Remediation of sediment and water contaminated by copper in small-scaled constructed wetlands: effect of bioaugmentation and phytoextraction

Enhanced removal of heavy metals and metalloids by constructed wetlands: A review of approaches and mechanisms

Constructed wetlands (CWs) are increasingly employed to remediate heavy metal and metalloid (HMM)-polluted water. However, the disadvantages of HMM removal by conventional CWs (without enhancement), such as an unstable and unpredictable removal efficiency, hinder the reliability of this technology. The objective of this study was to review research on enhanced CWs for HMM removal. In particular, we performed a bibliometric analysis to evaluate research trends, critical literature, and keyword evolution in recent years. Subsequently, we reviewed various enhanced approaches for the application of CWs for the removal of HMMs, including the use of improved substrates, aquatic macrophytes, microorganisms, bioelectrochemical coupling systems, hybrid CW, external additives, and operation parameters. Furthermore, the main mechanisms underlying HMM removal by these approaches are summarized. Our review clearly reveals that research on the remediation of HMM-polluted water via CW technology is receiving increased attention, with no apparent trends in topics. The selection of appropriate enhanced approaches or operation parameters as well as methodological improvements should be based on the dominant environmental conditions of the CW column and removal mechanisms for the targeted HMMs. Based on the established literature, several suggestions are proposed to guide the optimization of the design and operation of efficient CWs for the treatment of HMM-polluted water.

Siderophores: an alternative bioremediation strategy?

Siderophores are small molecular weight iron scavengers that are mainly produced by bacteria, fungi, and plants. Recently, they have attracted increasing attention because of their potential role in environmental bioremediation. Although siderophores are generally considered to exhibit high specificity for iron, they have also been reported to bind to various metal and metalloid ions. This unique ability allows siderophores to solubilise and mobilise heavy metals and metalloids from soil, thereby facilitating their bioremediation. In addition, because of their redox nature, they can mediate the production of reactive oxygen species (ROS), and thus promote the biodegradation of organic contaminants. The aim of this review is to summarise the existing knowledge on the developed strategies of siderophore-assisted bioremediation of metals, metalloids, and organic contaminants. Additionally, this review also includes the biosynthesis and classification of microbial and plant siderophores.

Advantages and limits to copper phytoextraction in vineyards

Copper (Cu) contamination of soils may alter the functioning and sustainability of vineyard ecosystems. Cultivating Cu-extracting plants in vineyard inter-rows, or phytoextraction, is one possible way currently under consideration in agroecology to reduce Cu contamination of vineyard topsoils. This option is rarely used, mainly because Cu phytoextraction yields are too low to significantly reduce contamination due to the relatively "low" phytoavailability of Cu in the soil (compared to other trace metals) and its preferential accumulation in the roots of most extracting plants. This article describes the main practices and associated constraints that could theoretically be used to maximize Cu phytoextraction at field scale, including the use of Cu-accumulating plants grown (i) with acidifying plants (e.g., leguminous plants), and/or (ii) in the presence of acidifying fertilizers (ammonium, elemental sulfur), or (iii) with soluble "biochelators" added to the soil such as natural humic substances or metabolites produced by rhizospheric bacteria such as siderophores, in the inter-rows. This discussion article also provides an overview of the possible ways to exploit Cu-enriched biomass, notably through ecocatalysis or biofortification of animal feed.

Effect of Pseudomonas putida-producing pyoverdine on copper uptake by Helianthus annuus cultivated on vineyard soils

Bioaugmentation-assisted phytoextraction was used to reduce the Cu load in vineyard soils. While performance is usually the endpoint of such studies, here we identified some mechanisms underlying Cu soil to plant transfer, particularly the role of siderophores in the extraction of Cu from the soil-bearing phases and its phytoavailability. Carbonated vs. non‑carbonated vineyard soils were cultivated with sunflower in rhizoboxes bioaugmented with Pseudomonas putida. gfp-Tagged P. putida was monitored in the soil and pyoverdine (Pvd), Cu, Fe, Mn, and Zn were measured in the soil solution. Trace elements (TE) were analysed in the roots and shoots. Plant growth and nutritional status were also measured. With bioaugmentation, the concentration of total Cu (vs. Cu²⁺) in the soil solution increased (decreased) by a factor of 1.6 to 2.6 (7 to 13) depending on the soil. The almost 1:1 relationship between the excess of Fe + Cu mobilized from the solid phase and the amount of Pvd in the soil solution in bioaugmented treatments suggests that Pvd mobilized Fe and Cu mainly by ligand-controlled dissolution via a 1:1 metal-Pvd complex. Bioaugmentation increased the Cu concentration by 17% in the shoots and by 93% in the roots, and by 30% to 60% the sunflower shoot biomass leading to an increase in the amount of Cu phytoextracted by up to 87%. The amount of Fe, Mn, Zn, and P also increased in the roots and shoots. Contrary to what was expected, carbonated soil did not increase the mobilization of TE. Our results showed that bioaugmentation increased phytoextraction, and its performance can be further improved by promoting the dissociation of Pvd-Cu complex in the solution at the soil-root interface.

Bioaugmented Phytoremediation of Metal-Contaminated Soils and Sediments by Hemp and Giant Reed

We assessed the effects of EDTA and selected plant growth-promoting rhizobacteria (PGPR) on the phytoremediation of soils and sediments historically contaminated by Cr, Ni, and Cu. A total of 42 bacterial strains resistant to these heavy metals (HMs) were isolated and screened for PGP traits and metal bioaccumulation, and two Enterobacter spp. strains were finally selected. Phytoremediation pot experiments of 2 months duration were carried out with hemp (Cannabis sativa L.) and giant reed (Arundo donax L.) grown on soils and sediments respectively, comparing in both cases the effects of bioaugmentation with a single PGPR and EDTA addition on plant and root growth, plant HM uptake, HM leaching, as well as the changes that occurred in soil microbial communities (structure, biomass, and activity). Good removal percentages on a dry mass basis of Cr (0.4%), Ni (0.6%), and Cu (0.9%) were observed in giant reed while negligible values (<100‰) in hemp. In giant reed, HMs accumulated differentially in plant (rhizomes > > roots > leaves > stems) with largest quantities in rhizomes (Cr 0.6, Ni 3.7, and Cu 2.2 g plant^–1). EDTA increased Ni and Cu translocation to aerial parts in both crops, despite that in sediments high HM concentrations in leachates were measured. PGPR did not impact fine root diameter distribution of both crops compared with control while EDTA negatively affected root diameter class length (DCL) distribution. Under HM contamination, giant reed roots become shorter (from 5.2 to 2.3 mm cm^–3) while hemp roots become shorter and thickened from 0.13 to 0.26 mm. A consistent indirect effect of HM levels on the soil microbiome (diversity and activity) mediated by plant response (root DCL distribution) was observed. Multivariate analysis of bacterial diversity and activity revealed not only significant effects of plant and soil type (rhizosphere vs. bulk) but also a clear and similar differentiation of communities between control, EDTA, and PGPR treatments. We propose root DCL distribution as a key plant trait to understand detrimental effect of HMs on microbial communities. Positive evidence of the soil-microbe-plant interactions occurring when bioaugmentation with PGPR is associated with deep-rooting perennial crops makes this combination preferable over the one with chelating agents. Such knowledge might help to yield better bioaugmented bioremediation results in contaminated sites.

Combination of plant-growth-promoting and fluoranthene-degrading microbes enhances phytoremediation efficiency in the ryegrass rhizosphere

Plant- and/or microbe-based systems can provide a cost-effective, sustainable means to remove contaminants from soil. Microbe-assisted phytoremediation has potential utility for polycyclic aromatic hydrocarbons such as fluoranthene (Flu) removal from soils; however, the efficiency varies with the plant and microbes used. This study evaluated the Flu removal efficiency in a system with ryegrass (Lolium multiflorum), an IAA-producing Arthrobacter pascens strain (ZZ21), and/or a Flu-degrading Bacillus cereus strain (Z21). Strain ZZ21 significantly enhanced the growth of ryegrass. Ryegrass in combination with both strains (FIP) was the most effective method for Flu removal. By day 60, 74.9% of the Flu was depleted in the FIP treatment, compared with 21.1% in the control (CK), 63.7% with ryegrass alone (P), 69.0% for ryegrass with ZZ21 (IP), and 72.6% for ryegrass with Z21 (FP). FIP treatment promoted ryegrass growth, accelerated Flu accumulation in plants, and increased soil microbial counts. Microbial carbon utilization was significantly higher in soil in the FIP than with the CK treatment. Principal component analysis of the distribution of carbon substrate utilization showed that microbial functional profiles diverged among treatments, and this divergence became more profound at day 60 than day 30. Microbial inoculation significantly enhanced microbial utilization of phenols. Microbes in the FIP soil dominantly utilized amines/amides and phenols at day 30 but shifted to carbohydrates by day 60. Together, the combination of IAA-producing microbes and Flu-degrading microbes could promote plant growth, facilitate Flu degradation, and change soil microbial functional structure.

Role of plant growth promoting rhizobacteria in the alleviation of lead toxicity to Pisum sativum L

Plant-microbe interaction is a significant tool to tackle heavy metals problem in the soil. A pot trial was conducted to evaluate the efficiency of lead tolerant rhizobacteria in improving pea growth under Pb stress. Lead sulfate (PbSO4) was used for spiking (250, 500, and 750 mg kg-1). Results indicated that inoculation with Pb-tolerant PGPR strain not only alleviated the harmful impacts of Pb on plant growth but also immobilized it in the soil. PGPR in the presence of Pb at concentrations of 0, 250, 500 and 750 mg kg-1, increased shoot and root lengths by 21, 15, 18% and 72, 80, 84%, respectively, than uninoculated control. Moreover, fresh biomass of shoots and roots were also increased by 51, 45, 35% and 57, 101, 139% respectively, at Pb concentrations of 250, 500 and 750 mg kg-1. In addition, PGPR inoculation also reduced Pb concentration in the roots and shoots by 57, 55, 49% and 70, 56 and 58% respectively, than uninoculated control. So, PGPR proved to be an efficient option for reducing Pb mobility and can be effectively used for its phytostabilization. Novelty statementLead (Pb) is highly noxious and second most toxic element in the nature having high persistence. It ranks 1st in the priority list of hazardous substances and causes adverse effects after its entry into the living system. So, its remediation is inevitable. Plant growth promoting rhizobacteria (PGPR) possess the potential to not only survive under stressed environments, but also promote plant growth on account of their different plant growth promoting mechanisms.Most researchers have worked on its bioaccumulation in plant body. This study however, used pea as a test crop and caused Pb phytostabilization and thereby, suppressed its entry in the above-ground plant parts.

Inoculation with abscisic acid (ABA)-catabolizing bacteria can improve phytoextraction of heavy metal in contaminated soil

Promotion of plant capacity for accumulation of heavy metals (HMs) is one of the key strategies in enhancing phytoremediation in contaminated soils. Here we report that, Rhodococcus qingshengii, an abscisic acid (ABA)-catabolizing bacteria, clearly boosts levels of Cd, Zn, and Ni in wild-type Arabidopsis by 47, 24, and 30%, respectively, but no increase in Cu was noted, when compared with non-inoculated Arabidopsis plants in contaminated growth substrate. Furthermore, when compared with wild-type plants, R.qingshengii-induced increases in Cd, Zn, and Ni concentrations were more pronounced in abi1/hab1/abi2 (ABA-sensitive mutant) strains of Arabidopsis, whereas little effect was observed in snrk2.2/2.3 (ABA insensitive mutant). This demonstrates that metabolizing ABA might be indispensable for R. qingshengii to improve metal accumulation in plants. Bacterial inoculation significantly elevated the expression of Cd, Zn, and Ni-related transporters; whereas the transcript levels of Cu transporters remained unchanged. This result may be a reasonable explanation for why the uptake of Cd, Zn, and Ni in plants was stimulated by bacterial inoculation, while no effect was observed on Cu levels. From our results, we clearly demonstrate that R. qingshengii can increase the accumulation of Cd, Zn, and Ni in plants via an ABA-mediated HM transporters-associated mechanism. Metabolizing ABA in the plants by ABA-catabolizing bacterial inoculation might be an alternative strategy to improve phytoremediation efficiency in HMs contaminated soil.

The Application of Different Biological Remediation Strategies to PCDDs/PCDFs Contaminated Urban Sediments

Our aim was to assess the efficacy of four different bioremediation strategies applied to soil treated with urban sediments for alleviating soil phytotoxicity (examined using Lepidium sativum), by removing polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and mitigating the toxic effect on plants by the applied sediment: (1) Natural attenuation, (2) phytoremediation with the use of two plants Tagetes patula L. and Festuca arundinacea, (3) rhizobacterial inoculation with Massilia niastensis p87 and Streptomyces costaricanus RP92 strains, (4) rhizobacteria-assisted phytoremediation with both plants and strains. The applied sediment had a positive influence on L. sativum growth (90% higher than in the unamended soil), mostly due to its high content of nutrients, mainly Ca and Fe, which immobilize pollutants. The positive effect of sediments continued for up to 10-week duration of the experiment; however, the rhizobacterial inoculated samples were characterized by higher growth of L. sativum. The application of rhizobacteria-assisted phytoremediation further increased the growth of L. sativum, and was also found to improve the efficiency of PCDD/PCDF removal, resulting in a maximum 44% reduction of its content. This strategy also alleviated the negative impact of urban sediments on T. patula and F. arundinacea biomass, and had a beneficial effect on protein and chlorophyll content in the studied plants.

A comparative study to evaluate natural attenuation, mycoaugmentation, phytoremediation, and microbial-assisted phytoremediation strategies for the bioremediation of an aged PAH-polluted soil

Biological treatments are considered an environmentally option to clean-up polluted soil with polycyclic aromatic hydrocarbons (PAHs). A pot experiment was conducted to comparatively evaluate four different strategies, including natural attenuation (NA), mycoaugmentation (M) by using Crucibulum leave, phytoremediation (P) using maize plants, and microbial-assisted phytoremediation (MAP) for the bioremediation of an aged PAH-polluted soil at 180 days. The P treatment had higher affinity degrading 2-3 and 4 ring compounds than NA and M treatments, respectively. However, M and P treatments were more efficient in regards to naphthalene, indeno[l,2,3-c,d]pyrene and benzo[g,h,i]perylene degradation respect to NA. However, 4, 5-6 rings undergo a strong decline during the microbe-assisted phytoremediation, being the treatment which determined the highest rates of PAHs degradation. Sixteen PAH compounds, except fluorene and dibenzo[a,h]anthracene, were found in maize roots, whereas the naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene were accumulated in the shoots, in both P and MAP treatments. However, higher PAH content in maize biomass was achieved during the MAP treatment respect to P treatment. The bioconversion and translocation factors were less than 1, indicating that phystabilization/phytodegradation processes occurred rather than phytoextraction. The microbial biomass, activity and ergosterol content were significantly boosted in the MAP treatment respect to the other treatments at 180 days. Ours results demonstrated that maize-C. laeve association was the most profitable technique for the treatment of an aged PAH-polluted soil when compared to other bioremediation approaches.

Bioremediation of copper-contaminated soils by bacteria

Although copper (Cu) is an essential micronutrient for all living organisms, it can be toxic at low concentrations. Its beneficial effects are therefore only observed for a narrow range of concentrations. Anthropogenic activities such as fungicide spraying and mining have resulted in the Cu contamination of environmental compartments (soil, water and sediment) at levels sometimes exceeding the toxicity threshold. This review focuses on the bioremediation of copper-contaminated soils. The mechanisms by which microorganisms, and in particular bacteria, can mobilize or immobilize Cu in soils are described and the corresponding bioremediation strategies-of varying levels of maturity-are addressed: (i) bioleaching as a process for the ex situ recovery of Cu from Cu-bearing solids, (ii) bioimmobilization to limit the in situ leaching of Cu into groundwater and (iii) bioaugmentation-assisted phytoextraction as an innovative process for in situ enhancement of Cu removal from soil. For each application, the specific conditions required to achieve the desired effect and the practical methods for control of the microbial processes were specified.

Comparative bioremediation of heavy metals and petroleum hydrocarbons co-contaminated soil by natural attenuation, phytoremediation, bioaugmentation and bioaugmentation-assisted phytoremediation

Biological remediation technologies are an environmentally friendly approach for the treatment of polluted soils. This study evaluated through a pot experiment four bioremediation strategies: a) natural attenuation, b) phytoremediation with alfalfa (Medicago sativa L.), c) bioaugmentation with Pseudomonas aeruginosa and d) bioaugmentation-assisted phytoremediation, for the treatment of a co-contaminated soil presenting moderate levels of heavy metals (Cu, Pb and Zn at 87, 100 and 110mgkg(-1) DW, respectively) and petroleum hydrocarbons (3800mgkg(-1) DW). As demonstrated by plant biomass and selected physiological parameters alfalfa plants were able to tolerate and grow in the co-contaminated soil, especially when soil was inoculated with P. aeruginosa, which promoted plant growth (56% and 105% increase for shoots and roots, respectively) and appeared to alleviate plant stress. The content of heavy metals in alfalfa plants was limited and followed the order: Zn>Cu>Pb. Heavy metals were mainly concentrated in plant roots and were poorly translocated, favouring their stabilization in the root zone. Bioaugmentation of planted soil with P. aeruginosa generally led to a decrease of plant metal concentration and translocation. The highest degree of total petroleum hydrocarbon removal was obtained for bioaugmentation-assisted phytoremediation treatment (68%), followed by bioaugmentation (59%), phytoremediation (47%) and natural attenuation (37%). The results of this study demonstrated that the combined use of plant and bacteria was the most advantageous option for the treatment of the present co-contaminated soil, as compared to natural attenuation, bioaugmentation or phytoremediation applied alone.

Bioturbation effects on heavy metals fluxes from sediment treated with activated carbon

Adding activated carbon (AC) to sediment has been proposed as an in situ sediment remediation technique. To date, it is not clear whether this technique is effective in the treatment of heavy metal-contaminated sediment in the presence of bioturbators. In the present study, we compare the ability of granular-activated carbon (GAC) and powder-activated carbon (PAC) to reduce Cu, Zn, and Pb pore water concentrations at environmentally relevant concentrations in the absence and presence of Chironomid larvae. Compared to untreated sediment, PAC and GAC addition in the absence of Chironomid larvae resulted in reductions of free Cu concentrations of 78 and 66 % just below the sediment-water interface after 28 days, respectively. While for Pb and Zn these concentration reductions were only 40 and 38, 19 and 25 %, respectively. The presence of Chironomid larvae in untreated, and GAC sediment generally increased the free heavy metals concentrations in pore water, especially in the deeper layers. In comparison with untreated sediment, the coexistence of AC enhanced the accumulation of heavy metals, especially for PAC. This increased bioaccumulation may decrease the survival of Chironomid larvae. The result indicated that ACs may not be suitable for the remediation of heavy metal-contaminated sediments.

Presence of pharmaceuticals and heavy metals in the waters of a Mediterranean coastal wetland: Potential interactions and the influence of the environment

The occurrence of 17 relevant pharmaceuticals and 7 heavy metals in the waters of the Pego-Oliva Marsh Natural Park (Valencia Community, Spain) were monitored. Thirty four zones (including the lagoon and the most important irrigation channels), covering the main land uses and water sources, were selected for sampling. Thirty three of them were contaminated with at least one pharmaceutical. Ibuprofen and codeine were the pharmaceuticals more frequently detected, in concentrations between 4.8 and 1.2 ng/L and a maximum of 59 ng/L and 63 ng/L, respectively. Regarding metals, Zn showed values under the detection limit in all the samples, while Cd, Co, Cr, Cu, Ni and Pb were detected at concentrations lower than the WHO and EU maximum levels for drinking waters. Ni showed significant direct correlations with diazepam, norfloxacin, ofloxacin and fenofibrate, and inverse relationships with ibuprofen, at 99 and 95% of significance. Cu, Co and Cr also showed significant correlations with some of the pharmaceuticals. These interactions could favor the synergistic/antagonistic interactions among pharmaceuticals and metals in the marsh, which can affect its aquatic fauna and flora or even human health. The influences of the water sources, land uses and spatial distribution of both types of contaminants were also studied.