Techno-economic analysis of hydrocarbon-polluted soil treatment by using ex situ microwave heating: influence of soil texture and soil moisture on electric field penetration,operating conditions and energy costs

Purpose

Microwave (MW) heating has been identified as a potential cost-effective technique to remediate hydrocarbon-polluted soils; however, the soil texture and properties could have a great impact on its full-scale treatment. In addition, very limited energy and economical data on MW treatment are available, and this lack makes its real application very limited. In this work, a first experimental phase was performed simulating a MW of several hydrocarbon-polluted soils. Obtained data were elaborated for a techno-economic analysis.

Materials and methods

Four soil textures, corresponding to medium, fine silica sand (at different soil moistures), silt as silica flour and clay as kaolin, were artificially contaminated with diesel fuel and irradiated by MWs using a bench scale apparatus. Soil samples were treated applying four specific power values at different times. At the end, soil temperature was measured, whereas residual contaminant concentrations were measured and fitted considering and exponential decay kinetic model. Temperature data, as well as kinetic parameters obtained, were used for the techno-economic analysis. The changing of the internal electric field was calculated for all the soils and operating conditions, then considering initial contamination values ranging from 750 to 5000 mg kg^?1, the minimal remediation time, specific energy and costs for the remediation were assessed.

Results and discussion

At low powers, MW effectiveness is limited by low soil moistures or fine soil textures due to a limitation of the electric field penetration, whereas when high powers are used soil properties have a limited effect. Remediation time, as a function of the initial contamination level, follows a linear trend, except for dry soils, for which an exponential trend was observed. For powers higher than 30 kW Kg^?1, remediation times lower than about 100 min are needed, for all the moisturized soils, in order to treat a contamination of 5000 mg kg^?1. The variation of soil moisture or soil texture results in the range 20–160 € ton^?1, and doubled costs are required for the treatment of clayey soils respect to sandy soils.

Conclusions

The analysis performed suggests that soil layers lower than 70 cm should be considered for ex situ remediation. MW has been shown as a quick technique also for high hydrocarbon concentrations; however, for energy saving, the application of some powers should be avoid. Unmoisturized or fine texture soil treatment results in higher costs; however, a maximum cost of 160 € ton^?1 generally makes MW heating a quick and cost-effective ex situ technique.

     

Remediation of a diesel-contaminated soil from a pipeline accidental spill: enhanced biodegradation and soil washing processes using natural gums and surfactants

Purpose

This paper addresses the application of bioproducts produced by plants (locust bean, guar, and mesquite seed gums) to enhance remediation processes of different nature: soil washing and biodegradation methodologies.

Materials and methods

These natural gums were tested at laboratory scale to remove total petroleum hydrocarbons-diesel fraction (TPH-diesel) from oil-contaminated volcanic soils sampled from a polluted site in an agricultural area of western Mexico. TPH-diesel removal by natural gums was compared to common synthetic surfactants.

Results and discussion

There is a strong evidence of contamination caused by the presence of TPH-diesel at a concentration of 32,100 mg/kg, which is above the legal limit of 1,200 mg/kg for agricultural soils in Mexico. Regarding the surfactant soil washing experiments, ionic surfactants showed removal rates above the control test of about 78.51 % (Maranil LAB), 71.27 % (Texapon 40), 60.13 % (SDS), and 48.19 % (Surfacpol G). In contrast, some nonionic surfactants showed removal rates below soil-washing background rate (40 %). On the other hand, natural gums showed interesting and promising results. Guar gum and locust bean gum showed efficiencies of 54.38 % and 53.46 %, respectively. Biodegradation experiments confirmed the effectiveness of natural gums as biodegradation enhancers in diesel-contaminated soils. Specifically, guar gum showed an excellent performance. An 82 % TPH-diesel removal rate was achieved for a very low gum concentration (2 ppm). In this particular context, reported surfactant concentrations to assist biodegradation are, in general, higher.

Conclusions

This work demonstrated the applicability of natural gums as soil remediation enhancers in diesel-contaminated systems. Particularly, guar gum might represent a cost-effective alternative for biodegradation enhancement processes.     

Modelling and preliminary technical,energy and economic considerations for full-scale in situ remediation of low-dielectric hydrocarbon-polluted soils by microwave heating (MWH) technique

Purpose

Microwave heating (MWH) has been recently proposed as a high-performance technique for the remediation of soils contaminated with organic pollutants. However, despite MWH potential advantages, it is scarcely applied due to the lack of full-scale in situ detailed studies. In this work, the in situ MWH applicability for the remediation of hydrocarbon-polluted soils was assessed by means of a specific energy and economic analysis. Essential technical information has also been purchased.

Materials and methods

Energy and economic analysis was performed using data obtained from modelling for which a dedicated equation-based process computer code simulating MWH phenomena was adopted. Elaborations involved the assessment of the influence of soil texture and moisture as well as operating conditions (supplied power and time) on electric field penetration into the soils and soil temperature variation as a function of time and radial distance from the irradiation source.

Results and discussion

Main results reveal that sandy soils are more penetrable by MW irradiation with respect to clayey ones. The soil MW penetrability was also observed to increase with decreasing the soil moisture. This was in turn reflected in the soil temperature profiles. However, the major effect on MWH effectiveness is ascribable by the changing of the operating power. In fact, the use of magnetrons with powers lower than 3 kW does not ensure enough microwave penetration into the soil and, therefore, is not suitable for in situ activities, whereas the application of a power of 6 kW led to a maximum treatable radius of 145 cm. In terms of energy consumption, calculation showed that almost 3 days more are in general required to remediate clayey soils with respect to sandy ones. Consequently, the economic analysis revealed that energy costs for sandy soils are about 3 € t^?1 lower than those required for clayey soils. Furthermore, the application of a power of 6 instead of 3 kW results in a higher total energy cost, which, jointly with the higher soil volume treatable, leads to almost equal specific costs.

Conclusions

The comparison of calculated costs with those of other available clean-up technologies for hydrocarbon-contaminated soils shows that very short remediation times and energy costs obtained (18–27 € t^?1) make in situ MWH a deliverable alternative to conventional thermal desorption or physical-chemical techniques.

     

Effects of iron-based amendments on soil structure: a lab experiment using soil micromorphology and image analysis of pores

Purpose

Recent trends in soil green and sustainable remediation require an increased attention on environmental effects. The physical consequences of remediation practices on soil structure are very rarely investigated.

Material and methods

A laboratory experiment was carried out by adding iron grit to a sand (S), a silt loam (L), and a clay (C) soil subjected to several wetting-drying cycles. The physical effects of the treatment on soil pore system were identified and quantified combining physical measurements on repacked samples with image analysis of pores on resin-impregnated soil blocks and micromorphological analysis on thin sections.

Results and discussion

A negligible reduction of total porosity (P) resulted in S, and a slight increase was observed in the L and C soils. However, an important impact on soil structure was identified in pore size range >10 μm for the L and C soils, with the formation of new pores related to the differential shrink-swell behavior between soil matrix and added iron grains. Different plasticity of these soils also played a role in planar pore formation.

Conclusions

Effects of the addition of iron grit on soil pore system are strongly dependent on soil physical properties. The performed experiment showed that iron-based amendments can improve soil structure in low-plastic shrink-swell soil increasing porosity in the range of transmission pores (50–500 μm). This study showed the high potential of soil micromorphology and pore image analysis in order to evaluate the environmental impact of soil remediation practices.     

Numerical approach to modelling pulse-mode soil flushing on a Pb-contaminated soil

Purpose

Soil flushing can represent a suitable technology in remediation of soils, sediments and sludge contaminated by persistent species (e.g. toxic metal). This paper presents a model specifically developed to evaluate the feasibility of chelating agent-enhanced flushing. The model, here applied to the remediation of real Pb-contaminated soils, was conceived also to simulate an innovative pulse-mode soil flushing technique.

Materials and methods

The soil flushing application was firstly carried out through columns laboratory experiments. Columns were filled with a real Pb-contaminated soil (3,000 mg kg^?1 of dry soil) and flushing was operated in a pulse mode with different chelating agent dosages (3 and 4.3 mmol kg^?1soil). Experimental results were used to calibrate and validate the developed reactive transport model that accounts for transport of ethylenediamine tetraacetic acid (EDTA) and EDTA–Pb chelate complexes, Pb residual concentration on soil and the reduction in permeability by soil dissolution. Determination of hydrodynamic and hydro-dispersive parameters was carried out through a numerical approach incorporating the use of neural network as interpolating function of breakthrough data obtained by a tracer test.

Results and discussion

The EDTA dosage strongly influenced the efficiency in Pb extraction and soil permeability. Cumulative extractions of Pb were found to be 20 and 29 % for the EDTA concentrations of 3 and 4.3 mmol/kg of dry soil, respectively. The soil dissolution caused a significant flow rate decrease, as a consequence of the increase in chelating agent concentration. Therefore the recovery phase duration increased from 738 to 2,080 h. The ability of the model in simulating all the examined phenomena is confirmed by a good fit with experimental results in terms of (a) soil permeability reduction, (b) eluted Pb and (c) residual Pb in the soil.

Conclusions

Results highlighted as the model, supported by a preliminary and careful characterization of the soil, can be useful to assess the feasibility of the flushing treatment (avoiding soil clogging) and to address the choice of the operating parameters (flow rate, chelating agent dosage and application method). On the basis of the present research results, a protocol is suggested for in situ soil pulse–flushing application.     

Fractions of Cu, Cd, and enzyme activities in a contaminated soil as affected by applications of micro- and nanohydroxyapatite

Purpose

With the rapid development of nanotechnology, hydroxyapatite-based nanoparticles have been applied in wastewater and soil remediation. However, limited studies have been conducted on the remediation of heavy metal-contaminated soils by microhydroxyapatite (MHA) and nanohydroxyapatite (NHA). Thus, we investigated the effects of MHA and NHA on soil pH values and fractions of copper (Cu) and cadmium (Cd). The changes of soil enzymes with application of MHA and NHA were also evaluated.

Materials and methods

Pots contained 200 g of the soil with MHA and NHA ranging from 1 % to 5 % incubated for 60 days under greenhouse condition, and maintained at 60 % of soil water holding capacity by adding deionized water. Soil pH, catalase, urease, and acid phosphatase were analyzed at incubation times of 7, 14, 30, and 60 days by chemical assays. The fractions of Cu and Cd were analyzed after 60 days by a sequential extraction procedure.

Results and discussion

Application of MHA and NHA significantly increased soil pH values. Especially, we found for the first time that soil pH values with 3 % (pH?>?7.90) and 5 % (pH?>?8.83) application rates of MHA were larger than that of MHA itself (pH?=?7.71). MHA was more effective than NHA in immobilizing Cu and Cd by significantly decreasing exchangeable fractions of Cu and Cd and transforming them from active to inactive fractions. Soil catalase and urease significantly increased, but acid phosphatase apparently decreased with increasing application rates of MHA. However, three enzymes activities changed slightly for NHA treatments.

Conclusions

MHA was more effective than NHA in immobilizing Cu and Cd. MHA had a more positive effect on soil catalase and urease activities than NHA. Furthermore, Pearson’s correlation coefficients showed that soil pH value was a key factor to influence the bioavailability of Cu and Cd and the activity of soil enzymes. The results of this study provided an efficient method for the remediation of heavy metal-contaminated soils.     

Metal removal from and microbial property improvement of a multiple heavy metals contaminated soil by phytoextraction with a cadmium hyperaccumulator Sedum alfredii H.

Purpose

Effects of phytoextraction by Sedum alfredii H., a native cadmium hyperaccumulator, on metal removal from and microbial property improvement of a multiple heavy metals contaminated soil were studied under greenhouse conditions.

Materials and methods

A rhizobox experiment with an ancient silver-mining ecotype of S. alfredii natively growing in Zhejiang Province, China, was conducted for remediation of a multiple heavy metals contaminated soil. The rhizobox was designed combining the root-shaking method for the separation of rhizospheric vs near-rhizospheric soils and prestratifying method for separation of sublayers rhizospheric soils (0–10 mm from the root) and bulk soil (>10 mm from the root). Soil and plant samplings were carried out after 3 and 6 months of plant growth.

Results and discussion

Cadmium (Cd), zinc (Zn), and lead (Pb) concentrations in shoots were 440.6, 11,893, and 91.2 mg kg^?1 after 6 months growth, and Cd, Zn, and Pb removed in the shoots were 0.862, 25.20, and 0.117 mg/plant. Microbial biomass C, basal respiration, urease, acid phosphatase, and invertase activities of the rhizospheric soils were significantly higher than that of unplanted soils after 6 months growth. Microbial biomass carbon (MBC) of 0–2 mm and basal respiration (BR) rate of 0–8 mm sublayer rhizospheric soils were significantly higher than that of bulk soil after 6 months growth. So were the three enzyme activities of 0–4 mm sublayer rhizospheric soils. BR rate and urease were significantly negatively correlated with soluble Cd, so were MBC, acid phosphatase, and intervase activities with soluble Zn, MBC, BR rate, and three enzyme activities with soluble Pb.

Conclusions

Harvesting shoots of S. alfredii could remove remarkable amounts of Cd, Zn, Pb, and lower water-soluble Cd, Zn, and Pb concentrations in the rhizospheric soils. MBC, BR rate, and enzyme activities of the metal polluted soil, especially the rhizospheric soils increased with phytoextraction process, which is attributed to the stimulation of soil microbes by planting as well as the decrease in soil-soluble metal concentration.     

Assessment of the stability of chromium in remedied soils by Pannonibacter phragmitetus BB and its risk to groundwater

Purpose

Acid rain can accelerate the acidification of the chromium-contaminated soils, resulting in chromium releasing into soil solution and causing ecological risk. The current study aims to investigate the release of chromium in the remedied soils by Pannonibacter phragmitetus BB under the simulated acid rain leaching and to assess its risk to groundwater.

Materials and methods

P. phragmitetus BB was utilized to remedy the Cr(VI)-contaminated soils at two levels (80 and 1,276 mg kg^?1) by the column leaching experiment, and the chemical remediation with ferrous sulfate was used as a control. The remedied soils by P. phragmitetus BB and ferrous sulfate were leached under the simulated acid rain to evaluate the release of chromium. Furthermore, the risk of chromium release from the remedied soils to the groundwater was assessed by a fuzzy comprehensive evaluation method.

Results and discussion

The average concentrations of water-soluble Cr(VI) in the remedied soils by P. phragmitetus BB were reduced to less than 5.0 mg kg^?1. Under leaching situation with the simulated acid rain, the release of total chromium and Cr(VI) from the remedied soils by P. phragmitetus BB and ferrous sulfate declined rapidly with the extended leaching time. However, the release amounts of total chromium and Cr(VI) from the remedied soil by P. phragmitetus BB more efficiently deceased as compared with that by ferrous sulfate remediation. Carbonate-bounded, exchangeable, and organics-bonded chromium were the major chromium-releasing sources under the simulated rain leaching. After microbial remediation with P. phragmitetus BB and chemical remediation with ferrous sulfate, the risk grades of the remedied soils to groundwater declined from classes 11 to 5 and 6, respectively.

Conclusions

The risks of the remedied soils by both microbial remediation with P. phragmitetus BB and chemical remediation with ferrous sulfate to groundwater effectively decreased and microbial remediation more significantly declined the chromium risk to groundwater than chemical remediation.     

Prediction of landfarming period using degradation kinetics of petroleum hydrocarbons: test with artificially contaminated and field-aged soils and commercially available bacterial cultures

Purpose

This study predicts the optimal landfarming period for the total petroleum hydrocarbons (TPHs)-contaminated field soils that are subject to the combined landfarming and Fenton oxidation treatment.

Materials and methods

The TPHs degradation in the artificially contaminated model soils and the field-aged contaminated soils were compared in a laboratory scale. The soils were bioaugmented with the 16 petroleum hydrocarbon-degrading microbial cultures that are commercially available in Korea.

Results and discussion

The TPHs degradation by bioaugmentation in the model soils was 81?±?2% after 56 days, while it was only 27?±?2% after 74 days in the field-aged soils. The lower degradation in the field-aged soils can be attributed to the removal of a large part of the biodegradable and volatile TPHs fractions during the weathering process. The two-compartment model that can depict a fast-degradation phase followed by a slow-degradation phase predicted that the remedial goal of 475 mg kg^?1 of TPHs could be achieved in the model soils within the conventional landfarming period of 60–120 days in Korea; however, the predicted period for the field-aged soils (710–4,086 days) was not practical requiring a combined biological and chemical treatment approach. Based on the kinetics study, the optimal landfarming period for the field-aged soils was 3 weeks and pre-Fenton oxidation can be used to meet the remedial goal.

Conclusions

The two-compartment model can be useful for predicting landfarming period for the combined landfarming and Fenton oxidation treatment of field-aged TPHs contaminated soils. The prediction of the optimal landfarming period could lead to the reduction in both the treatment cost and period.     

Toxicity of arsenic in relation to soil properties: implications to regulatory purposes

Purpose

The present work evaluates the influence of different soil properties and constituents on As solubility in laboratory-contaminated soils, with the aim of assessing the toxicity of this element from the use of bioassays to evaluate the soil leachate toxicity and thereby propose soil guideline values for studies of environmental risk assessment in soil contamination.

Materials and methods

Seven soils with contrasting properties were artificially contaminated in laboratory with increasing concentrations of As. Samples were incubated for 4 weeks, and afterwards, soil solution (1:1) was obtained after shaking for 24 h. The soil leachate toxicity was assessed with two commonly used bioassays (seed germination test with Lactuca sativa and Microtox ® test with Vibrio fischeri).

Results and discussion

The relationship between soluble As and soil properties indicated that iron oxides and organic matter content were the variables most closely related to the reduction of the As solubility, while pH and CaCO₃ increased As solubility in the soil solutions. Toxicity bioassays showed significant differences between soils depending on their properties, with a reduction of the toxicity in the iron-rich soil (no observed effect concentration (NOEC)?=?150 mg kg^?1) and a significant increase in the highly carbonate samples (NOEC between 15 and 25 mg kg^?1).

Conclusions

Soil guideline values for regulatory purposes usually set a single value for large areas (regions or countries) which can produce over- or underestimation of efforts in soil remediation actions. These values should consider different levels according to the main soil properties controlling arsenic mobility and the soil leachate toxicity.     

Retention and release of diethyl phthalate in biochar-amended vegetable garden soils

Purpose

Diethyl phthalate (DEP) is one of the most commonly used plasticizers as well as a soil contaminant. Using biochar to remediate soils contaminated with DEP can potentially reduce the bioavailability of DEP and improve soil properties. Therefore, a laboratory study was conducted to evaluate the effect of biochar on soil adsorption and desorption of DEP.

Materials and methods

Two surface soils (0–20 cm) with contrasting organic carbon (OC) contents were collected from a vegetable garden. Biochars were derived from bamboo (BB) and rice straw (SB) that were pyrolyzed at 350 and 650 °C. Biochars were added to two types of soil at rates of 0.1 and 0.5 % (w/w). A batch equilibration method was used to measure DEP adsorption-desorption in biochar treated and untreated soils at 25 °C. The adsorption and desorption isotherms of DEP in the soils with or without biochar were evaluated using the Freundlich model.

Results and discussion

The biochar treatments significantly enhanced the soil adsorption of DEP. Compared to the untreated low organic matter soil, the soils treated with 0.5 % 650BB increased the adsorption by more than 19,000 times. For the straw biochar treated soils, the increase of DEP adsorption followed the order 350SB?>?650SB. However, for the bamboo biochars, the order was 650BB?>?350BB. Bamboo biochars were more effective than the straw biochars in improving soils’ adsorption capacity and reducing the desorption ability of DEP.

Conclusions

Adding biochar to soil can significantly enhance soil’s adsorption capacity on DEP. The 650BB amended soil showed the highest adsorption capacity for DEP. The native soil OC contents had significant effects on the soils’ sorption capacity treated with 650BB, whereas they had negligible effects on the other biochar treatments. The sorption capacity was affected by many factors such as the feedstock materials and pyrolysis temperature of biochars, the pH value of biochar, and the soil organic carbon levels.     

Impact of organic matter addition on pH change of paddy soils

Purpose

The objective of the present study was to explore the effect of initial pH on the decomposition rate of plant residues and the effect of residue type on soil pH change in three different paddy soils.

Materials and methods

Two variable charge paddy soils (Psammaquent soil and Plinthudult soil) and one constant charge paddy soil (Paleudalfs soil) were used to be incubated at 45 % of field capacity for 105 days at 25 °C in the dark after three plant residues (Chinese milk vetch, wheat straw, and rice straw) were separately added at a level of 12 g?kg^?1 soil. Soil pH, CO₂ escaped, DOC, DON, MBC, MBN, NH ₄ ⁺ , and NO ₃ ^? during the incubation period were dynamically determined.

Results and discussion

Addition of the residues increased soil pH by 0.1–0.8 U, and pH reached a maximum in the Psammaquent and Plinthudult soils with low initial pH at day 105 but at day 3 in the Paleudalfs soil with high initial pH. Incorporation of Chinese milk vetch which had higher concentration of alkalinity (excess cations) and nitrogen increased soil pH more as compared with incorporation of rice and wheat straws. Microbial activity was the highest in Chinese milk vetch treatment, which resulted in the highest increase of soil pH as compared with addition of rice and wheat straws. However, nitrification seemed to be inhibited in the variable charge soils of Psammaquent and Plinthudult but not in the constant charge soil of Paleudalfs.

Conclusions

The effectiveness of increasing soil pH after incorporation of the plant materials would be longer in low initial pH soils of Psammaquent and Plinthudult than in high initial pH soil of Paleudalfs. In order to achieve the same degree of pH improvement, higher amounts of plant residues should be applied in constant charge soils than in variable charge soils.     

Sorption isotherms and kinetics of Sb(V) on several Chinese soils with different physicochemical properties

Purpose

Sorption of antimony on soils is the primary factor that influences its immobilization and migration in the environment. In the present study, the sorption of Sb(V) onto seven Chinese soils with different physicochemical properties was investigated for exploring the relationship between the sorption capacity of Sb(V) and the physicochemical properties of the soils.

Materials and methods

Sorption isotherms and kinetics experiments were performed to ascertain the sorption capacity and the kinetic rate, respectively. The relationship between the sorption capacity of Sb(V) and the physicochemical properties of the soils was analyzed by multiple linear regressions.

Results and discussion

The results showed that the sorption isotherms fitted with both the Langmuir and Freundlich equations very well (R ²?=?0.936–0.997), and the sorption kinetic of Sb(V) onto the seven Chinese soils followed a pseudo-second-order reaction. The maximum sorption capacity of Sb(V) on the soils ranged from 134 to 1,333 mg?kg^?1. Nearly 94 % of the variability in maximum sorption of Sb(V) modeled by Freundlich equation could be described by Fe_DCB (dithionite–citrate–bicarbonicum extractable), and nearly 98 % of the variability could be described by Fe_DCB and Al_DCB.

Conclusions

Multiple linear regressions can be successfully applied to analyzing the relationship between sorption capacity and soil properties. Fe_DCB and Al_DCB played important roles in Sb(V) sorption onto soils. It would be useful to understand the environmental behaviors of Sb and for the implementation of risk assessment management and remediation strategies of Sb.     

The bioaccumulation of Cd in rice grains in paddy soils as affected and predicted by soil properties

Purpose

The area of cadmium (Cd)-contaminated soil in China is increasing due to the rapid development of the Chinese economy. To ensure that the rice produced in China meets current food safety and quality standards, the current soil quality standards for paddy soils urgently need to be updated.

Materials and methods

We conducted a pot experiment with 19 representative paddy soils from different parts of China to study the effects of soil properties on bioaccumulation of Cd in rice grains. The experiment included a control, a low treatment concentration (0.3 mg kg^–1 for pH?<?6.5 and 0.6 mg kg^–1 for pH?≥?6.5), and a high treatment concentration (0.6 mg kg^–1 for pH?<?6.5 and 1.2 mg kg^–1 for pH?≥?6.5) of Cd salt added to soils.

Results and discussion

The results showed that the Cd content in grains of the control and low and high Cd treatments ranged from 0.021 to 0.14, 0.07 to 0.27, and 0.12 to 0.33 mg kg^–1, respectively. Stepwise multiple regression analysis indicated that soil pH and organic carbon (OC) content could explain over 60 % of the variance in the (log-transformed) bioaccumulation coefficient (BCF) of Cd in grains across soils. Aggregated boosted trees analysis showed that soil pH and OC were the main factors controlling Cd bioavailability in paddy soils. Validation of the models against data from recent literature indicated that they were able to accurately predict the BCF in paddy soils.

Conclusions

These quantitative relationships between the BCF of Cd in grains and soil properties are helpful for developing soil-specific guidance on Cd safety threshold value for paddy soils.     

Comparing chemical-enhanced washing and waste-based stabilisation approach for soil remediation

Purpose

This study aimed to compare the effectiveness of chemical-enhanced soil washing (with chelating agents, humic substances and inorganic acids) and soil stabilisation by inorganic industrial by-products (coal fly ash, acid mine drainage sludge and zero-valent iron) and organic resource (lignite) for timber treatment site remediation.

Materials and methods

Both remediation options were assessed in terms of extraction/leaching kinetics and residual leachability (toxicity characteristic leaching procedure, TCLP) of the major risk drivers, i.e. Cu and As.

Results and discussion

In chemical-enhanced soil washing, chelating agents only minimised the Cu leachability. Humic substances were ineffective while inorganic acids reduced the As leachability to the detriment of the soil quality. For the waste-stabilised soil, the short-term leaching potential (72 h) and long-term TCLP leachability (9 months) revealed that Fe-/Al-/Ca-rich AMD sludge and coal fly ash sequestered As through adsorption and (co-)precipitation, while carbonaceous lignite stabilised Cu with oxygen-containing functional groups. The short-term and long-term leaching of Cu and As into the soil solution was negligible in the presence of the waste materials. However, the waste-stabilised soil did not maintain sufficient Cu stability in the TCLP tests, in which acetate buffer induced significant mineral dissolution of the waste materials.

Conclusions

These results suggest that chelant-enhanced washing (significant reduction of Cu leachability) may be augmented with subsequent stabilisation with inorganic waste materials (effective control of As leachability), thus minimising the environmental risks of both Cu (heavy metal) and As (metalloid) while preserving the reuse value of the soil. Additional tests under field-relevant conditions are required to provide a holistic performance evaluation.     

Remediation of lead-, arsenic-, and cesium-contaminated soil using consecutive washing enhanced with electro-kinetic field

Purpose

Extensive deposition of Pb, As, and Cs in soils may damage ecosystems and human’s health. Soil washing is the most conventional remediation method, and its efficiency depends on metal solubility in soil. This study aims to optimize operating variables of electro-kinetic field (EKF)-enhanced soil washing procedures.

Materials and methods

Soil samples from a Mississippi River Delta rice field were homogeneously spiked with Pb, As, and Cs, and contaminated soil was aged for 3 months. The remediation involved a first stage electro-kinetic process, followed by a soil washing procedure. Soil pH changes under EKF were studied. Effects of citric acid and reversed EKF were investigated for alleviating possible alkaline precipitation. In the washing procedure, soil washing time and cycles with different extractants were examined. The overall EKF-enhanced soil washing efficiencies were discussed as well.

Results and discussion

The implement of EKF offered an acidic soil environment around the anode areas for solubilizing metal(loid)s. Combined with EKF, citric acid was more conductive to desorb metal(loid)s. In addition, the reversed EKF effectively alleviated metal(loid) precipitation caused by alkalization in the first stage cathode areas. The EKF significantly enhanced metal(loid) extractions in the anode area of soils using Na₂EDTA, CaCl₂, and citric acid at pH of 2. The most preferable removal of Pb (80–98 %), As (48–63 %), and Cs (10–13 %) was achieved with three extractants. CaCl₂ and citric acid were proved to be suitable alternatives to Na₂EDTA for Pb extraction. A washing process of 2 h extraction with double washing cycles was optimized.

Conclusions

Soil washing time and cycles were major factors governing the metal(loid) removal from soil. Washing process of 2 h extraction with double cycles was optimized for further extraction based on higher washing efficiency. The EKF effectively improved washing efficiency while some electrical parameters need further studies for cost performance consideration.

     

Effects of drinking water treatment residuals on nickel retention in soils: a macroscopic and thermodynamic study

Purpose

Recent research has focused on using water treatment residuals (WTRs) as cost-effective materials to remove potential environmental contaminants. To better understand and predict how WTRs affect the mobility and retention of nickel (Ni) in soils with time, it is crucial that the kinetics and thermodynamics of these reactions be understood. Such information is lacking in the literature and would aid in evaluating the suitability of WTR as a soil amendment for adsorbing Ni contaminant. Accordingly, we focused on investigating the retention of Ni in differing soils and the subsequent influence of WTR application on Ni retention.

Materials and methods

To examine the effects of WTR application on the characteristics of Ni retention, equilibrium, and kinetics, sorption batch experiments were performed on three soils having different properties. The sorption data were applied to the first-order kinetic model, and the Arrhenius equation was used to determine the thermodynamic parameters.

Results and discussion

The quantity of Ni sorbed by the soils followed the trend Typic Torrifluvent > Typic Calciorthids > Typic Torripsamment. Soil sorption isotherms shift toward a higher sorption of Ni indicating addition of more sorption sites as a result of WTRs’ application. Data generated at different temperatures for soils and WTR-amended soils fitted well to Freundlich isotherm and first-order kinetic models. The energy of activation (E _a) and enthalpy (ΔH ^#), entropy (ΔS ^#), and free energy of activation (ΔG ^#) related to Ni sorption were calculated using the Arrhenius equation. The activation energy (E _a) values (51.65–130.0 kJ mol^?1) and the positive ΔH ^# values characterize Ni sorption process onto the sorbents studied as chemisorption with an endothermic nature. The large negative ΔS ^# values (?262 to ?290 J?mol^?1) and the large positive ΔG ^# values (88.11–89.14 kJ mol^?1) indicate the involvement of an associative mechanism in the Ni sorption process.

Conclusions

WTR addition has led to an overall increase in Ni sorption by the amended soils. Such increase in Ni sorption provides evidence that WTR has the potential for land application as a Ni sorbent in soil remediation techniques. The sorption capacity of the soils and WTR-amended soils enhanced with an increase in temperature. Therefore, to truly understand the potential fate and mobility of Ni in the natural environment, temperature, in particular, should be considered.     

Contribution of bricks to urban soil properties

Purpose

Bricks are regularly found in urban soils where they can strongly impact soil properties. The purpose of this study is to investigate abundance, especially in the fine earth fraction, and properties of bricks in urban soils, focusing on rooting, plant nutrition and contamination.

Materials and methods

Three different urban soils from the city of Berlin have been studied for their brick contents in the coarse and fine earth fractions by hand sorting. Light (LM) and scanning electron microscopy (SEM) was employed to investigate bricks for proofs of rooting. Third, CEC, pH, EC, C_org, nutrient and contaminant storage and availability have been investigated for bricks and the fine earth fractions of the corresponding soil horizons.

Results and discussion

The fine earth fractions of the investigated soils contain 3 to 5 % of bricks, while the coarse fractions contain up to 50 %. The LM and SEM micrographs made the proof that roots enter brick pores or attach to brick surfaces. Therefore, they can use the water and nutrients stored in bricks and bypass pore system discontinuities between bricks and surrounding soil. The CEC of bricks is grain size dependent and reaches a maximum of 6 cmol_c kg^?1 for particles smaller than 0.063 mm. This dependency is the result of the restricted diffusion into the brick pore system due to the short shaking time in the CEC analysis protocol and of the rising surface with decreasing particle size. From the nutrient storage and availability, we conclude that bricks can better supply plants with K, Mg, Ca and S than the investigated sandy bulk soil.

Conclusions

The nutrient availability from bricks is low compared to control soils, except for Ca and S. Because of the water and nutrient storage, low contamination status and the possible rooting of bricks, they can be used for amelioration of poor sandy soils and for constructed Technosols, preferably employed in small grain sizes.     

Immobilization of Cu and Cd in a contaminated soil: one- and four-year field effects

Purpose

Many amendments have been applied to immobilize heavy metals in soil. However, little information is available on the changes of immobilization efficiencies of heavy metals in contaminated soils over time. This work investigated the immobilization efficiencies of copper (Cu) and cadmium (Cd) in contaminated soils in situ remediated with one-time application of three amendments for 1 year and 4 years.

Materials and methods

Apatite, lime, and charcoal were mixed with the topsoil of each plot with the amounts of 22.3, 4.45, and 66.8 t/ha, respectively. Soil chemical properties and fractions of Cu and Cd were examined after in situ remediation for 1 year and 4 years. Soil sorption and retention capacities and desorption proportions for Cu and Cd were investigated by batch experiments.

Results and discussion

The addition of amendments significantly increased soil pH, but decreased exchange acid and aluminum (Al). The amendments significantly decreased the CaCl₂ extractable Cu and Cd and transformed them from active to inactive fractions. After the application of amendments for 1 year, the maximum sorption capacities ranged from 35.6 to 38.8 mmol/kg for Cu and from 14.4 to 17.0 mmol/kg for Cd, which were markedly higher than those of the application of amendments for 4 years (Cu, 29.6–34.7 mmol/kg; Cd, 10.9–16.4 mmol/kg). Desorption proportions (D) of Cu and Cd using three extractants followed the order of \( {D}_{{\mathrm{NaNO}}_3}<{D}_{{\mathrm{CaCI}}_2}<{D}_{{\mathrm{MgCI}}_2} \) . Moreover, the retention capacities (R) of Cu and Cd both increased and followed the order of R _apatite?>?R _lime?>?R _charcoal, resulting in higher Cu and Cd in the amended soils than the untreated soil.

Conclusions

Apatite, lime, and charcoal increased the soil sorption and retention capacities of Cu and Cd and resulted in higher immobilization efficiencies in the amended soils than the untreated soil. However, the immobilization efficiencies of Cu and Cd decreased with the decrease of sorption capacities after 4 years. It was concluded that apatite had the best effect on the long-term stability of immobilized Cu and Cd and can be applied to immobilize heavy metals in contaminated soils.     

Simultaneous removal of Cu and PAHs from dredged sediments using flotation

Purpose

This study addresses the feasibility of a flotation technique, using a lab-scale flotation cell, to simultaneously remove both metals and polyaromatic hydrocarbons (PAHs) from fine sediment fractions (<250 μm) that are potentially contaminated with copper (Cu).

Materials and methods

A multiple flotation process with three consecutive flotation stages was performed on three sediments (13S, 14B, and 24A) with different particle size distributions, Cu and PAH concentrations, and organic matter contents.

Results and discussion

Flotations performed under selected conditions allowed for significant removal of both Cu (61–70 %) and PAHs (75–83 %) with acceptable froth recoveries of approximately 23–29 %. Removal rates for arsenic, lead, and zinc were 48–61, 40–48, and 32–36 %, respectively. Flotation selectivity of Cu was greatly influenced by the contents of fine particles and organic matter of the sediments. The maximum flotation selectivity was obtained for the 53–125-μm size fraction. The high flotation selectivity of Cu (2.5–3.2) and PAHs (3.0–3.6) demonstrated the feasibility of flotation to treat soils or sediments containing both organic and inorganic pollutants.

Conclusions

Overall, the flotation results showed a high selectivity for both Cu and PAHs and demonstrated the feasibility of flotation to treat media contaminated with organic and inorganic contaminants.