The cup plant (Silphium perfoliatum L.) – a viable solution for bioremediating soils polluted with heavy metals

Heavy metal pollution, manifested by the accumulation, toxicity and persistence in soil, water, air, and living organisms, is a major environmental problem that requires energetic resolution. Mining tailing areas contain metal minerals such as Cu, Zn, Pb, Cr and Cd in high concentrations that pollute the environment and pose threats to human health. Phytoremediation represents a sustainable, long-term, and relatively inexpensive strategy, thus proving to be convenient for stabilizing and improving the environment in former heavy metal-polluted mining sites. This study presents the bioremediation potential of Silphium perfoliatum L. plants, in the vegetative stages of leaf rosette formation, grown on soil polluted with heavy metals from mining dumps in Moldova-Noua, in the Western part of Romania. The bioaccumulation factor (BAF), translocation factor (TF), metal uptake (MU) and removal efficiency (RE) of Cu, Zn, Cr and Pb by S. perfoliatum plants were determined in a potted experiment in controlled environmental conditions. The reference quantities of heavy metals have been determined in the studied soil sample. The experiment followed the dynamics of the translocation and accumulation of heavy metals in the soil, in the various organs of the silphium plants, during the formation of the leaf rosette (13-18 BBCH). The determination of the amount of heavy metals in soil and plants was achieved by the method of digestion with hydrochloric and nitric acid 3/1 (v/v) quantified by atomic absorption spectroscopy (AAS). The obtained experimental results demonstrate that the substrate has a high heavy metal content being at the alert threshold for Zn (260.01 mg kg-1 in substrate compared with alert threshold 300 mg kg-1) and at intervention thresholds for other metals (Cu -234.66 mg kg-1/200 mg kg-1; 299.08 mg kg-1/300 mg kg-1 and Pb-175.18 mg kg-1/100 mg kg-1). The average concentration of the metals determined in dynamics in the dry biomass of plants varied between roots, petioles, and laminas. The root is the main accumulator for Cu and Cr (Cu – 37.32 mg kg-1 -13 BBCH to 43.89 mg kg-1-15 BBCH and 80.71 mg kg-1 – 18 BBCH; Cr – 57.43 mg kg-1 – 13 BBCH to 93.36 mg kg-1 -18 BBCH), and for Zn and Pb the lamina seems to carry the same function. Preliminary results show that Silphium perfoliatum may be a viable alternative in the bioremediation and treatment of heavy metal-contaminated area

Progress in Understanding the Mechanism of CrVI Removal in Fe0-Based Filtration Systems

Hexavalent chromium (CrVI) compounds are used in a variety of industrial applications and, as a result, large quantities of CrVI have been released into the environment due to inadequate precautionary measures or accidental releases. CrVI is highly toxic to most living organisms and a known human carcinogen by inhalation route of exposure. Another major issue of concern about CrVI compounds is their high mobility, which easily leads to contamination of surface waters, soil, and ground waters. In recent years, attention has been focused on the use of metallic iron (Fe0) for the abatement of CrVI polluted waters. Despite a great deal of research, the mechanisms behind the efficient aqueous CrVI removal in the presence of Fe0 (Fe0/H2O systems) remain deeply controversial. The introduction of the Fe0-based filtration technology, at the beginning of 1990s, was coupled with the broad consensus that direct reduction of CrVI by Fe0 was followed by co-precipitation of resulted cations (CrIII, FeIII). This view is still the dominant removal mechanism (reductive-precipitation mechanism) within the Fe0 remediation industry. An overview on the literature on the Cr geochemistry suggests that the reductive-precipitation theory should never have been adopted. Moreover, recent investigations recalling that a Fe0/H2O system is an ion-selective one in which electrostatic interactions are of primordial importance is generally overlooked. The present work critically reviews existing knowledge on the Fe0/CrVI/H2O and CrVI/H2O systems, and clearly demonstrates that direct reduction with Fe0 followed by precipitation is not acceptable, under environmental relevant conditions, as the sole/main mechanism of CrVI removal in the presence of Fe0

Progress in Understanding the Mechanism of CrVI Removal in Fe0-Based Filtration Systems

Hexavalent chromium (CrVI) compounds are used in a variety of industrial applications and, as a result, large quantities of CrVI have been released into the environment due to inadequate precautionary measures or accidental releases. CrVI is highly toxic to most living organisms and a known human carcinogen by inhalation route of exposure. Another major issue of concern about CrVI compounds is their high mobility, which easily leads to contamination of surface waters, soil, and ground waters. In recent years, attention has been focused on the use of metallic iron (Fe0) for the abatement of CrVI polluted waters. Despite a great deal of research, the mechanisms behind the efficient aqueous CrVI removal in the presence of Fe0 (Fe0/H2O systems) remain deeply controversial. The introduction of the Fe0-based filtration technology, at the beginning of 1990s, was coupled with the broad consensus that direct reduction of CrVI by Fe0 was followed by co-precipitation of resulted cations (CrIII, FeIII). This view is still the dominant removal mechanism (reductive-precipitation mechanism) within the Fe0 remediation industry. An overview on the literature on the Cr geochemistry suggests that the reductive-precipitation theory should never have been adopted. Moreover, recent investigations recalling that a Fe0/H2O system is an ion-selective one in which electrostatic interactions are of primordial importance is generally overlooked. The present work critically reviews existing knowledge on the Fe0/CrVI/H2O and CrVI/H2O systems, and clearly demonstrates that direct reduction with Fe0 followed by precipitation is not acceptable, under environmental relevant conditions, as the sole/main mechanism of CrVI removal in the presence of Fe0

Effect of Sand Co-Presence on Cr^VI Removal in Fe⁰-H₂O System

The aim of the present study was to provide new knowledge regarding the effect of non-expansive inert material addition on anionic pollutant removal efficiency in Fe0-H2O system. Non-disturbed batch experiments and continuous-flow-through column tests were conducted using CrVI as a redox–active contaminant in three different systems: “Fe0 + sand”, “Fe0 only” and ”sand only”. Both experimental procedures have the advantage that formation of (hydr)oxide layers on Fe0 is not altered, which makes them appropriate proxies for real Fe0-based filter technologies. Batch experiments carried out at pH 6.5 showed a slight improvement of CrVI removal in a 20% Fe0 system, compared to 50, 80 and 100% Fe0 systems. Column tests conducted at pH 6.5 supported results of batch experiments, revealing highest CrVI removal efficiencies for “Fe0 + sand” systems with lowest Fe0 ratio. However, the positive effect of sand co-presence decreases with increasing pH from 6.5 to 7.1. Scanning electron microscopy—energy dispersive angle X-ray spectrometry and X-ray diffraction spectroscopy employed for the characterization of Fe0 before and after experiments indicated that the higher the volumetric ratio of sand in “Fe0 + sand” system, the more intense the corrosion processes affecting the Fe0 grains. Results presented herein indicate the capacity of sand at sustaining the efficiency of CrVI removal in Fe0-H2O system. The outcomes of the present study suggest that a volumetric ratio Fe0:sand = 1:3 could assure not only the long-term permeability of Fe0-based filters, but also enhanced removal efficiency of CrVI from contaminated water

Mitigation of Cr(VI) Aqueous Pollution by the Reuse of Iron-Contaminated Water Treatment Residues

Reducing the levels of heavy metals in wastewaters below the permissible limits is imperative before they are discharged into the environment. At the same time, water treatment technologies should be not only efficient, but also affordable. In accordance with these principles, this study assessed the possibility of recovering iron-contaminated residues, resulting from the treatment of synthetic acid mine drainage, for the subsequent remediation of Cr(VI) polluted aqueous solutions. Bentonite, an inexpensive and available natural material, was used as an adsorbent for the removal of Fe(II) from synthetic acid mine drainage (AMD). Then, Fe(II)-contaminated bentonite, the residue generated during the treatment of AMD, was recovered and activated with sodium borohydride in order to convert the adsorbed Fe(II) to Fe(0). Subsequently, the Fe(0)-containing bentonite (Be-Fe(0)) was further used for the treatment of Cr(VI) contaminated aqueous solutions. Reactive materials investigated in this work were characterized by means of scanning electron microscopy-energy dispersive angle X-ray spectrometry (SEM-EDX), X-ray diffraction spectroscopy (XRD), point of zero charge, and image photographs. The effect of several important parameters (pH, temperature, metal concentration, and ionic strength) on both treatment processes was investigated and discussed. It was shown that the efficiency of Cr(VI) removal with Be-Fe(0) was much higher than with bentonite. On the basis of the present study it can be concluded that residues generated during the treatment of AMD with bentonite can be used as a cheap precursor for the production an Fe(0)-based reagent, with good Cr(VI) removal potential

Sequential Abatement of FeII and CrVI Water Pollution by Use of Walnut Shell-Based Adsorbents

In this study walnut shells, an inexpensive and readily available waste, were used as carbonaceous precursor for preparation of an innovative adsorbent (walnut-shell powder (WSP)) which was successfully tested for the removal of FeII from synthetic acid mine drainage (AMD). Then, the exhausted iron-contaminated adsorbent (WSP-FeII) was recovered and treated with sodium borohydride for the reduction of adsorbed FeII to Fe0. The resulting material (WSP-Fe0) was subsequently tested for the removal of CrVI from aqueous solutions. Treatability batch experiments were employed for both FeII and CrVI-contaminated solutions, and the influence of some important experimental parameters was studied. In addition, the experimental data was interpreted by applying three kinetic models and the mechanism of heavy metal removal was discussed. The overall data presented in this study indicated that fresh WSP and WSP-Fe0 can be considered as promising materials for the removal of FeII and CrVI, respectively. Furthermore, the present work clearly showed that water treatment residuals may be converted in upgraded materials, which can be successfully applied in subsequent water treatment processes. This is an example of sustainable and environmentally-friendly solution that may reduce the adverse effects associated with wastes and delay expensive disposal methods such as landfilling or incineration