This paper reviews current remediation technologies that use chelating agents for the mobilization and removal of potentially toxic metals from contaminated soils. These processes can be done
in situ ...as enhanced phytoextraction, chelant enhanced electrokinetic extraction and soil flushing, or
ex situ as the extraction of soil slurry and soil heap/column leaching. Current proposals on how to treat and recycle waste washing solutions after soil is washed are discussed. The major controlling factors in phytoextraction and possible strategies for reducing the leaching of metals associated with the application of chelants are also reviewed. Finally, the possible impact of abiotic and biotic soil factors on the toxicity of metals left after the washing of soil and enhanced phytoextraction are briefly addressed.
The use of synthetic chelants for soil washing and enhanced phytoextraction by plants has been well studied for the remediation of metal-contaminated soils in the last two decades.
► Calcium aluminate cement was efficient for multi-element contaminated soil S/S. ► Surface wash-off was a mechanism of toxic elements release from S/S soil. ► Multi-point pH leaching gives an ...insight into element release after soil S/S. ► TCLP significantly underrates the S/S efficiency under given field pH conditions.
The toxicity characteristics leaching procedure (TCLP) is commonly used to assess the efficiency of solidification/stabilization (S/S) of pollutants in wastes, despite recent objections to this method. In this study, formulations of 7, 10, 15 and 20% (w/w) of calcium aluminate cement (CAC) and sulfate resistant Portland cement (SRC) were used for S/S of soil from brownfield contaminated with 43,149, 10,115, 7631, 6130, 90, 82mgkg−1 of Zn, Pb, Cu, As, Cd and Ni, respectively. CAC produced S/S soil monoliths of higher mechanical strength (up to 7.65Nmm−2). Mass-transfer analysis indicated surface wash-off as a mechanism of toxic elements release, and equilibrium leaching as a crucial parameter of S/S efficiency assessment. In the expected range of field soil pH after S/S (pH 7–9), the TCLP gave markedly different results than the multi-point pH equilibrium leaching method (using nine targeted pH values): up to 2953-, 94-, 483-, 1.3-, 27- and 1.5-times more Zn, Pb, Cu, As, Cd and Ni, respectively, was determined in the TCLP leachate. S/S with CAC reduced leachability of toxic elements more effectively than SRC. Our results indicate that, under given field conditions, the TCLP significantly underrates the efficiency of S/S of contaminated soil with cementitious binders.
In a laboratory study, Portland cement (15%, w/w) was used for solidification/stabilisation (S/S) of Cd, Pb, Zn, Cu, Ni and As contaminated soils from the former industrial site. Soils formed solid ...monoliths with cement. S/S effectiveness was assessed by measuring the mechanical strength of the monoliths, concentrations of metals in deionised water and TCLP (toxicity characteristic leaching procedure) soil extracts, and mass transfer of metals. Concentrations of Cd, Pb, Zn and Ni in water extracts from S/S soils generally decreased, concentrations of As remained unchanged, while concentrations of Cu increased. Concentrations of Cd, Pb, Zn and Ni in the TCLP extracts from S/S soils were lower than from original soils. Cu extractability was lower in most soil samples, while the extractability of As from S/S soils increased. Overall, the concentration of metals in deionised water and TCLP solution, obtained after extraction of the S/S soils, was below the regulatory limits. S/S greatly reduced the mass transfer of Cd (up to 83-times), Pb (up to 13.7-times) and Zn (up to 294-times). Mass transfer of Ni and As was generally also reduced, while that of Cu increased in some S/S soils. Based on the findings of mass-transfer mechanism analysis the predominant mechanism of release was surface wash-off of metals otherwise physically encapsulated within the cementous soil matrix.
Odpadno blato iz čistilnih naprav (ČN) ima zaradi velike hranilne vrednosti potencial za uporabo v kmetijstvu, vendar je pogosto onesnaženo s potencialno strupenimi kovinami (PSK). V raziskavi smo ...preverili ali ReSoil® tehnologija, ki temelji na uporabi EDTA (50, 70 in 100 mmol l-1), omogoča učinkovito odstranjevanje PSK iz odpadnega blata, vzetega po aerobni (blato1) in anaerobni (blato2) biološki obdelavi. Največji delež odstranitve Pb smo dosegli v blatu1 (do 60 %), največji delež odstranitve Zn in Cu pa v blatu2 (do 55 in 29 %). Vsebnost hranil se po remediaciji ni bistveno spremenila, le vsebnost dostopnega fosforja se je v blatu2 po remediaciji do 1,7-krat zmanjšala, vendar je še vedno ostala velika (489-510 mg 100-1 g-1). Po remediaciji so bile koncentracije v izpirkih za vse kovine, razen za Zn, pod mejo za nenevarne snovi. Za dokazovanje možnosti recikliranja procesnih raztopin in EDTA smo blato2 oprali s 50 mmol l-1 pralno raztopino v seriji 5 zaporednih remediacij in pri tem v povprečju odstranili 28 % Pb, 48 % Zn, 35 % Cu, 30 % Mn in 10 % Fe. ReSoil® tehnologija sicer omogoča odstranjevanje PSK iz blata in pri tem ohrani njegovo hranilno vrednost, vendar je učinkovitost tehnologije odvisna od postopkov obdelave blata na ČN.
► We assess the feasibility of using soil S/S for industrial land reclamation. ► Retarders, accelerators, plasticizers were used in S/S cementitious formulation. ► We proposed novel S/S efficiency ...model for multi-metal contaminated soils.
In a laboratory study, formulations of 15% (w/w) of ordinary Portland cement (OPC), calcium aluminate cement (CAC) and pozzolanic cement (PC) and additives: plasticizers cementol delta ekstra (PCDE) and cementol antikorodin (PCA), polypropylene fibers (PPF), polyoxyethylene-sorbitan monooleate (Tween 80) and aqueous acrylic polymer dispersion (Akrimal) were used for solidification/stabilization (S/S) of soils from an industrial brownfield contaminated with up to 157, 32,175, 44,074, 7614, 253 and 7085
mg
kg
−1 of Cd, Pb, Zn, Cu, Ni and As, respectively. Soils formed solid monoliths with all cementitious formulations tested, with a maximum mechanical strength of 12
N
mm
−2 achieved after S/S with CAC
+
PCA. To assess the S/S efficiency of the used formulations for multi-element contaminated soils, we propose an empirical model in which data on equilibrium leaching of toxic elements into deionized water and
TCLP (toxicity characteristic leaching procedure) solution and the mass transfer of elements from soil monoliths were weighed against the relative potential hazard of the particular toxic element. Based on the model calculation, the most efficient S/S formulation was CAC
+
Akrimal, which reduced soil leachability of Cd, Pb, Zn, Cu, Ni and As into deionized water below the limit of quantification and into
TCLP solution by up to 55, 185, 8750, 214, 4.7 and 1.2-times, respectively; and the mass transfer of elements from soil monoliths by up to 740, 746, 104,000, 4.7, 343 and 181-times, respectively.
The efficiency of multi-step leaching of heavy metal contaminated soils was evaluated in a laboratory scale study. Four different soils contaminated with Pb (1136
±
16–4424
±
313
mg
kg
−1) and Zn ...(288
±
5–5489
±
471
mg
kg
−1) were obtained from industrial sites in the Mežica Valley, Slovenia and Príbram district, Czech Republic. Different dosages (2.5–40
mmol
kg
−1) of ethylenediamine tetraacetate (EDTA) were used to treat soils in 1–10 leaching steps. Higher EDTA dosages did not result in a proportional gain in Pb and Zn removal. EDTA extracted Pb more efficiently than Zn from three of four tested soils. The percentage of removed Zn did not exceed 75% regardless of the soil, EDTA dosage and leaching steps. Significantly more Pb (in three of four soils) and Zn were removed from soils when the same amount of EDTA was applied in several leaching steps. The interference of major soil cations Fe and Ca with EDTA complexation as a possible factor affecting Pb and Zn removal efficiency with multi-step heap leaching was examined and is discussed. The results of our study indicate that, for some soils, using multi-step leaching instead of the more traditionally used single dose EDTA treatment could improve heavy metal removal efficiency and thus the economics of soil remediation.
In this study, we investigated the importance of plant cover for secondary succession and soil fungal community development in remediated substrates after EDTA washing of metal-contaminated soils. ...The abundance of the total fungal community, determined by ITS fungal marker genes (Internal Transcribed Spacer region), and root colonisation by arbuscular mycorrhizal (AM) fungi were monitored in two types of soil material (
calcareous and acidic
) sown with perennial ryegrass (
Lolium perenne
L.) and without plant cover (
bulk soil
). Four months after the start of the experiment, the abundance of ITS genes in the soil clearly showed that the presence of plants was the main factor affecting the total fungal community, which increased in the rhizosphere soil in most treatments, while it remained at a low level in the bulk soil (without plants). Interestingly, the addition of environmental inoculum, i.e., rhizosphere soil from a semi-natural meadow, did not have a positive effect on the abundance of the total fungal community. While fungal ITS genes were detected in soils at the end of the first growing season, arbuscular mycorrhizal (AM) structures were scarce in
Lolium
roots in all treatments throughout the first season. However, in the second season, more than a year after the start of the experiment, AM fungal colonisation was detected in
Lolium roots
in virtually all treatments, with the frequency of colonised root length ranging from 30% to >75% in some treatments, the latter also in remediated soil. This study demonstrates the importance of plants and rhizosphere in the development and secondary succession of fungal communities in soil, which has important implications for the revitalisation of remediated soils and regenerative agriculture.
The feasibility of a novel EDTA-based soil heap leaching method with treatment and reuse of extractants in a closed process loop was evaluated on a laboratory scale. Ozone and UV irradiation were ...used for oxidative decomposition of EDTA-metal complexes in extractants from Pb (1243
mg
kg
−1) and Zn (1190
mg
kg
−1) contaminated soil. Released metals were absorbed in a commercial metal absorbent Slovakite. Six-consecutive additions of 2.5
mmol
kg
−1 EDTA (total 15
mmol
kg
−1 EDTA) removed 49.6
±
0.6% and 19.7
±
1.7% of initial total Pb and Zn from soil (4.6
kg) packed in 22
cm high columns. The efficiency of extraction was similar to small-scale simulations of heap leaching (150
g of soil), where EDTA used in the same manner removed 49.7
±
1.0% and 13.7
±
0.4% of Pb and Zn. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98
±
2.17
mg
l
−1, 4.55
±
2.36
mg
l
−1, and 0.05
±
0.04
mM, respectively), which could presumably be reduced even further with continuation of treatment. The results of our study indicate that for soils contaminated primarily with Pb, treating the EDTA extractants with ozone/UV and reuse of extractants enables efficient soil heap leaching with very little or no wastewater generation, easy control over emissions, and lowers the requirements for process water.
In a soil column experiment, we investigated the effect of 5
mmol
kg
−1 soil addition of citric acid, ethylenediamine tetraacetate (EDTA), diethylenetriamine-pentaacetate (DTPA) and S,S-stereoisomer ...of ethylenediamine-disuccinate (EDDS) on phytoextraction of Cu from a vineyard soil with 162.6
mg
kg
−1 Cu, into the test plant
Brassica rapa var.
pekinensis. We also examined the use of a horizontal permeable barrier, composed of layers of nutrient enriched sawdust and apatite, for reduction of chelator induced Cu leaching. The addition of all chelators, except citric acid, enhanced Cu mobility and caused leaching of 19.5–23% of initial total Cu from the soil column. However, Cu plant uptake did not increase accordingly; the most effective was the EDDS treatment, in which plant Cu concentration reached
37.8±1.3
mg
kg
−1
Cu and increased by 3.3-times over the control treatment. The addition of none of the chelators in the concentration range from 5 to 15
mmol
kg
−1 exerted any toxic effect on respiratory soil microorganisms. When EDDS was applied into the columns with horizontal permeable barriers, only 0.53±0.32% of the initial total Cu was leached. Cu (36.7%) was washed from the 18
cm soil layer above the barrier and accumulated in the barrier. Our results indicate that rather than for a reduction of Cu leaching during rather ineffective chelate induced Cu phytoextraction, horizontal permeable barriers could be more effective in a new remediation technique of controlled in situ soil washing of Cu with biodegradable chelates.
Contamination with toxic metals prevents the use of sewage sludge (SS) as a soil fertilizer. Hydrodynamic cavitation, thermal microwaving, microwave-assisted alkaline, and acid hydrolysis coupled ...with ethylenediaminetetraacetate (EDTA) washing were tested as a method to remove toxic metals from SS. Acid hydrolysis coupled with EDTA washing was most effective and was used in a closed-loop process based on ReSoil technology. EDTA and process solutions were recycled at a pH gradient of 12.5-2, which was imposed by the addition of quicklime (CaO) and H
SO
. An average of 78%-Pb, 76%-Zn, 1%-Cu, and 17%-Cr were removed from SS in five consecutive batches. No wastewater was generated, only solid waste (40%). The EDTA lost in the process (42%) was resupplied in each batch. In a series of batches, the process solutions retained metal removal efficiency and quality. The treatment removed 70% and 23% of P and N, respectively, from SS and increased the leachability of Zn, Cu, Mn, and Fe in the washed SS by 11.7, 6.8, 1.4, and 5.2 times, respectively. Acid hydrolysis coupled with EDTA washing proved to be a technically feasible, closed-loop process but needs further development to reduce reagent, material, and nutrient loss and to reduce toxic emissions from the washed sludge.
