Metal Forming Research Articles

Green synthesis of silver nanoparticles from southern Eucalyptus globulus: Potent antioxidants and photocatalysts for rhodamine B dye degradation

Nanotechnology and nanoscience provide innovative foundations for delivering a wide range of new and enhanced technologies to the modern world. To break down Rhodamine B (RhB) dye solutions, this work describes the creation of silver nanoparticles (Ag-NPs) using the aqueous leaf extract of Eucalyptus globulus (E. globulus). The biosynthesized Ag-NPs were studied using UV–vis spectroscopy and SEM in conjunction with EDX, TEM, FTIR, HPLC and XRD. Stable Ag-NPs were synthesized using E. globulus, as evidenced by the absorption peaks at 431 nm seen in the UV–vis spectrum analysis. Spherical-shaped particles were visible in the SEM images, and the metallic form of the synthesised Ag-NPs was verified by EDX. The initial concentration of RhB, reaction duration, temperatures, and pH were all adjusted to optimise the photocatalytic degradation of RhB using Ag-NPs. The results indicated that photocatalytic effectiveness increased with rising pH. After 80 min at room temperature, the degradation reached 91.20 % at pH 12. The antioxidant EC50 results demonstrate that Ag-NPs exhibited a higher level of protective antioxidants (20.15 ± 0.3 µg/mL for DPPH and 10.24 ± 0.2 µg/mL for ABTS) compared to the E. globulus leaf extracts.

Influence of material in sheet-bulk metal forming from coil

AbstractSheet-bulk metal forming (SBMF) represents an innovative approach for the efficient manufacturing of functional integrated parts from sheet metal by applying bulk forming processes to sheet metal. It combines the advantages of part weight optimization and process chain shortening. In view of the increasing demand for functionally integrated components, the forming from coil offers an economical possibility to produce high output rates. Nevertheless, an underfilling of functional elements as well as an asymmetric part forming are current challenges. In order to apply SBMF from coil in industrial contexts, it is necessary to understand the causes of these process limits and to develop measures for improving the forming of functional elements and thereby push existing forming limits. In this paper, both a lateral and a backward extrusion process from coil for the forming of internal and external geared SBMF parts are investigated to develop a fundamental process understanding. In a combined numerical-experimental approach, the influence of the flow behavior of different materials as well as the impact of the main material flow direction of the process on the dimensional part accuracy is analyzed. This enables the evaluation of fundamental material-related dependencies and influences regarding the geometrical part accuracy, the mechanical component properties and the tool load for forming of the conventional deep-drawing steel DC04 (t0 = 2 mm), a high-strength steel material (HC260LA) and an aluminum alloy (AA6082 T6). Based on these findings, measures for material flow control are identified and investigated to counteract the challenges of SBMF from coil. These findings permit the identification of potential transferability of the identified cause-effect mechanisms to different processes, component geometries and materials. Thereby, in particular, an increase in coil width as well as lower a strain hardening behavior of the material offers the possibility to improve the geometrical accuracy of the components.

Evaluating the performance of Bi58Sn42 mold produced by material extrusion additive manufacturing system for agile manufacturing

PurposeThe purpose of this study is to produce a low-cost sheet metal forming mold made from the low melting point Bi58Sn42 (bismuth) alloy by using an open-source desktop-type material extrusion additive manufacturing system and to evaluate the performance of the additively manufactured mold for low volume sheet metal forming. Thus, it was aimed to develop a fast and inexpensive die tooling methodology for low-volume batch production.Design/methodology/approachInitially, the three-dimensional printing experiments were performed to produce the sheet metal forming mold. The encountered problems during the performed three-dimensional printing experiments were analyzed. Accordingly, both tunings in process parameters (extrusion temperature, extrusion multiplier, printing speed, infill percentage, etc.) and customizations on the extruder head of the available material extrusion additive manufacturing system were made to print the Bi58Sn42 alloy properly. Subsequently, the performance of the additively manufactured mold was evaluated according to the dimensional change that occurred on it during the performed pressing operations.FindingsResults showed that the additively manufactured mold was rigid enough and proved to have sufficient strength in sheet metal forming operations for low-volume production.Originality/valueAlternative mold production was carried out using open-source material extrusion system for low volume sheet metal part production. Thus, cost effective solution was presented for agile manufacturing.

Fertility and ecotoxicological condition of light gray forest soils of the Ivanovo region

The results oflong-term monitoring of light gray forest soil for agricultural purposesin theIvanovo region are presented, which was carried outto establish the level of fertility according to the mainagrochemical indicators, the content of mobile forms of trace elementsand sulfur, the ecotoxicological state according to the content ofgross and mobile forms of heavy metals, arsenic, caesium-137 andstrontium-90.The average values of the indicators, their levels ofchange and trends in changes in the metabolic and hydrolyticacidity of the soil, availability of organic matter, mobile formsof nitrogen, phosphorus, potassium, exchangeable calcium, magnesium and other bases,mobile forms of boron, copper, zinc, molybdenum, cobalt, manganese andsulfur have been established.According to the content of traceelements in the soil, the needs for the use ofmicronutrients are determined.Changes, concentrations and trends in the mobileforms of cadmium, lead, copper, zinc, nickel, chromium, mercury andarsenic in the soil have been determined.The study establishedbackground values, changes and trends in the specific activities ofcaesium-137 and strontium-90, the density of their contamination of thesoil of the site and the power of the exposuredose of gamma radiation.According to thePearson–Spearman correlation coefficients,the peculiarities of the influence of the content of organicmatter, the level of acidity and the capacity of cationexchange on the content of mobile forms of trace elements,sulfur, gross and mobile forms of metals and radionuclides wereestablished.

Model based analysis of trace metal dosing strategies to improve methane yield in anaerobic digestion systems

Favourable effects of trace metals (TMs) on regulating anaerobic digestion (AD) performance are extensively utilised to improve methane yield. This study discusses a model-based approach to find out the best TM dosing strategies. The model has been applied to compare continuous, preloading, pulse dosing and in-situ loading. Simulations were also carried out to comprehend appropriate dosing form, dosing time and quantity of metals to be dosed. Model results show that the best way to dose TMs is repeated pulse dosing at low concentration levels in the optimum range with high frequency. Best dosing strategy for the system in this study was found to be 5 µM pulse loading at 5 days intervals as it gave maximum methane production and low effluent metal loss. Preferable dosing form depends on reactor configuration and this has been verified after model calibration with experimental data. Easily dissociable metal chlorides are ideal for continuous reactors.

Microbial electrochemical enhanced composting of sludge and kitchen waste: Electricity generation, composting efficiency and health risk assessment for land use

To realize the energy and resource utilization from organic solid waste, a two-phase microbial desalination cell (TPMDC) was constructed using dewatered sludge and kitchen waste as the anode substrate. The performance of electricity generation and composting efficacy was investigated, along with a comprehensive assessment of the potential health risks associated with the land use of the resulting mixed compost products. Experimental outcomes revealed a maximum open-circuit voltage of 0.893 ± 0.005 V and a maximum volumetric power density of 0.797 ± 0.009 W/m³. After 90 days of composting enhanced by microbial electrochemistry, a significant organic matter removal rate of 31.13 ± 0.44 % was obtained, and the anode substrate electric conductivity was reduced by 30.02 ± 0.04 % based on the anode desalination. Simultaneously, there was an increase in the content of available nitrogen, phosphorus, and potassium, as well as an improvement in the seed germination index. The forms of heavy metals shifted from bioavailable to stable residual states. The non-carcinogenic hazard index (HI) values for heavy metals and polycyclic aromatic hydrocarbons (PAHs) during the land use of compost products were less than 1, and the total carcinogenic risk (TCR) values for heavy metals and PAHs were below the acceptable threshold of 10−4. The occupational population risk of infection from five pathogens was higher than that of the general public, with all risk values ranging from 8.67 × 10−8 to 1, where the highest risk was attributed to occupational exposure to Legionella. These outcomes demonstrated that the mixture of dewatered sludge and kitchen waste was an appropriate anode substrate to enhance TPMDC stability for electricity generation, and its compost products have promising land use suitability and acceptable land use risk, which will provide important guidance for the safe treatment and disposal of organic solid waste.

INTEGRATING NEURAL NETWORKS INTO SHEET METAL FORMING: A REVIEW OF RECENT ADVANCES AND APPLICATIONS

In order to predict defects, improve performance, and streamline operations, machine learning techniques are becoming ever more indispensable in manufacturing processes, mainly in sheet metal forming. Incorporating neural networks into the process of sheet metal forming is the subject of this article's exhaustive examination of recent developments and applications. Exploring datasets from a variety of sheet metal forming processes, numerous machine learning models, including ensemble and single learning techniques are investigated. The functionality of this method extends to various tasks, including the prediction of springback in cold-rolled anisotropic steel sheets. The review provides a conclusion section that presents the main implementation methodologies and how they address to some manufacturing issues.

Effect of increased soil available phosphorus from vermicompost application on the bioavailability, chemical form, and bioaccessibility of heavy metals.

Phosphorus (P) plays an important role in immobilizing heavy metals (HMs), thereby preventing their accumulation, especially in edible parts of crops. In this study, vermicompost (VM) and chemical fertilizers (CFs) were used as soil amendments to increase the available P concentration in soil contaminated with cadmium (Cd) and nickel (Ni), with the aim of reducing their bioavailability, uptake, and bioaccessibility. Using CF and VM as soil amendments substantially increased the available P and exchangeable potassium concentrations in the soil. Furthermore, VM addition led to an increase in OM content and in exchangeable calcium and magnesium, resulting in the improved growth of lettuce. It also reduced the uptake of Cd and Ni in the two lettuce cultivars tested in the study. However, CF addition boosted the accumulation of Cd and Ni by increasing the soil acidity. CF addition, and especially VM addition, altered the chemical forms of Cd and Ni from active to inactive. Overall, the results of this study underscore the positive impact of using VM as a soil amendment on lettuce growth and the prevention of HM accumulation in edible parts of lettuce. VM addition led to decreased bioavailability, uptake, and bioaccessibility of HMs in soil, which could improve food safety and reduce potential risks associated with HM contamination.

Neutron spectroscopy of plutonium using a handheld detection system

The ability to distinguish multiple forms of plutonium from one another, such as oxide and metal, is paramount in areas of nuclear nonproliferation and international safeguards. In its metal form, plutonium can be readily used in a nuclear weapon, while oxide forms are associated with nuclear reactor fuel. Oxide-based plutonium forms emit neutrons with an energy spectrum that is significantly different from the fission neutrons that are emitted from plutonium metal. Organic scintillation detectors output pulses that are proportional to the neutron energy deposited, and therefore present a means of distinguishing these plutonium forms based on their energy spectra. In this work, metal and oxide forms of plutonium were measured using a handheld detection system based on an organic glass scintillator. Monte Carlo modeling of these experiments was performed to provide insight into the origin of the features in the observed light output spectra. Through analysis of multiple regions of these spectra, in a matter of minutes we were able to unambiguously discriminate oxide and metal plutonium forms from one another and from a plutonium-beryllium neutron source, which was considered for comparison because these sources are commonly used in industrial applications. The ability to discriminate weapons-usable material from nuclear reactor fuel has applications in nuclear treaty verification and safeguards.

Effects of voltage stress conditions on degradation of iridium-based catalysts occurring during high-frequency operation of PEM water electrolysis

Proton exchange membrane water electrolysis (PEMWE) is needed to store renewable energy in the form of hydrogen. To improve the performance of PEMWE, elucidating the degradation mechanism of iridium (Ir)-based catalysts used for oxygen evolution reaction (OER) that is a rate-determining step is important. To date, it is known that one important reason for that is undesirable change in crystal structure of Ir. To further evaluate its degradation mechanism, in this study, effects of operating voltage conditions on crystal structure of Ir are investigated because the voltage conditions can accelerate changes in crystal structure of Ir. Especially how Ir was changed into an unfavorable metallic form was analyzed. PEMWE single cells under conditions of (i) a constant voltage of 1.9 V, (ii) a voltage cycle of 1.7–1.9 V, then a constant voltage of 1.9 V, and (iii) an applied voltage cycle with open circuit voltage (OCV). In the OCV condition, the crystalline structure of Ir was irreversibly transformed into a metallic form. This transformation induced lower OER activity than other crystalline states of Ir, followed by lower performance of PEMWE single cells. These findings help to establish baseline protocols for the stable operation of PEMWEs using Ir-based catalysts.

Material Parameter Identification for a Stress-State-Dependent Ductile Damage and Failure Model Applied to Clinch Joining

Similar to bulk metal forming, clinch joining is characterised by large plastic deformations and a variety of different 3D stress states, including severe compression. However, inherent to plastic forming is the nucleation and growth of defects, whose detrimental effects on the material behaviour can be described by continuum damage models and eventually lead to material failure. As the damage evolution strongly depends on the stress state, a stress-state-dependent model is utilised to correctly track the accumulation. To formulate and parameterise this model, besides classical experiments, so-called modified punch tests are also integrated herein to enhance the calibration of the failure model by capturing a larger range of stress states and metal-forming-specific loading conditions. Moreover, when highly ductile materials are considered, such as the dual-phase steel HCT590X and the aluminium alloy EN AW-6014 T4 investigated here, strong necking and localisation might occur prior to fracture. This can alter the stress state and affect the actual strain at failure. This influence is captured by coupling plasticity and damage to incorporate the damage-induced softening effect. Its relative importance is shown by conducting inverse parameter identifications to determine damage and failure parameters for both mentioned ductile metals based on up to 12 different experiments.

Effect of Countersample Coatings on the Friction Behaviour of DC01 Steel Sheets in Bending-under-Tension Friction Tests.

The aim of this article is to provide an analysis of the influence of the type of hard anti-wear coatings on the friction behaviour of DC01 deep-drawing steel sheets. DC01 steel sheets exhibit high formability, and they are widely used in sheet metal forming operations. The tribological properties of the tool surface, especially the coating used, determine the friction conditions in sheet metal forming. In order to carry out the research, this study developed and manufactured a special bending-under-tension (BUT) friction tribometer that models the friction phenomenon on the rounded edges of tools in the deep-drawing process. The rationale for building the tribotester was that there are no commercial tribotesters available that can be used to model the phenomenon of friction on the rounded edges of tools in sheet forming processes. The influence of the type of coating and sheet deformation on the coefficient of friction (CoF) and the change in the topography of the sheet surface were analysed. Countersamples with surfaces prepared using titanium + nitrogen ion implantation, nitrogen ion implantation and electron beam remelting were tested. The tests were carried out in conditions of dry friction and lubrication with oils with different kinematic viscosities. Under dry friction conditions, a clear increase in the CoF value, with the elongation of the samples for all analysed types of countersamples, was observed. Under lubricated conditions, the uncoated countersample showed the most favourable friction conditions. Furthermore, oil with a lower viscosity provided more favourable conditions for reducing the coefficient of friction. Within the entire range of sample elongation, the most favourable conditions for reducing the CoF were provided by uncoated samples and lubrication with S100+ oil. During the friction process, the average roughness decreased as a result of flattening the phenomenon. Under dry friction conditions, the value of the Sa parameter during the BUT test decreased by 20.3-30.2%, depending on the type of countersample. As a result of the friction process, the kurtosis and skewness increased and decreased, respectively, compared to as-received sheet metal.

Divalent metal ion in the active site of purple acid phosphatase modulates substrate binding: Kinetic and thermodynamic properties

The purple acid phosphatase was purified from 5.9-fold to apparent homogeneity from Anagelis arvensis seeds using SP-Sephadex C-50 and Sephadex G-100 chromatography. The results of residual activity tests conducted using different temperature ranges (50–70 °C) were calculated as the activation energy (Ed = 72 kJ/mol), enthalpy (69.31 ≤ (ΔH° ≤ 69.10 kJ/mol), entropy (−122.48 ≤ ΔS° ≤ −121.13 J/mol·K), and Gibbs free energy (108.87 ≤ ΔG° ≤ 111.25 kJ/mol) of the enzyme irreversible denaturation. These thermodynamic parameters indicate that this novel PAP is highly thermostable and may be significant for use in industrial applications. However, it may be confirmed by stopped-flow measurements that this substitution produces a chromophoric Fe3+ site and a Pi-substrate interaction that is about ten times faster. Additionally, these data show that phenyl phosphate hydrolysis proceeds more rapidly in metal form of A. arvensis PAP than the creation of a μ-1,3 phosphate complex. The Fe3+ site in the native Fe3+-Mn2+ derivative interacts with it at a faster rate than in the Fe3+-Fe2+ form. This is most likely caused by a network of hydrogen bonds between the first and second coordination spheres. This suggests that the choice of metal ions plays a significant role in regulating the activity of this enzyme.

Sheet Forming via Limiting Dome Height (LDH) Test: Influence of the Application of Lubricants, Location and Sheet Thickness on the Micro-Mechanical Properties of X8CrMnNi19-6-3

This work is concerned with forming, specifically deep drawing, and its influence on the micro-mechanical properties of sheet metal. In practice, there are several applications in which fractions can occur due to weak spots in the deep-drawn sheet metal, especially after long-term use. The deep drawing process was carried out on BUP–600 machines using the LHD (Limiting Dome Height) method, which uses a forming tool with a diameter of 100 mm and bead groove. Sheet metals X8CrMnNi19-6-3 (1.4376) with thicknesses of 1, 1.5, and 3 mm were selected for this process. To study the effect of a lubricant on the formability of the sheet metal, deep drawing without and with a lubricant was compared. An FEM analysis was conducted to identify critical points in the deep drawing process, and the results were later compared with real results. The analysis was conducted using the AutoForm program. The micro-mechanical properties of these points were subsequently examined. The specified points on the formed part showed significant differences in their micro-mechanical properties, suggesting a higher strength but also less resistance to fractures. The difference in micro-mechanical properties (indentation and Vickers hardness) in points that were not deep-drawn and points located in critical areas was up to 86%. Significant changes in behavior were found in the indentation modulus and plastic/elastic deformation work as well. This study demonstrates the significant effect of the use of a lubricant in achieving the deep drawing of the sheet metal. The application of a lubricant resulted in a 33% increase in drawing range compared to drawing without lubrication. This study has a significant influence on the deep drawing of sheet metals in practice, showing the fundamental influence of the lubricant on the drawing process and also showing the problem of critical points that need to be eliminated.

Wrinkling suppression in cryogenic forming of high-strength Al-alloy ultra-thin shells by controlling interface shear stress

Avoiding wrinkling defects is extremely difficult in the sheet metal forming of ultra-thin components made from high-strength aluminum alloys. A novel cryogenic forming is thus proposed for solving this very challenging problem, wherein a thicker cladding blank is stacked above the thinner target blank to establish interface shear stress for reducing the hoop compressive stress, so that the critical wrinkling stress is increased. The enlarged radial deformation is transferred and withstood by the increasing hardening and ductility at cryogenic temperatures. The wrinkling suppression mechanism is revealed through mechanical and numerical analyses. Systematic experiments were conducted for studying the feasibility of high-strength AA7075 in cryogenic forming with different stacking sequences of cladding and target blanks, cladding blanks including three material strength levels (AA1060-O, AA5052-O, and SUS-304), two thicknesses (3.0, and 2.0 mm), and blank-holder forces. The effects of the cladding blank and blank-holder forces were clarified as reflected by forming defects, thickness, and strain distributions. The mechanical and numerical analyses can indicate that contact pressure can be produced by the wrinkling tendency of thinner target blank and limitation of cladding blank to wrinkling, which is accompanied by a decrease in the hoop compressive stress and increase in the critical wrinkling stress. Therefore, applying a relatively thicker blank on the punch side can prevent the wrinkling of ultra-thin components. The wrinkling tendency decreases with increasing strength and thickness of the cladding blank, which results in an increase in deformation or even splitting, which can be solved by the cryogenic temperature. The thickness of the cladding blank can be reduced by increasing the blank-holder force, which further reduces the material cost. AA5052-O, which has a strength similar to that of the target blank, is more suitable as a cladding blank for AA7075-W because the balance preventing wrinkling, improving thickness uniformity, and controlling the forming force. A Φ200 mm hemispherical shell with an initial thickness of 0.3 mm was formed successfully, and the corresponding thickness-to-diameter ratio reached 0.8‰, which almost increased one time on the basis of direct cryogenic forming. This new approach can be used for fabricating ultra-thin components from high-strength aluminum alloys.

Simulation-based data reduction and data processing for sheet metal forming in the hybrid twin framework

In sheet metal forming, the interaction between virtual models and the real world remains challenging. Process simulations can exhibit significant errors, and reliable measurements are often scarce during early production stages. This study presents a hybrid twin framework that systematically unifies computer-aided design, simulation, and measurement data in an adaptive manner. Central to this framework is a reverse engineering algorithm that reconstructs and transforms the geometry of deep-drawn components from optical scan data into B-spline surfaces. The algorithm demonstrated high precision, indicating its suitability for process control and geometric analysis. The hybrid twin framework integrates virtual data from simulations and real-world data, as evidenced by a sensor concept for inline surface measurement. The framework ensures robust and redundant measurement concepts by estimating complete geometries from a few systematically preselected measuring points. This adaptive approach permits continuous updates and extensions to the database, accommodating both sparse inline signals and offline inspection data. This framework provides a conceptual model for integrating direct feedback interactions between virtual and physical environments, thereby enhancing the precision of analytical and predictive models in sheet metal forming processes.

Dissolve organic matter of mature chicken compost contributes to the protection of microorganisms from the stress of heavy metals

Dissolved organic matter (DOM), as the most active component of manure and compost, can influence heavy metal forms and microbial community structure by changing the physicochemical properties of soils. This study employed DOM extracted from fresh chicken manure (FDOM) and mature compost (MDOM) and evaluated their impacts on the physicochemical properties, heavy metal forms, and microbial communities of chernozem and dark brown soil through UV-Vis, EEM-PARAFAC, and high-throughput sequencing. The results showed that all DOM treatments decreased soil pH except for the MDOM at 150 mg C/kg applied to chernozem (CLM). MDOM treatments were more favorable in increasing soil available phosphorus content and the degree of humification of soil DOM, and FDOM treatments were more favorable in increasing soil dissolved organic carbon content and decreasing the degree of humification of soil DOM. Except for CLM treatment, the other DOM treatments decreased the content of stable state Pb, increased the risk of Pb migration and biotoxicity, and FDOM treatments were more pronounced. In addition, DOM treatments decreased the Shannon diversity of two soils and the Chao1 index in chernozem. However, DOM at 300 mg C/kg could increase the abundance of soil eutrophic and heavy metals tolerant dominant phyla (Proteobacteria and Bacteroidota) as well as dominant genera (Sphingomonas, Lysobacter, and Arthrobacter). This study provides a comprehensive understanding of the risk of manure and compost application in heavy metals-polluted soils and its interaction with microorganisms.

In-situ exploration of divergent methane coupling pathways in dry and aqueous environments on silver and palladium heteropolyacid-titania photocatalysts

Methane, abundant but inert, contributes significantly to greenhouse gases, primarily through combustion, which emits vast amounts of CO2. Photocatalytic methane coupling at ambient temperature offers a method to convert it into ethane, a more versatile hydrocarbon. This study examines selective methane-to-ethane coupling using heterogeneous photocatalysts comprising silver and palladium salts dispersed on titania. Through a combination of ex-situ and in-situ techniques along with DFT simulations, distinct mechanisms and active phases in these catalysts are revealed. In silver catalysts, dispersed cationic Ag+ species are crucial for methane activation, while in palladium catalysts, palladium primarily exists in metallic form during coupling. Water strongly enhances coupling rates with both catalysts. DFT modeling identified methane adsorption sites, suggesting methane activation via •OH radicals, experimentally supported by EPR under in-situ conditions.

THE INFLUENCE OF PERCEIVED EASE OF USE AND PERCEIVED USEFULNESS ON THE DECISION TO USE OF QRIS AS A DIGITAL PAYMENT IN GENERATION Z IN THE CITY OF BANDUNG

This study aims to examine the effect of perceived ease of use and perceived usefulness on the decision to use QRIS as a digital payment for Generation Z in Bandung City. Money, both in paper and metal form, has an important role in economic transactions. However, with the development of financial technology (fintech) technology, payment methods are starting to shift from cash to digital. Quick Response Code Indonesian Standard (QRIS) is a QR code standard implemented to facilitate digital payments in Indonesia. This research uses a quantitative method with a descriptive approach, and data is collected through questionnaires to Generation Z QRIS users in Bandung City. The results showed that perceived ease of use and perceived usefulness have a positive and significant influence on the decision to use QRIS. The practicality and efficiency of QRIS make it popular among Generation Z, who tend to prioritize convenience and usefulness in financial transactions. These findings confirm the importance of perceived convenience and benefits in influencing the decision to use digital payment technology.

Effect of Bamboo Charcoal with Earthworms on the Form and Bioavailability of Heavy Metals in Steel Sludge

Effect of Bamboo Charcoal with Earthworms on the Form and Bioavailability of Heavy Metals in Steel Sludge