Leaching variations of heavy metals in chelator-assisted phytoextraction by Zea mays L. exposed to acid rainfall

Critical evaluation of the performance of rhamnolipids as surfactants for (phyto)extraction of Cd, Cu, Fe, Pb and Zn from copper smelter-affected soil

Rhamnolipids are biosurfactants produced by bacteria belonging to the Pseudomonas genus. They are discussed to complex heavy metal cations stronger than cations of Fe, Ca, Mg. It is therefore suggested to employ rhamnolipids in phytoextraction where their addition to soil should result in preferential complexation of heavy metals that can be taken up by plants, thus enabling rapid and ecological clean-up of contaminated soil. In order to test this concept, we evaluated the rhamnolipid-mediated phytoextraction of heavy metal from soil collected from the vicinity of a copper smelter. The following aspects were investigated: i) selectivity of rhamnolipids towards Cu, Zn, Pb, Cd and Fe during soil washing; ii) phytoextraction efficiency of each ion with respect to the effective concentration of rhamnolipids; iii) possible phytotoxic effects; iv) effect of micro-sized polystyrene amendment. The experiments evaluated soil washing efficiency, BCR (Community Bureau of Reference) sequential extraction to determine the impact of rhamnolipids on the mobility of metal ions, phytoextraction with maize (Zea mays L.) and phytotoxic effects based on dry matter, chlorophyll fluorescence and content. The obtained results indicated that rhamnolipids lack desired selectivity towards heavy metal ions as Fe was complexed more efficiently by 80 % of the available rhamnolipids compared to priority pollutants like Zn, Cu, Pb, which were complexed by only 20 % of the tested rhamnolipids. With increased concentration of rhamnolipids, the soil washing efficiency increased and shifted in favour of Fe, reaching values of approx. 469 mg for Fe and only 118 mg in total of all tested heavy metals. Phytoextraction also favoured the accumulation of Fe, while Cd was not removed from the soil even at the highest applied rhamnolipid concentrations. Considering the selectivity of rhamnolipids and the costs associated with their production, our results suggest the need to search for other alternative (bio)surfactants with better selectivity and lower price.

Basic principles for biosurfactant-assisted (bio)remediation of soils contaminated by heavy metals and petroleum hydrocarbons - A critical evaluation of the performance of rhamnolipids

Despite the fact that rhamnolipids are among the most studied biosurfactants, there are still several gaps which must be filled. The aim of this review is to emphasize and to indicate which issues should be taken into account in order to achieve efficient rhamnolipids-assisted biodegradation or phytoextraction of soils contaminated by heavy metals and petroleum hydrocarbons without harmful side effects. Four main topics have been elucidated in the review: effective concentration of rhamnolipids in soil, their potential phytotoxicity, susceptibility to biodegradation and interaction with soil microorganisms. The discussed elements are often closely associated and often overlap, thus making the interpretation of research results all the more challenging. Each dedicated section of this review includes a description of potential issues and questions, an explanation of the background and rationale for each problem, analysis of relevant literature reports and a short summary with possible application guidelines. The main conclusion is that there is a necessity to establish regulations regarding effective concentrations for rhamnolipids-assisted remediation of soil. The use of an improper concentration is the direct cause of all the other discussed phenomena.

Comparative Activation Process of Pb, Cd and Tl Using Chelating Agents from Contaminated Red Soils

Adding chelating agents is a critical technique of heavy metal activation for enhancing phytoextraction through the formation of soluble metal complexes which will be more readily available for extraction. The preliminary, dynamic, equilibrium activation experiments and speciation analysis of Pb, Cd and Tl in contaminated red soils were used to select six chelates with relatively good activation performance from nine chelates, and the effects of dosage and pH on the heavy metals activation were studied systematically. Results showed that the activation of Pb, Cd and Tl by chelates reached equilibrium within 2 h, and the activation process showed three stages. Under neutral conditions, chelates had better activation performance on Pb- and Cd-contaminated soils. Except for S,S-ethylenediamine disuccinic acid (S,S-EDDS) and citric acid (CA), the maximum equilibrium activation effect (MEAE) of ethylenediaminetetraacetic acid (EDTA), N,N-bis (carboxymethyl) glutamic acid (GLDA), diethylenetriaminepentaacetic acid (DTPA) and aminotriacetic acid (NTA) was over 81%. The MEAE of Tl-contaminated soil was less than 15%. The decreasing order of the dosage of chelating agents corresponding to MEAE for three types of contaminated soils was Pb-, Cd- and Tl-contaminated soil, relating to the forms of heavy metals, the stability constants of metal–chelates and the activation of non-target elements Fe in red soil. Under acidic conditions, the activation efficiencies of chelates decreased to differing degrees in Pb- and Cd-contaminated soils, whereas the activation efficiencies of chelating agents in Tl-contaminated soils were slightly enhanced.

Insights into Heavy Metals Leakage in Chelator-Induced Phytoextraction of Pb- and Tl-Contaminated Soil

Chelators including DTPA (diethylene triamine pentaacetic acid) and oxalic acid were selected for inducing phytoextraction of heavy metals (HMs) from Pb-, Tl-, and Pb-Tl- contaminated soil, in which heavy metals leakage was highly remarkable. Results showed that compared with the control group without chelating agent under planting conditions, the extraction efficiency (i.e., uptake coefficient) of Pb, Tl increased by 86%, 43% from Pb-Tl- contaminated soil in the presence of oxalic acid, and there was no significant change in heavy metal leakage under rainfall conditions. It was the best phytoremediation scheme in this work. Under rainfall conditions, the HMs concentration in the leachate showed a linear decreasing trend. Acid rain promoted the leakage of heavy metals, and the average leached amount of Tl increased by 1.47 times under acid rain conditions. However, for Pb, DTPA was the main influencing factor, followed by acid rain.

Natural and assisted dissipation of polycyclic aromatic hydrocarbons in a long-term co-contaminated soil with creosote and potentially toxic elements

An enhanced bioremediation strategy was applied to an industrial soil co-contaminated with Polycyclic Aromatic Hydrocarbons (PAHs) and Potentially Toxic Elements (PTEs). Hydroxypropyl-β-cyclodextrin (HPBCD) and a natural mixture of two rhamnolipids (RL) were added to increase PAHs bioavailability, and combined with a microbial consortium (MC) to biodegrade soil PAHs. Bioavailability of only six PAHs (3-, 4-ring PAHs) increased when using HPBCD, with a maximum increase about 2.8-fold higher. The highest dose of HPBCD (5%) enhanced PAH degradation, with the best results for 4-ring PAHs with treatments of HPBCD + MC (up to 48% degradation for pyrene and 43% for fluoranthene), whereas dissipation for 5-ring PAHs was very low and for 6-ring was negligible. The use of RL increased the bioavailability of 13 of the 16 PAHs studied, reaching up to 60-fold higher values for phenanthrene or 18-fold higher for acenaphtene. RL addition did not show degradation improvement in any situation, and even inhibited the scarce degradation observed in the control treatment. The high increase in availability of both PAHs and mainly PTEs when using RL as amendment could make them toxic for microorganisms. In fact, Microtox Acute Toxicity test using Aliivibrio fischeri and the absence of colony forming units (CFUs) of indigenous bacteria demonstrated the extremely high levels of toxicity in RL treated soil.