Lead phytoextraction, using plants to extract Pb from contaminated soils, is an emerging technology. Calculations of soil Pb mass balance suggest that this technology will be economically feasible only if systems can be developed to employ high biomass plants that can accumulate greater than 1% Pb in their shoots. In this study, we investigated the potential of adding chelates to Pb-contaminated soils to increase Pb accumulation in plants. The addition of chelates to a Pb-contaminated soil (total soil Pb 2500 mg kg-1) increased shoot Pb concentrations of corn (Zea mays L. cv. Fiesta) and pea (Pisum sativum L. cv. Sparkle) from less than 500 mg kg-1 to more than 10000 mg kg-1. The surge of Pb accumulation in these plants was associated with the surge of Pb level in the soil solution due to the addition of chelates to the soil. For the chelates tested, the order of the effectiveness in increasing Pb desorption from the soil was EDTA > HEDTA > DTPA > EGTA >EDDHA. We also found that EDTA significantly increased Pb translocation from roots to shoots. Within 24 h after applying EDTA solution [1.0 g of EDTA (kg soil)-1] to the contaminated soil, Pb concentration in the corn xylem sap increased 140-fold, and net Pb translocation from roots to shoots increased 120-fold as compared to the control (no EDTA). These results indicate that chelates enhanced Pb desorption from soil to soil solution, facilitated Pb transport into the xylem, and increased Pb translocation from roots to shoots. Results from this study suggest that with careful management, chelate-assisted Pb phytoextraction may provide a cost-effective soil decontamination strategy.

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SUMM.^RYLead transport has been characterized in corn {Zea mays L. cv. Fiesta) and ragweed (Ambrosia artemisiifolia L.), and the Pb phytoextraction efficiency of these species has been compared with that of Triticum aestivum, Thlaspi rotundifolium, Thlaspi caerutescens and Brassica juncea, using both nutrient solutions and Pb-contaminated soils. Our results demonstrated that plant species differ significantly in Pb uptake and translocation. In short term (60 min) experiments, Pb uptake by ragweed roots was threefold higher than that hy corn roots, .'^fter 2 w^k of Pb (100/^M) exposure in hydroponics, root-Pb concentration was 24000 mg kg~* for ragweed and 4900 mg kg~^ for corn. In contrast to root-Pb concentration, shoot-Pb concentration was significantly higher in corn (560 mg kg"^) than in ragweed (30 mg kg"'), .-^t an external Pb concentration of 20/IM, corn concentrated Pb in shoots hy 90-fold, and ragweed concentrated Pb in shoots by 2G-fold over the solution Pb concentration. Of the 11 species/cultivars tested using botb nutrient solutions and Ph-contaminated soils, corn accumulated the highest shoot-Pb concentration. Using this corn cultivar. we investigated the role of synthetic chelates in Pb phytoextraction. Addition of HEDT.Ai (2-0 g kg"' soil) to a Pb-contaminated soil (total soil Pb 2500 mg kg"') resulted in a surge of Pb accumulation in corn. The shoot Pb-concentration was increased from 40 mg kg"' for the control ( -HE^DT.-^) to 10600 mg kg"' for the HEDTA-treated soil. To our knowledge, this is the highest shoot Pb concentration reported m the literature for plants grown on Ph-contaminated soils. Our results suggest that in combination with soil amendment, some agronomic crops, such as corn, might be used for the clean-up of Pbcontaminated soil.

The main performance bottleneck of modern Digital Subscriber Line (DSL) networks is the crosstalk among different lines (users). By deploying Dynamic Spectrum Management (DSM) techniques and reducing excess crosstalks among users, a network operator can dramatically increase the data rates and service reach of broadband access. However, current DSM algorithms suffer from either substantial suboptimality in typical deployment scenarios or prohibitively high complexity due to centralized computation. This paper develops, analyzes, and simulates a new suite of DSM algorithms for DSL interferencechannel models called Autonomous Spectrum Balancing (ASB), for both synchronous and asynchronous transmission cases. In the synchronous case, the transmissions over different tones are orthogonal to each other. In the asynchronous case, the transmissions on different tones are coupled together due to intercarrier-interference. In both cases, ASB utilizes the concept of a reference line, which mimics a typical victim line in the interference channel. The basic procedure in ASB algorithms is simple: each user optimizes the weighted sum of the achievable rates on its own line and the reference line while assuming the interferences from other users as noise. Users then iterate until the target rate constraints are met. Good choices of reference line parameters are already available in industry standards, and the ASB algorithm makes the intuitions completely rigorous and theoretically sound. ASB is the rst set of algorithms that is fully autonomous, has low complexity, and yet achieves nearoptimal performance. It effectively solves the nonconvex and coupled optimization problem of DSL spectrum management, and overcomes the bottleneck of all previous DSM algorithms.