Jan Japenga

Feasibility of phytoextraction to remediate cadmium and zinc contaminated soils

A Cd and Zn contaminated soil was mixed and equilibrated with an uncontaminated, but otherwise similar soil to establish a gradient in soil contamination levels. Growth of Thlaspi caerulescens (Ganges ecotype) significantly decreased the metal concentrations in soil solution. Plant uptake of Cd and Zn exceeded the decrease of the soluble metal concentrations by several orders of magnitude. Hence, desorption of metals must have occurred to maintain the soil solution concentrations. A coupled regression model was developed to describe the transfer of metals from soil to solution and plant shoots. This model was applied to estimate the phytoextraction duration required to decrease the soil Cd concentration from 10 to 0.5 mg kg(-1). A biomass production of 1 and 5 t dm ha(-1) yr(-1) yields a duration of 42 and 11 yr, respectively. Successful phytoextraction operations based on T. caerulescens require an increased biomass production.

Predictions of spatially averaged cadmium contents in rice grains in the Fuyang valley, P.R. China.

Soils in the Fuyang valley (Zhejiang province, southeast China) have been contaminated by heavy metals. Since rice (Oryza sativa L.) is the dominant crop in the valley and because of its tendency to accumulate Cd in its grains, assessment of the human health risk resulting from consumption of locally produced rice is needed. In this study, we used a regression model to predict the average Cd content in rice grains for paddy fields. The multiple linear model for log(Cd) content in rice grains with log(HNO(3)-Cd), pH, log(clay), and log(soil organic matter, SOM) as predictors performed much better (R(2)(adj) = 66.1%) than the model with log(CaCl(2)-Cd) as a single predictor (R(2)(adj) = 28.1%). This can be explained by the sensitivity of CaCl(2)-extracted Cd for changes in redox potential and as a result of the drying of the soil samples in the laboratory. Consequently, the multiple linear model was used to predict the average Cd contents in rice grains for paddy fields, and to estimate the probability that the FAO/WHO standard of 0.2 mg kg(-1) will be exceeded. Eleven blocks had a probability smaller than 10% of exceeding this standard (safe blocks). If a lognormal distribution is assumed, 35 blocks had a probability larger than 90% (blocks at risk). Hence, risk reduction measures should be undertaken for the blocks at risk. For 27 blocks the probability was between 10 and 90%. For these blocks the uncertainty should be reduced via improvement of the regression model and/or increasing the number of sample locations within blocks.

A feasibility test to estimate the duration of phytoextraction of heavy metals from polluted soils.

The practical applicability of heavy metal (HM) phytoextraction depends heavily on its duration. Phytoextraction duration is the main cost factor for phytoextraction, both referring to recurring economic costs during phytoextraction and to the cost of the soil having no economic value during phytoextraction. An experiment is described here, which is meant as a preliminary feasibility test before starting a phytoextraction scheme in practice, to obtain a more realistic estimate of the phytoextraction duration of a specific HM-polluted soil. In the experiment, HM-polluted soil is mixed at different ratios with unpolluted soil of comparable composition to mimic the gradual decrease of the HM content in the target HM-polluted soil during phytoextraction. After equilibrating the soil mixtures, one cropping cycle is carried out with the plant species of interest. At harvest, the adsorbed HM contents in the soil and the HM contents in the plant shoots are determined. The adsorbed HM contents in the soil are then related to the HM contents in the plant shoots by a log–log linear relationship that can then be used to estimate the phytoextraction duration of a specific HM-polluted soil. This article describes and evaluates the merits of such a feasibility experiment. Potential drawbacks regarding the accuracy of the described approach are discussed and a greenhouse–field extrapolation procedure is proposed.

Protocols for Applying Phytotechnologies in Metal-Contaminated Soils

Contamination with heavy metals continues to pose a serious challenge for the remediation of polluted soil, as they are not degradable and must be physically removed. At present, most technologies used for removing heavy metals from the soil greatly affect the biogeochemical characteristics of the soil. In many cases, the soil can no longer be considered a useful and productive soil resource, and the treated soil has to be disposed of in landfills. Phytoremediation is the only solution that approaches the problem from an eco-sustainable point of view—it is environmentally friendly and relatively cheap. In this chapter, two phytotechnology approaches for remediating heavy metal-contaminated soil will be discussed, along with protocols for their implementation: phytoextraction and phytostabilization. Phytoremediation as a technique for rehabilitating heavy metal-polluted land therefore requires protocols and decision-support tools to assess the most appropriate approach, based on site-specific characteristics and requirements for soil status during and after remediation. Decisions have to be made on whether to use phytoextraction or phytostabilization, or even reject phytoremediation as a whole. Protocols and decision tools, from modeling and laboratory tests to full-blown feasibility studies, will be discussed.