Combining biochar and grass-legume mixture to improve the phytoremediation of soils contaminated with potentially toxic elements (PTEs)

The combination of soil amendments with plants can be a viable option for restoring the functionality of PTEs-contaminated soils. Soil recovery could be further optimized through the mixed cropping of plant species (e.g. legumes and grasses) with different physiological characteristics. The aim of this study was to assess the phytoremediation ability of Vicia villosa Roth. And Lolium rigidum Gaud. Grown alone or in mixture in a soil contaminated with PTEs (C), i.e. Cd (23 mg kg-1), Pb (4473 mg kg-1) and Zn (3147 mg kg-1), and amended with 3% biochar (C + B). Biochar improved soil fertility and changed PTEs distribution, reducing soluble fractions and increasing the more stable ones. The addition of biochar increased the plant biomass of hairy vetch and annual ryegrass, both in monoculture and when in mixture. For example, shoot and root biomass of the C + B intercropped hairy vetch and annual ryegrass increased 9- and 7-fold, and ∼3-fold respectively, compared to the respective C plants. The biochar addition decreased PTE-uptake by both plants, while mixed cropping increased the uptake of PTEs by shoots of hairy vetch grown in C and C + B. The bioaccumulation, translocation factors, and mineralomass showed that hairy vetch and annual ryegrass behaved as phytostabilising plants. PTE mineralomasses proved that mixed cropping in C + B increased the overall capacity of PTE accumulation by plant tissues, particularly the root system. Therefore, the combination of biochar and legumes/grasses mixed cropping could be an effective solution for the recovery of PTEs-contaminated soils and the mitigation of their environmental hazard.

Steam assisted slow pyrolysis of contaminated biomasses: Effect of plant parts and process temperature on heavy metals fate

The post-treatment of biomass from phytoremediation is not yet a well-established practice due to the risk induced by the presence of Potentially Toxic Elements (PTEs). Pyrolysis is a thermochemical treatment that reduces the volume and weight of contaminated matter producing a combustible vapor phase and a solid residue (char). A key factor enhancing the economic and the environmental sustainability of biomass valorization through pyrolysis is the production of a market value char. A proper choice of the pyrolysis operating conditions should take into account the effect of final temperature on PTEs release, on the char physicochemical properties as well as on the mobility of retained PTEs. In particular, in this work the influence of both the temperature and the plant parts is discussed (branches and leaves of Populus Nigra L. and rhizomes and culms of Arundo donax L.) on the release of Cd, Pb, Cu, and Zn in the temperature range 653-873 K under steam assisted slow pyrolysis conditions. The mobility of the heavy metals retained in the chars was also studied as well as the product yields, the gas composition and char porosity. The results suggested that in presence of Cd it is necessary to operate at low-temperature (lower than 703 K) to obtain a heavy metals free vapor phase fuel, whereas in presence of one or more metals among Pb, Cu, and Zn, it is possible to conduct a pyrolytic treatment at higher temperatures, thus obtaining a char with high BET surface area and lower metals mobility.

Pyrolysis for exploitation of biomasses selected for soil phytoremediation: Characterization of gaseous and solid products

Biomasses to be used in the phytoremediation process are generally selected to match agronomic parameters and heavy metals uptake ability. A proper selection can be made greatly effective if knowledge of the properties of the residual char from pyrolysis is available to identify possible valorization routes. In this study a comparative analysis of the yields and characteristics of char obtained from slow pyrolysis of five uncontaminated biomasses (Populus nigra, Salix alba, Fraxinus oxyphylla, Eucalyptus occidentalis and Arundo donax) was carried out under steam atmosphere to better develop char porosity. Moreover, the dependence of the properties of solid residue on the process final temperature was studied for E. occidentalis in the temperature range of 688-967K. The results demonstrate that, among the studied biomasses, chars from P. nigra and E. occidentalis have to be preferred for applications regulated by surface phenomena given their highest surface area (270-300m²/g), whereas char from E. occidentalis is the best choice when the goal is to maximize energy recovery.