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Different light combinations can improve phytoremediation efficiency by increasing the biomass yield and metal concentrations of plants. However, there has been rare research of using hyperaccumulators to change metal fractions and its possible leaching risk during phytoremediation. It was investigated in this study the impacts of different intensities of blue and red light mixed on the biomass production and metal uptake of Noccaea caerulescens and the changes of water soluble and exchangeable metal fractions in soil. The biomass of N. caerulescens increased with light intensity. The increment was relatively slow at 50 m -2  s -1 , dramatically increased at 200 m -2  s -1 and decreased significantly when beyond. Under optimal light condition, N. caerulescens produced less biomass than Thlaspi arvense, but the former is significantly more efficient in phytoremediation than the latter because it can accumulate significantly more metals per unit biomass. Without light irradiation, N. caerulescens can deteriorate the potential leaching risk of Cu and Pb by increasing their water soluble and exchangeable fractions in soil comparing with T. arvense. The proportions of bioavailable fractions did not change under the treatment of light at an intensity of 50 m -2  s -1 , but decreased obviously when the intensity exceeded 100 m -2  s -1 . Therefore, using hyperaccumulator for multiple metal contaminated soil remediation should be conducted with caution since the species can mobilize all metals in soil but only hyperaccumulate part of them, and proper intensity of light can improve the phytoremediation effect and alleviate the leaching risk through decreasing bioactive metal fractions in soil., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The influence of combined red and blue light on the capacity of Eucalyptus globulus to phytoremediate a metal-polluted soil was evaluated in this study. Five combinations of blue and red light (0%, 10%, 25%, 50% and 100% blue) at the same intensity were used to treat E. globulus, and its biomass generation, metal uptake and water absorption in phytoremediation under different light treatments were assessed. The plant produced significantly more biomass under blue light, regardless of the ratio, than under single red or white light. The highest biomass was generated under the light ratio of B 10 R 90 . In addition, light combination influenced the metal concentrations in different plant tissues. The highest concentrations of Cd and Cu in roots appeared under the light ratio of B 0 . All metals in plant shoots achieved their highest concentrations under the light ratio of B 100 , except Pb. Comparing with control, red and blue light combined in varying proportions increased the efficiency remove Cd, Pb and Cu by 50.6-65.6, 71.1-88.7 and 28.9-70.6%, respectively,. The leachate volume under blue and red light combinations was 46.7-66.0% less than control with the combination of B 10 R 90 mitigating the most metal loss. Light sources with different spectra combinations can enhance the phytoremediation efficiency of Eucalyptus globulus and alleviate leaching risk at the same time., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Hyperaccumulators can accumulate high amounts of specific metals and have been widely used to remediate metal polluted soil. However, organic acid secretion and soil acidification (two important mechanisms for hyperaccumulators to mobilize and extract metals) can also activate non-hyperaccumulated metals and then increase the leaching risk. The decontamination efficiency and leaching risk of using Noccaea caerulescens (formerly Thlaspi caerulescens) and Thlaspi arvense were compared in the present study. Although N. caerulescens accumulated significantly more Cd and Zn than T. arvense, it increased the leaching risk of Pb and Cu as well. Under magnetic fields of 30, 60, 120 and 150 mT, the biomass production of N. caerulescens was increased by 18.5, 48.9, 80.4, and 29.3% respectively, but decreased by 21.7% under 400 mT. Comparing with the control, plants raised from seeds pre-treated by magnetic fields accumulated 37.8-250.1% more metals and reduced the leachate volume and leached metals by 1.1-32.9% and 4.6-48.1% respectively. Considering remediation efficiency, environmental risk alleviation and energy consumption, N. caerulescens treated by 120 mT magnetic field is suited to remediate multi-metal polluted soil., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Direct and alternating current electric fields with various voltages were used to improve the decontamination efficiency of chelator assisted phytoremediation for multi-metal polluted soil. The alleviation effect of electric field on leaching risk caused by chelator application during phytoremediation process was also evaluated. Biomass yield, pollutant uptake and metal leaching retardation under alternating current (AC) and direct current (DC) electric fields were compared. The biomass yield of Eucalyptus globulus under AC fields with various voltages (2, 4 and 10 V) were 3.91, 4.16 and 3.67kg, respectively, significantly higher than the chelator treatment without electric field (2.71kg). Besides growth stimulation, AC fields increased the metal concentrations of plant tissues especially in aerial parts manifested by the raised translocation factor of different metals. Direct current electric fields with low and moderate voltages increased the biomass production of the species to 3.45 and 3.12kg, respectively, while high voltage on the contrary suppressed the growth of the plants (2.66kg). Under DC fields, metal concentrations elevated obviously with increasing voltages and the metal translocation factors were similar under all voltages. Metal extraction per plant achieved the maximum value under moderate voltage due to the greatest biomass production. DC field with high voltage (10V) decreased the volume of leachate from the chelator treatment without electric field from 1224 to 56mL, while the leachate gathered from AC field treatments raised from 512 to 670mL. DC field can retard the downward movement of metals caused by chelator application more effectively relative to AC field due to the constant water flow and electroosmosis direction. Alternating current field had more promotive effect on chelator assisted phytoremediation efficiency than DC field illustrated by more metal accumulation in the species. However, with the consideration of leaching risk, DC field with moderate voltage was the optimal supplementary technique for phytoremediation., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Multiple techniques for soil decontamination were combined to enhance the phytoremediation efficiency of Eucalyptus globulese and alleviate the corresponding environmental risks. The approach constituted of chelating agent using, electrokinetic remediation, plant hormone foliar application and phytoremediation was designed to remediate multi-metal contaminated soils from a notorious e-waste recycling town. The decontamination ability of E. globulese increased from 1.35, 58.47 and 119.18 mg per plant for Cd, Pb and Cu in planting controls to 7.57, 198.68 and 174.34 mg per plant in individual EDTA treatments, respectively, but simultaneously, 0.9-11.5 times more metals leached from chelator treatments relative to controls. Low (2 V) and moderate (4 V) voltage electric fields provoked the growth of the species while high voltage (10 V) had an opposite effect and metal concentrations of the plants elevated with the increment of voltage. Volumes of the leachate decreased from 1224 to 134 mL with voltage increasing from 0 to 10 V due to electroosmosis and electrolysis. Comparing with individual phytoremediation, foliar cytokinin treatments produced 56% more biomass and intercepted 2.5 times more leachate attributed to the enhanced transpiration rate. The synergistic combination of the individuals resulted in the most biomass production and metal accumulation of the species under the stress condition relative to other methods. Time required for the multi-technique approach to decontaminate Cd, Pb and Cu from soil was 2.1-10.4 times less than individual chelator addition, electric field application or plant hormone utilization. It's especially important that nearly no leachate (60 mL in total) was collected from the multi-technique system. This approach is a suitable method to remediate metal polluted site considering its decontamination efficiency and associated environmental negligible risk., (Copyright © 2017 Elsevier Ltd. All rights reserved.)