Soil Washing Solution Research Articles

Combination of anodic oxidation and biological treatment for the removal of phenanthrene and Tween 80 from soil washing solution

Surfactant enhanced soil washing (SW) and soil flushing are efficient processes for polycyclic aromatic hydrocarbons removal from contaminated soils. However, management of soil washing solutions is a complex challenge for environmental engineering. This study investigated the implementation of a combined process coupling anodic oxidation (AO) – an electrochemical advanced oxidation process – and aerobic biological treatment (BT) for proper handling of a synthetic SW solution containing phenanthrene, Tween® 80 and humic acids. As a sole process, AO required a treatment duration of 5h and a current intensity of 1A to remove 95% of phenanthrene and Tween® 80 as well as 71% of COD. Such operating conditions resulted in high electrical energy consumption (67kWh (kgCOD)−1). In contrast, BT alone allowed a total degradation of both phenanthrene and Tween® 80, but an insufficient removal of COD (44%) was observed due to the production of non-biodegradable by-products. In order to promote synergistic effects, AO and BT were combined and two different treatment strategies were investigated depending on whether AO was used as pre- or post-treatment. The production of more biodegradable by-products was observed after short AO pre-treatment (3h, 500mA), resulting in 80% COD removal after BT. The opposite combination (with AO post-treatment) led to 76% and 93% COD removal after the AO post-treatment at 500mA for 3h and 5h, respectively. Therefore, such combined process appears as a sustainable way to enhance both quality of treated effluent and process cost-effectiveness.

Impact of electrochemical treatment of soil washing solution on PAH degradation efficiency and soil respirometry

The remediation of a genuinely PAH-contaminated soil was performed, for the first time, through a new and complete investigation, including PAH extraction followed by advanced oxidation treatment of the washing solution and its recirculation, and an analysis of the impact of the PAH extraction on soil respirometry. The study has been performed on the remediation of genuine PAH-contaminated soil, in the following three steps: (i) PAH extraction with soil washing (SW) techniques, (ii) PAH degradation with an electro-Fenton (EF) process, and (iii) recirculation of the partially oxidized effluent for another SW cycle. The following criteria were monitored during the successive washing cycles: PAH extraction efficiency, PAH oxidation rates and yields, extracting agent recovery, soil microbial activity, and pH of soil. Two representative extracting agents were compared: hydroxypropyl-beta-cyclodextrin (HPCD) and a non-ionic surfactant, Tween® 80. Six PAH with different numbers of rings were monitored: acenaphthene (ACE), phenanthrene (PHE), fluoranthene (FLA), pyrene (PYR), benzo(a)pyrene (BaP), and benzo(g,h,i)perylene (BghiP). Tween® 80 showed much better PAH extraction efficiency (after several SW cycles) than HPCD, regardless of the number of washing cycles. Based on successive SW experiments, a new mathematical relation taking into account the soil/water partition coefficient (Kd*) was established, and could predict the amount of each PAH extracted by the surfactant with a good correlation with experimental results (R2 > 0.975). More HPCD was recovered (89%) than Tween® 80 (79%), while the monitored pollutants were completely degraded (>99%) after 4 h and 8 h, respectively. Even after being washed with partially oxidized solutions, the Tween® 80 solutions extracted significantly more PAH than HPCD and promoted better soil microbial activity, with higher oxygen consumption rates. Moreover, neither the oxidation by-products nor the acidic media (pH approximately 3) of the partially oxidized solution inhibited the general soil microbial activity during the washing cycle.

Na<sub>2</sub>S를 이용한 EDTA 토양세척수로부터의 중금속 분리

Soil washing with ethylenediaminetetraacetic acid (EDTA) is highly effective in the remediation of soils contaminated with heavy metals. The EDTA recycling process is a requisite for reducing the operating cost. The applicability of Na2S addition on the precipitation of heavy metals from the spent soil washing solution and thereby recycling of EDTA was investigated. Addition of Na2S into the single metal-EDTA and the mixed metal-EDTA solutions ([Na2S]/[metal-EDTA] ratio = 30, reaction time = 30 min and pH = 7~9) was highly effective in the separation of Cu and Pb from metal-EDTA complexes, but not for Ni. The Zn removal efficiency varied with pH and slightly increased upto 40% as the reaction time increased from 0 to 240 min which was longer than those for Cu and Pb. Ca(OH)2 was subsequently added to induce further precipitation of Zn and Ni and to reduce the Na2S dose. At the [Na2S]/[metal-EDTA] ratio of 10, the removal efficiencies of all heavy metals excluding Ni were above 98% with the dose of Ca(OH)2 at 0.002, 0.006 and 0.008 g into 100 mL of Cu-, Pb- and Zn-EDTA solutions, respectively. However, Ca(OH)2 addition was not effective for Ni-EDTA solution. A further research is needed to improve metal removal efficiency and subsequent EDTA recycling for the real application in field-contaminated soils.

Simultaneous removal of heavy metals from field-polluted soils and treatment of soil washing effluents through combined adsorption and artificial sunlight-driven photocatalytic processes

This paper proposes a process for reducing the content of copper and zinc in polluted soils and a combined photocatalytic-physical process for the treatment of the soil washing effluents. For this purpose, real soil samples were taken from the “Land of Fires”, a region in Southern Italy which is known for its high incidence of cancer mortality. Ethylenediamine-N,N′-disuccinic acid (EDDS) was used to extract the heavy metals from the contaminated soil. The soil washing effluents were treated through a sequence of photocatalytic and adsorption processes to lower the concentration values of metals (Cu, Zn, Fe and Mn) below the limits of national legislation for discharge in municipal sewers and to remove the EDDS from the soil washing solutions. Ecotoxicological tests, using different living organisms (Daphnia magna, Vibrio fischeri, Pseudokirchneriella subcapitata and Lepidium sativum), were performed on the soil washing effluents before and after the treatments to assess the effects of the proposed combined process on the ecotoxicity of the soil washing solutions.

Application of an electrochemical treatment for EDDS soil washing solution regeneration and reuse in a multi-step soil washing process: Case of a Cu contaminated soil

Soil washing is an extensively used process for remediation of heavy metals contaminated soils. However the amount of fresh washing solution to be used represents a significant economical drawback of this process. This paper investigates the application of an electrochemical process (Fe/Fe electrodes couple) for the regeneration of a spent EDDS solution, containing Cu and major competitor cations (Ca, Fe, Mg, and Mn). The effect of current density, pH and conductivity of the washing solution on the recovery process performances was investigated. Current density showed the highest influence on Cu, Mg and Mn removal yields. Maximum removal yields reached 99% for Cu, 77% for Mn and 49% for Mg. No influence of the investigated parameters on Ca removal was observed, while an increase of Fe concentration due to anode dissolution occurred. Characterization of sludge produced from the 2 h electrochemical test (5 mA cm−2, pH = 8, 8 mS cm−1) displayed concentrations of 2.8 g kg−1 for Ca, 0.4 g kg−1 for Cu, 535.6 g kg−1 for Fe, 2.6 g kg−1 for Mg. TCLP tests at pH 2.88 and 4.93 showed a low leaching percentage (Ca, 10–21%; Cu, 6–12%; Fe, 0.22% Mg, 27–36%). Multi-washing tests were carried out to assess the decrease of the chelating ability of the regenerated washing solution and the Cu extraction efficiency.

Effect of different soil washing solutions on bioavailability of residual arsenic in soils and soil properties

The effect of soil washing used for arsenic (As)-contaminated soil remediation on soil properties and bioavailability of residual As in soil is receiving increasing attention due to increasing interest in conserving soil qualities after remediation. This study investigates the effect of different washing solutions on bioavailability of residual As in soils and soil properties after soil washing. Regardless of washing solutions, the sequential extraction revealed that the residual As concentrations and the amount of readily labile As in soils were reduced after soil washing. However, the bioassay tests showed that the washed soils exhibited ecotoxicological effects – lower seed germination, shoot growth, and enzyme activities – and this could largely be attributed to the acidic pH and/or excessive nutrient contents of the washed soils depending on washing solutions. Overall, this study showed that treated soils having lower levels of contaminants could still exhibit toxic effects due to changes in soil properties, which highly depended on washing solutions. This study also emphasizes that data on the As concentrations, the soil properties, and the ecotoxicological effects are necessary to properly manage the washed soils for reuses. The results of this study can, thus, be utilized to select proper post-treatment techniques for the washed soils.

Characterizing the Photodegradation Ability of Pentachlorophenol in β-Cyclodextrin Soil Washing Solution

The objective of this study was to characterize the photodegradation ability of pentachlorophenol (PCP) in β-cyclodextrin (β-CD) soil washing solution. After contaminated soil remediation by β-CD soil washing method, solution contained target contaminant was further treated with photodegradation technique. Degradation processes with Xe lamp to simulate natural sun light was investigated under different temperature, and various aeration rate. TiO2employed as photocatalyst was also tested in the soil washing system. The result demonstrated that photodegradation rate of PCP in soil washing solution was related to the aeration rate. The reaction could be slightly accelerated by increasing the temperature. TiO2adding will decrease the degradation speed, while using it together with H2O2will extremely increase the degradation rate of the target contaminant in this condition.

The comparative photodegradation activities of pentachlorophenol (PCP) and polychlorinated biphenyls (PCBs) using UV alone and TiO2-derived photocatalysts in methanol soil washing solution.

Photochemical treatment is increasingly being applied to remedy environmental problems. TiO2-derived catalysts are efficiently and widely used in photodegradation applications. The efficiency of various photochemical treatments, namely, the use of UV irradiation without catalyst or with TiO2/graphene-TiO2 photodegradation methods was determined by comparing the photodegadation of two main types of hydrophobic chlorinated aromatic pollutants, namely, pentachlorophenol (PCP) and polychlorinated biphenyls (PCBs). Results show that photodegradation in methanol solution under pure UV irradiation was more efficient than that with either one of the catalysts tested, contrary to previous results in which photodegradation rates were enhanced using TiO2-derived catalysts. The effects of various factors, such as UV light illumination, addition of methanol to the solution, catalyst dosage, and the pH of the reaction mixture, were examined. The degradation pathway was deduced. The photochemical treatment in methanol soil washing solution did not benefit from the use of the catalysts tested. Pure UV irradiation was sufficient for the dechlorination and degradation of the PCP and PCBs.

Simulated solar photocatalytic processes for the simultaneous removal of EDDS, Cu(II), Fe(III) and Zn(II) in synthetic and real contaminated soil washing solutions

Abstract Metal contamination of natural soil is a global problem. In many cases the contamination is due to anthropogenic activities, and, sometimes, it comes from illegal waste disposal due to uncontrolled industrial development. One of the ways of solving metal soil contamination is by soil washing using a chelating biodegradable agent (EDDS). However, the resulting liquid wastes that contain the extracted metals and chelating agents have to be submitted to further treatments for a complete detoxification before being discharged into the environment. The simultaneous removal of EDDS and three metals (Cu, Fe and Zn) from both synthetic waters (home prepared) and real contaminated soil washing mixtures at neutral pH has been investigated, a lab-scale, considering four artificial solar driven photocatalytic technologies (TiO 2 /hν/N 2 , TiO 2 /hν/air, Fe(III)-EDDS/hν/air and Fe(III)-EDDS/hν/H 2 O 2 /air). The soils were sampled in the Province of Naples (South of Italy) in a zone infamously known for its high incidence of cancer mortality. The results indicate that both the type and the sequence of photocatalytic technologies that need to be apply should be chosen prudently considering both the nature and the content of the soil washing solutions.

Treatment of synthetic soil washing solutions containing phenanthrene and cyclodextrin by electro-oxidation. Influence of anode materials on toxicity removal and biodegradability enhancement

Electrochemical advanced oxidation processes were applied to treatment of highly loaded synthetic soil washing solution. Phenanthrene (PHE) and hydroxypropyl-beta-cyclodextrin (HPCD) were chosen respectively as a polycyclic aromatic hydrocarbon (PAH) representative and as a solubilizing agent. Different anode materials such as platinum (Pt), dimensionally stable anode (DSA;) and boron-doped diamond (BDD) were employed to carry out the treatment in a electrocatalytic way. Two electrochemical processes were compared: electro-Fenton (EF) and anodic oxidation (AO) with BDD as an anode. Toxicity (Microtox®) and biodegradability (BOD5/COD) of treated solutions during the treatment were assessed.Pt anode was found to be the most efficient one in degradation of PHE, while BDD anode showed better ability to degrade HPCD and to mineralize the solution. This confirms the different ways to treat the effluent, which are related to the O2 evolution overpotential of selected anode: mainly electro-oxidation in the case of Pt and DSA, mineralization (conversion of organics to CO2) mechanism in AO with BDD anode and paired electrocatalysis in the case of EF with BDD anode. Toxicity and biodegradability assays corroborate these mechanisms. After a complete degradation of PHE and HPCD corresponding to 60% mineralization of solution with EF and AO treatments using BDD as anode, the toxicity starts to decrease and the biodegradability reaches its maximal value (100%). In these conditions, the complete mineralization needs longer treatment times. But in this study electrochemical treatment is conceived to enhance the biodegradability for a biological post-treatment in a reasonably short time. Therefore six factors including energy consumptions values were compared after reaching a biodegradability of 33% and after reaching complete mineralization. Performing electrochemical treatment until reaching a BOD5/COD ratio of 33% appears to be the best option, since EF and AO displayed most of the time a similar behaviour.

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

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.

Influence of solubilizing agents (cyclodextrin or surfactant) on phenanthrene degradation by electro-Fenton process – Study of soil washing recycling possibilities and environmental impact

One of the aims in soil washing treatment is to reuse the extracting agent and to remove the pollutant in the meantime. Thus, electro-Fenton (EF) degradation of synthetic soil washing solutions heavily loaded with phenanthrene was suggested for the first time. Two solubilising agents hydroxypropyl-beta-cyclodextrin (HPCD) and Tween 80® (TW 80) were chosen as cyclodextrin (CD) and surfactant representatives, respectively. In order to reuse HPCD and to degrade the pollutant simultaneously, the following optimal parameters were determined: [Fe2+] = 0.05 mM (catalyst), I = 2000 mA, and natural solution pH (around 6), without any adjustment. Only 50% of TW 80 (still higher than the critical micelle concentration (CMC)) can be reused against 90% in the case of HPCD while phenanthrene is completely degraded in the meantime, after only 180 min of treatment. This can be explained by the ternary complex formation (Fe2+-HPCD-organic pollutant) (equilibrium constant K = 56 mM−1) that allows OH to directly degrade the contaminant. This confirms that Fe2+ plays an important role as a catalyst since it can promote formation of hydroxyl radicals near the pollutant and minimize HPCD degradation. After 2 h of treatment, HPCD/phenanthrene solution got better biodegradability (BOD5/COD = 0.1) and lower toxicity (80% inhibition of luminescence of Vibrio fischeri bacteria) than TW 80/phenanthrene (BOD5/COD = 0.08; 99% inhibition of V. fischeri bacteria). According to these data, HPCD employed in this integrated (soil washing + EF degradation) approach gave promising results in order to be reused whereas the pollutant is degraded in the meanwhile.

A new micelle-based method to quantify the Tween 80® surfactant for soil remediation

Tween 80® is a surfactant widely employed to enhance remediation of contaminated soils. Actually, there are few convenient and selective methods to quantify Tween 80® at low concentrations. Here, we report a new and simple quantification method to monitor Tween 80® analysis. Our method is based on the enhancement of the fluorescence of 6-(p-toluidino)naphthalene-2-sulfonic acid (TNS) by formation of micelles with Tween 80®. Results show a linear calibration curve with a correlation coefficient R2 of 0.99, a detection limit of 0.13 mM and a quantification limit of 0.19 mM. This method showed better performances compared to actual methods such as UV absorbance and total organic carbon measurements. In addition, we demonstrated that the measurements using this new technique are impacted only at 3.5 % maximum by the presence of oxidation by-products formed during oxidation of Tween 80® by the electro-Fenton process or by hydrophobic organic contaminants present in solution. Fluorescence measurements of soil washing solution with real contaminated soil show almost no impact on Tween 80®–TNS micelle analysis. The analytical method proposed for Tween 80® analysis could replace the currently used conventional method because it is quite simple, highly sensitive and more selective.

Adsorption of mixed polycyclic aromatic hydrocarbons in surfactant solutions by activated carbon

The adsorption behavior of three polycyclic aromatic hydrocarbons (PAH) in TX100 solution by activated carbon was studied aiming at surfactant recovery. Adsorption experiments were conducted in batch and column models. The adsorption data fitted well with the Langmuir isotherm model, Dubinin–Radushkevich isotherm models and pseudo-second-order kinetics model. Regarding column adsorption, the exhausting time of TX100 was shorter than the breakthrough time of PAH in the fixed AC column. This finding indicates the feasibility of reusing surfactants from soil-washing solutions. The total costs can reduce about $ 0.57 per 10L washing solution with AC adsorption.

Pilot-scale washing of metal contaminated garden soil using EDTA

Ten batches (75kg each) of garden soil with >50% of silt and clay and average 1935mgkg−1 Pb, 800mgkg−1 Zn, 10mgkg−1 Cd and 120mgkg−1 As were remediated in a pilot-scale chemical extraction plant. Washing with 60mmol ethylenediaminetetraacetic acid (EDTA) per kg of soil on average removed 79, 38, 70, and 80% of Pb, Zn, Cd and As, respectively, and significantly reduced the leachability, phyto-accessibility and oral-availability of residual toxic metals, as assessed using deionised water, toxicity characteristic leaching procedure (TCLP), diethylenetriamine pentaacetic acid extraction (DTPA) and physiologically based extraction test (PBET) tests. The used soil washing solution was treated before discharge using an electrochemical advanced oxidation process with graphite anode: EDTA was removed by degradation and toxic metals were electro-precipitated onto a stainless steel cathode. The novelty of the remediation technique is separation of the soil from the washing solution and soil rinsing (removal of mobilized contaminants) carried out in the same process step. Another novelty is the reuse of the soil rinsing solution from the previous batch for cleansing the soil sand, soil rinsing and for preparation of the washing solution in subsequent batches. The cost of energy and material expenses and disposal of waste products amounted to approximately 75€ton−1 of soil.

Electrochemical treatment of spent solution after EDTA-based soil washing

The use of EDTA in soil washing technologies to remediate soils contaminated with toxic metals is prohibitive because of the large volumes of waste washing solution generated, which must be treated before disposal. Degradation of EDTA in the waste solution and the removal of Pb, Zn and Cd were investigated using electrochemical advanced oxidation processes (EAOP) with a boron-doped diamond anode (BDDA), graphite and iron anodes and a stainless-steel cathode. In addition to EAOP, the efficiency of electro-Fenton reactions, induced by the addition of H2O2 and the regulation of electrochemical systems to pH 3, was also investigated. Soil extraction with 15 mmol kg−1 of soil EDTA yielded waste washing solution with 566 ± 1, 152 ± 1 and 5.5 ± 0.1 mg L−1 of Pb, Zn and Cd, respectively. Treatments of the waste solution in pH unregulated electrochemical systems with a BDDA and graphite anode (current density 67 mA cm−2) were the most efficient and removed up to 98 ± 1, 96 ± 1, 99 ± 1% of Pb, Zn and Cd, respectively, by electrodeposition on the cathode and oxidatively degraded up to 99 ± 1% of chelant. In the electrochemical system with an Fe anode operated at pH 3, the chelant remained preserved in the treated solution, while metals were removed by electrodeposition. This separation opens up the possibility of a new EDTA recycling method from waste soil washing solution.

Novel EDTA and process water recycling method after soil washing of multi-metal contaminated soil

The development of EDTA-based soil washing technologies is hampered by the lack of treatment methods of the spent solution, particularly when multi-metal contaminated soils have to be remediated. Extraction of Pb (5329 mg kg −1), Zn (3401 mg kg −1), Cd (35 mg kg −1) and As (279 mg kg −1) contaminated soil with 60 mmol EDTA kg −1 of soil removed 72%, 27%, 71%, and 80% of contaminants, respectively. We demonstrate here, on a laboratory scale experiment, the feasibility of using acid precipitation with HCl and H 2SO 4, coupled to initial alkaline Fe removal, to recover up to 88% of EDTA from a spent soil washing solution containing 11,578 mg L −1 of EDTA and 1109, 267, 7.1 and 64 mg L −1 of Pb, Zn, Cd and As, respectively. An electrochemical advanced oxidation process with a graphite anode was subsequently used to degrade 99.9% of the remaining EDTA in the spent washing solution and remove 99.7% Pb, 100% Zn, 96.6% Cd and 100% of As as (electro)precipitate. The cleansed process water obtained after electrochemical treatment was then used to prepare recycled washing solution by re-dissolving the recovered/recycled part of the EDTA. Washing solutions prepared from recycled EDTA had the same potential to extract Pb, Zn, Cd and As from soil as washing solution prepared from fresh EDTA of the same molarity. The novel recycling method is simple and robust and enables reuse of both EDTA and process water in a closed process loop.

Recycling of EDTA solution after soil washing of Pb, Zn, Cd and As contaminated soil

Soil washing with EDTA is known to be an effective means of removing toxic metals from contaminated soil. A practical way of recycling of used soil washing solution remains, however, an unsolved technical problem. We demonstrate here, in a laboratory scale experiment, the feasibility of using acid precipitation to recover up to 50% of EDTA from used soil washing solution obtained after extraction of Pb (5330mgkg−1), Zn (3400mgkg−1), Cd (35mgkg−1) and As (279mgkg−1) contaminated soil. Up to 100% of EDTA residual in the washing solution and 100%, 97%, 98% and 100% of initial Pb, Zn, Cd and As concentration in the solution, respectively, were removed in an electrolytic cell using a graphite anode. We employed the recovered EDTA and treated washing solution to prepare recycled soil washing solution with the same potential for extracting toxic metals from soil as the original. The efficiency of soil washing depends on the EDTA concentration. Using twice recycled 30mmol EDTA kg−1 soil, we removed 44%, 20%, 53% and 61% of Pb, Zn, Cd and As, respectively, from contaminated soil.

Effect of coagulant addition on the sedimentation of a surfactant-containing washing solution used for phenanthrene-contaminated soil

The coagulation of soil particles from a soil-washing solution containing the nonionic surfactant Triton X-100 (TX100) was investigated using various coagulants, such as chitosan (CS), polyacrylamide (PAA), and polyaluminum chloride (PAC). Soil contaminated with phenanthrene (PHE) was washed with aqueous TX100 solutions at concentrations ranging from 0.1 to 20 g/L. The effectiveness of CS (5 mg/L) as a coagulant in the system was compared with PAA (50mg/L) and PAC (50 mg/L), and the results indicated that the system with CS exhibited a more effective separation of soil than those with PAA and PAC. The removal efficiency of PHE (Rj=81.7%) and the selective separation factor (SSF=14.2) at 10 g/L TX100 were the highest for the system with CS (5 mg/L), indicating that the selection of CS as a coagulant in surfactant-mediated soil washing markedly improved both PHE removal and soil separation.

Electrochemical EDTA recycling after soil washing of Pb, Zn and Cd contaminated soil

Recycling of chelant decreases the cost of EDTA-based soil washing. Current methods, however, are not effective when the spent soil washing solution contains more than one contaminating metal. In this study, we applied electrochemical treatment of the washing solution obtained after EDTA extraction of Pb, Zn and Cd contaminated soil. A sacrificial Al anode and stainless steel cathode in a conventional electrolytic cell at pH 10 efficiently removed Pb from the solution. The method efficiency, specific electricity and Al consumption were significantly higher for solutions with a higher initial metal concentration. Partial replacement of NaCl with KNO 3 as an electrolyte (aggressive Cl − are required to prevent passivisation of the Al anode) prevented EDTA degradation during the electrolysis. The addition of FeCl 3 to the acidified washing solution prior to electrolysis improved Zn removal. Using the novel method 98, 73 and 66% of Pb, Zn and Cd, respectively, were removed, while 88% of EDTA was preserved in the treated washing solution. The recycled EDTA retained 86, 84 and 85% of Pb, Zn and Cd extraction potential from contaminated soil, respectively.