosphate with soapnut. Therefore, the addition of phosphates would have positive impact on soil washing using soapnut saponin.
KeywordsArsenic Soapnut Sapindus mukorossi Soil remediation Colloidal gas aphrons
USEPA has classified arsenic as a human carcinogen. Soil and water pollution through mining, smelting, coal burning, wood preservation and illegal waste dumping activities releases arsenic in ecosystem (Tokunaga and Hakuta 2002). In a recent study, soil of golf course was found to be contaminated with arsenic sourced from seaweed fertiliser (O’Neill et al. 2014). Usual screening concentration of arsenic in soil usually varies in the range of 10–16 mg Kg−1 for industrial soil (DOE Malaysia 2009). Arsenic and other contaminants can be removed from soil through a host of technologies including bioremediation and soil washing with surfactants and other chemicals (Jang et al. 2005; Majumder et al. 2013; Mukhopadhyay et al. 2015a).
CGAs have shown excellent potential as separation agents for contaminant removal from liquid and soil matrices (Hashim et al. 2012). Saponin, a natural surfactant obtained from the fruit pericarp of Sapindus mukorossi or soapnut has been used for removing organic and inorganic contaminants including arsenic from soil (Mukhopadhyay et al. 2013, 2015a; Roy et al. 1997; Song et al. 2008). Phosphates (PO43−) share chemical properties and structural similarities with arsenates (AsO43−) and have been frequently used for removing arsenic from soil. Alam et al. (2001) extracted up to 40 % arsenic from a yellow/brown forest soil by 0.9 M phosphate solution. Other researchers (Tokunaga and Hakuta 2002) also removed 80 %–99 % of the bound arsenic from contaminated soil by phosphate and phosphoric acids. Zeng et al. (2008) found that KH2PO4 at lower concentration was less corrosive than H3PO4. Hence, KH2PO4 was used in the present work in the range of 50–150 mM in combination with saponin extracted from soapnut to extract arsenic from soil.
In this work, soil As(V) concentration of up to 85 mg Kg−1 has been washed using CGAs generated from saponin extracted from soapnut pericarp mixed with phosphate (referred to as Ph + SN now on). The enhancement of arsenic removal performance by saponin CGAs in presence of phosphate has been investigated. The sustainability of the process has been assessed using SEM imagery to probe the soil corrosion. The mechanism of the soil washing process has also been described. Presence of large amount of phosphate in the terrestrial and aquatic environment originating from NPK fertilisers signify that an in situ soil washing process involving saponin and CGAs will need to consider phosphate’s effect on the process.
Materials and Methods
(a) Characterization of natural soil sample; (b) arsenic speciation in spiked soil; (c) characterization of washing agents
(a) Characterization of spiked soil sample
USEPA SW-846 Method 9045D
ASTM D 854 – Water pycnometer method
Ammonium acetate method for acidic soil
Organic matter content
Loss of weight on ignition (Storer 1984)
Bulk density (gm cc−1)
Total porosity (%)
Total As (mg kg−1)
As (III) (mg kg−1)
As (V) (mg kg−1)
Total iron (mg kg−1)
Total silicon (mg kg−1)
Aluminium (mg kg−1)
Total manganese (mg kg−1)
Magnesium (mg kg−1)
Lead (mg kg−1)
Zinc (mg kg−1)
Soil particle size distribution
Sand (<50 μm)
Sandy soil according to USDA Soil Classification
Silt (50–2 μm)
Clay (>2 μm)
CMC at 25°C
Surface tension (mN m−1)
Viscosity (at 25°C) cP
(b) Characterization of washing agents
0.5 % + 50 mM
1 % + 100 mM
1.5 % + 150 mM
Experimental conditions and variables
300 gm soil in steel column
Constant flow rate of 10 mL min−1
Temperature = 25°C
Pore volume = 80 mL
No of pore volumes = 6
Washing agent: Soapnut, Soapnut + Phosphate
Flow modes: upflow (UF), downflow (DF)
pH: 5, 6
Substrate: solution, CGAs
Concentration of wash solutions:
Soapnut = L = SN 0.5 %, LM = SN 0.75 %, M = SN1 %, MH = 1.25 %, H = SN 1.5 %
Soapnut + Phosphate: L = SN 1 % + Ph 50 mM, LM = SN 1 % + Ph 75 mM, M = SN 1 % + Ph 100 mM, MH = SN 1 % + Ph 125 mM, H = SN 1 % + Ph 150 mM)
Results and Discussion
Stability of CGAs: Separation of CGAs from the liquid phase with respect to time can be measured from the movement of CGAs-liquid front and CGAs half-life (t1/2). Ph + SN CGAs were found to have the longest half-life among all extractants followed by soapnut and SDS. At low strength, i.e. 0.5 % soapnut- 50 mM phosphate, the CGAs had half-life of 900 s and at 1 % soapnut-100 mM phosphate concentration, it increased to 1200 s. Ph + SN CGAs had air hold-up of 35 %–39 % v/v and they were kept under constant stirring with magnetic stirrer at 1000 rpm to maintain homogeneity.
Arsenic removal by CGAs and solution of soapnut, Ph + SN and SDS at soil pH of 6 are illustrated in Fig. 1b. In down flow mode, 1 % soapnut solution removed more than 63 % of arsenic while CGAs removed only 41 % of arsenic. In up flow mode, CGAs removed 70 % arsenic while solution removed 76 % arsenic. For Ph + SN mixtures, CGAs performed better in up flow mode than solution, but fared slightly worse in down flow mode. In up flow mode, a highest arsenic removal of 93 % was achieved by Ph + SN CGAs where its solution showed 88.71 % arsenic removal. These results show that CGAs and solution had similar functional efficiencies. CGAs perform especially better in up flow modes than down flow mode due to higher buoyancy of the microbubbles on their introduction from the base of the column. Since CGAs entrap up to 35 % of air by volume, it is more economical than using surfactant solution of same volume. Addition of phosphate boosted the performance of soapnut CGAs from 70 % to 93 % in up flow mode and from 41 % to 78 % in down flow mode. The flow mode of wash solutions is another important factor in arsenic removal from soil column as shown in Fig. 1c. Two different wash modes, viz. down flow and up flow modes were used in this study. While in down flow mode, soapnut CGAs achieved up to 41.3 % arsenic removal, Ph + SN CGAs removed up to 78 % arsenic. The solutions, on the other hand, performed better under natural flow of gravity, the removal by soapnut and Ph + SN solutions being 63.63 % and 84.7 % respectively. It was observed that in down flow mode, tiny channels transmitting solutions and CGAs open up and close, by passing some parts of contaminated soil matrix. However, in up flow mode, this problem of flow channelling was not prevalent. In up flow mode, CGAs and solutions of soapnut removed 70 % and 76.21 % arsenic respectively and the corresponding values for Ph + SN mixture are 93 % and 88.7 %. Thus, up flow mode is found to perform better than down flow mode and phosphate addition definitely improved the performance of soapnut in both flow modes. Although Fig. 1c shows different flow modes at soil pH of 6, similar trends were observed for soil at pH 5. The concentration of washing solution and CGAs largely influenced the arsenic removal efficiency. In all cases, higher concentration solutions and CGAs remove more arsenic. Although the data for soil pH of 6 is shown, the trend is similar for soil of pH 5. Higher concentration solutions and CGAs contain more micelles compared to their lower concentration counter parts resulting in increased solubility of arsenic in the surfactant micelles. This observation is again supported by other research studies which reported that the number of micelles usually increase above CMC, resulting in enhanced mobility of arsenic, heavy metals and organic substances from soil (Mulligan 2005). Also, an increase in the arsenic removal was observed when only phosphate concentration was increased in 1 % soapnut solution. This is expected due to increase in the number of exchangeable phosphate molecules in the solution which can replace the arsenate from the soil (Alam et al. 2001; Zeng et al. 2008). Another notable feature that emerged from the experiments is that, the results were not influenced by variation of soil pH. In some cases, soil pH of 5 exhibited slightly higher removal of arsenic than soil pH of 6. However, the trends of arsenic removal under all the other conditions were similar for both soil pH values of 5 and 6. This signifies that soapnut can be used in conjunction with phosphate without altering the solution pH for treating slightly acidic soil.
Mechanistic studies and soil corrosion: Desorption kinetic studies were performed using soapnut in combination with phosphate. The linear forms of the kinetic models e.g. Elovich equation, two-constant rate equation, parabolic diffusion equation and first order equation were explored. Elovich equation with highest R2 and lowest SE values of 0.969 and 0.017 describes desorption kinetics by Ph + SN solution appropriately. A decrease in “α” indicates a reduction in the desorption rate (Chien and Clayton 1980). Ph + SN has a higher value of α than soapnut, indicating its higher efficiency for desorbing arsenic.
Phosphate had a significant effect on soapnut solutions and CGAs for removal of low level arsenic residues from soil. Longer half-life, slower rate of liquid-bubble separation and higher air-holdup make Ph + SN CGAs highly suitable for soil washing. In general, CGAs comprising up to 35 % of its volume of air showed similar soil washing performance with solutions, indicating economic advantage. High arsenic removals up to 93 % were achieved by Ph + SN CGAs in up flow mode at soil pH of 6. Soapnut CGAs removed only 70 % arsenic in up flow mode at pH 6. This indicates a 33 % increase in efficiency of soapnut on phosphate addition. Phosphate replaced arsenate from soil due to its similarities in pK1, pK2, pK3 values and is found to be an advantage during competitive adsorption–desorption phenomenon (Mukhopadhyay et al. 2015a). SEM image of the washed soil reveals only minor corrosion of the soil particles in presence of Ph + SN washing agent. Therefore, Ph + SN solutions and CGAs are safe, efficient and environment friendly means of remediating arsenic-contaminated soil. The presence of phosphate in the natural soil is likely to have a synergistic effect on the performance of soapnut.
The authors acknowledge the funding provided by University of Malaya, Kuala Lumpur (Project UM-QUB6A-2011) for carrying out this research (Brookins 1986).
- DOE Malaysia (2009) Contaminated Land Management and Control Guidelines No. 1: Malaysian Recommended Site Screening Levels for Contaminated LandGoogle Scholar