Marc Verloo

Characteristics of wood ash and influence on soil properties and nutrient uptake: an overview

Wood industries and power plants generate enormous quantities of wood ash. Disposal in landfills has been for long a common method for removal. New regulations for conserving the environment have raised the costs of landfill disposal and added to the difficulties for acquiring new sites for disposal. Over a few decades a number of studies have been carried out on the utilization of wood ashes in agriculture and forestry as an alternative method for disposal. Because of their properties and their influence on soil chemistry the utilization of wood ashes is particularly suited for the fertility management of tropical acid soils and forest soils. This review principally focuses on ash from the wood industry and power plants and considers its physical, chemical and mineralogical characteristics, its effect on soil properties, on the availability of nutrient elements and on the growth and chemical composition of crops and trees, as well as its impact on the environment.

Chemical speciation and fractionation in soil and sediment heavy metal analysis: a review

Abstract Today it is generally recognized that the particular behaviour of trace metals in the environment is determined by their specific physicochemical forms rather than by their total concentration. Several chemical speciation and fractionation methods for heavy metal analysis in soils and sediments have been and are still being developed and applied. They primarily are intended to understand the particular environmental behaviour of metals, present in a variety of forms and in a variety of matrices. Analytical developments, modifications of existing methods, and recent new approaches are reviewed and discussed. Techniques used include chemical extractions, ion-exchange/gel chromatography, filtration, centrifugation and sieving, selective solvent extraction. Moreover, the application of these various techniques in different research fields over the last years is explored. The value and the limitations of speciation and fractionation techniques applied in specific experimental work is outlined. It is d...

Heavy Metal Fractionation and Extractability in Dredged Sediment Derived Surface Soils

The objective of this study was to characterise pollution with heavy metals in surface soils sampled at various dredged sediment disposal sites in the Flemish region (Belgium). The sites selected varied in the period since sediment disposal ceased and in current vegetation and land use.Total metal contents (Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn) in the surface soils varied widely. For some disposal sites Cd and Zn concentrations greatly exceeded reference values for clean soil. The distribution of the metals as determined by sequential extraction differed between elements, but was simular for all the soils. This suggested that metals in these sediment derived surface soils were accumulated and transformed in a similar way. Residual fractions were low compared to total contents (2 – 4% for Cd, 25% – 35% for Co, 7 – 18% for Mn, 4 – 22% for Zn, 12 – 41% for Ni, 11 – 42% for Pb, 20 – 45% for Cu, < 10% for Zn). High metal concentrations in the acid-extractable and reducible fractions may indicate pollution from anthropogenic sources.DTPA-extractable metals, which may be considered indicative of plant-available contents, were relatively high compared to the total contents. The relative extractability, expressed as the ratio of DTPA-extractable to total contents, decreased in the order Cd (38%) > Cu (28%) = Zn (26%) > Pb (13%) > Ni (10%) > Co (3%). Most of the sites studied would be of concern if they were used for agricultural activities. No trends in metal availability in the period following disposal were apparent from the data.

Enhanced phytoextraction: in search of EDTA alternatives.

ABSTRACT Enhanced phytoextraction proposes the use of soil amendments to increase the heavy-metal content of above-ground harvestable plant tissues. This study compares the effect of synthetic aminopolycarboxylic acids [ethylenediamine tetraacetatic acid (EDTA), nitriloacetic acid (NTA), and diethylenetriamine pentaacetic acid (DTPA)] with a number of biodegradable, low-molecular weight, organic acids (citric acid, ascorbic acid, oxalic acid, salicylic acid, and NH4 acetate) as potential soil amendments for enhancing phytoextraction of heavy metals (Cu, Zn, Cd, Pb, and Ni) by Zea mays. The treatments in this study were applied at a dose of 2 mmol/kg−1 1 d before sowing. To compare possible effects between presow and postgermination treatments, a second smaller experiment was conducted in which EDTA, citric acid, and NH4 acetate were added 10 d after germination as opposed to 1 d before sowing. The soil used in this screening was a moderately contaminated topsoil derived from a dredged sediment disposal site. This site has been in an oxidized state for more than 8 years before being used in this research. The high carbonate, high organic matter, and high clay content characteristic to this type of sediment are thought to suppress heavy-metal phytoavailability. Both EDTA and DTPA resulted in increased levels of heavy metals in the above-ground biomass. However, the observed increases in uptake were not as large as reported in the literature. Neither the NTA nor organic acid treatments had any significant effect on uptake when applied prior to sowing. This was attributed to the rapid mineralization of these substances and the relatively low doses applied. The generally low extraction observed in this experiment restricts the use of phytoextraction as an effective remediation alternative under the current conditions, with regard to amendments used, applied dose (2 mmol/kg−1 soil), application time (presow), plant species (Zea mays), and sediment (calcareous clayey soil) under study.

Metal solubility as a function of pH in a contaminated, dredged sediment affected by oxidation.

The solubility as a function of pH for metals in a reduced dredged sediment, subjected to different redox conditions, was studied in a laboratory experiment. The redox conditions imposed simulated (i) the undisturbed sediment (flooded), (ii) a dredged material stored in a confined pond (aerated once and then flooded), (iii) an upland stored dredged material (drained and dried), and (iv) an upland stored sediment subjected to tillage (drained, dried and mixed). Minor differences in the solubility as a function of pH were observed between the treatments after two weeks. After three months, the solubility of Cd, Cu, Pb and Zn increased strongly in the oxidized sediments. Leachability of Fe decreased, while Mn, Ni and Co were mostly unaffected. Both short- and long-term mobility of metals (except Fe) is expected to be lowest when a reduced sediment remains in reduced conditions. Studying the solubility as a function of pH may provide additional information on the chemical association of metals in sediments.

Baseline concentration levels of trace elements as a function of clay and organic carbon contents in soils in Flanders (Belgium)

Abstract Baseline concentration levels of trace elements in the soils of Flanders (Belgium) were assessed as a function of clay and organic carbon contents. Outliers in the data were identified and rejected according to statistical criteria. Median trace element concentrations in mg/kg dry wt. were As 7, Cd 0.18, Co 0.05, Cr 24.6, Cu 9.6, Hg 0.17, Ni 3.5, Pb 21.5, Zn 34.5. Clay and organic carbon contents were found to be significant in predicting soil trace element contents in unpolluted soils. The coefficients of determination ranged from 0.07 (Hg) to 0.69 (Ni). Based on confidence intervals of the observations, trace element concentrations that have a known probability of being exceeded can be assessed as a function of clay and organic carbon content and used as soil quality reference values.

Leaching behaviour of Cd, Cu, Pb and Zn in surface soils derived from dredged sediments

Leaching of heavy metals from land-disposed dredged sediment spoils is a potential environmental hazard. The leaching behavior of Cd, Cu, Pb and Zn in surface soils sampled from abandoned dredged sediment disposal sites was assessed. Using simple mass-balance calculations, the significance of the leaching test results with respect to metal migration into underlying clean soil was appraised. The potential leachability, defined as the amounts released at constant pH 4, decreased in the order (% of total contents): Zn (58%) approximately equal to Cd (49%)>Cu (5%) approximately equal to Pb (2%). The kinetics of metal release were determined in a cascade shaking test using de-mineralized water acidified to pH 4 (HNO(3)). Metal concentrations in the leachates were low and metal migration was, assuming uniform convective flow, predicted to be of no environmental concern. It is emphasized that any long-term prediction of metal migration is uncertain.

Chemically Assisted Phytoextraction: A Review of Potential Soil Amendments for Increasing Plant Uptake of Heavy Metals

The contamination of soils by trace metals has been an unfortunate sideeffect of industrialization. Some of these contaminants can interfere with vulnerable enduses of soil, such as agriculture or nature, already at relatively low levels of contamination. Reversely, conventional civil–technical soil-remediation techniques are too expensive to remediate extended areas of moderately contaminated soil. Phytoextraction has been proposed as a more economic complementary approach to deal with this specific niche of soil contamination. However, phytoextraction has been shown to be a slow-working process due to the low amounts of metals that can be annually removed from the soil under normal agronomic conditions. Therefore, extensive research has been conducted on process optimization by means of chemically improving plant availability and the uptake of heavy metals. A wide range of potential amendments has been proposed in the literature, with considerable attention being spent on aminopolycarboxylic acids such as ethylenediaminetetraacetic acid (EDTA). However, these compounds have received increasing criticism due to their environmental persistence and associated risks for leaching. This review presents an overview of potential soil amendments that can be employed for enhancing metal uptake by phytoextraction crops, with a distinct focus on more degradable alternatives to persistent compounds such as EDTA.

Characterisation of Malian and Belgian solid waste composts with respect to fertility and suitability for land application

Two composts, a Malian (C1) and a Belgian (C2), and a peat substrate (C3) were analyzed for their suitability for land application. The results revealed that the materials can be used in agriculture but only the application of the peat substrate may create N immobilization. Composts had higher P, Ca, and Mg contents and lower C and K contents and C/N ratio than the peat substrate. The available P extracted from the three materials ranged from 15% (for C2) to 48% (for C3) and available K from 36% (in the composts) to 48% (for C3) of the total elements respectively. The fractionation of Mn, Fe, Zn and Cu in operationally defined fractions (water soluble, exchangeable, complexed, organically bound and residual) allows an estimation of the availability of metals for uptake. For example, 16% of the total Mn in the composts (C1 and C2) and 22% in the peat substrate would be plant available. Available Fe in the three materials was less than 2%. Available Zn varied between 10 and 25%. The fractionation allowed an estimation of the potential for contamination of groundwater following the applications of composts to agricultural lands.

Chemical effects of wood ash on plant growth in tropical acid soils

Abstract A greenhouse study was conducted to compare chemical effects of wood ash and CaCO3 on the growth of rye grass (Lolium perenne L.) in three tropical acid soils (Kandiudult). Amendments were applied at rates to attain target pH values of 5.5, 6.0 and 6.5. Generally, plants grown on ash-amended soils showed higher biomass production than did plants grown on lime and control treatments. After wood ash application, plants benefited from Ca and K supplementation, the synergy K-NO−3 and from changes in soil chemistry similar to lime application: higher ECEC and reduced Al and Mn toxicity. Therefore, wood ash appeared at the same time as a neutralizer of soil acidity and as a supplier of nutrients for tropical acid soils.