MXene based non-noble metal catalyst for overall water splitting in alkaline conditions

MXene,the two-dimensional transition metal carbide or nitride material,was first discovered in 2011.They possess superior characteristics such as stability,electric conductivity,and electrochemical properties,that make them attract the attention of the energy engineering field.Overall water splitting which generates hydrogen and oxygen,not only serves as a clean energy supply technology but also demonstrates the capacity for redistribution and integration of renewable energy.MXene based non-noble metal has demonstrated significant potential in terms of cost-effectiveness.Therefore,the current focus is implementing targeted regulation at the micro level to render it effective comparable to the precious metals.In this context,the mechanisms of the hydrogen evolution reaction(HER) and the oxygen evolution reaction(OER) under the influence of MXene can be elucidated in terms of electron and ion transfer processes,hydrogen coverage,and regulation of terminal groups.Certainly,the composition,structure,synthesis,and stability strategies of MXene are the subjects of comprehensive investigation from both theoretical calculations using density functional theory(DFT) and experimental perspectives.In addition,this review provides a comprehensive summary of MXene based non-noble metal and various modification methods.These methods encompass doping,vacancy engineering,hybrid structures,heterojunction formation,multi-scale engineering,surface engineering,and phase engineering.The review also presents suggestions for designing high-performance MXene based on non-noble metals.It offers guidance on employing construction strategies for electrocatalysts.By leveraging the unique properties and tunability of MXene and implementing these modification methods,researchers can enhance the catalytic activity,stability,selectivity,and efficiency of MXene based non-noble metal catalysts.

Screening non-noble metal oxides to boost the low-temperature combustion of polyethylene waste in air

Globally,the efficient utilization of polymer wastes is one of the most important issues for current sustainable development topics.Herein,a green and efficient low-temperature combustion approach is proposed to deal with polymer wastes and recover heat energy,simultaneously alleviating the environment and energy crisis.Non-noble metal oxides(Al_(2)O_(3),Fe_(2)O_(3),NiO_(2),ZrO_(2),La_(2)O_(3)and CeO_(2)) were prepared,characterized and screened to boost the low-temperature combustion of polyethylene waste at 300℃ in air.The mass change,heat release and CO_(x) formation were studied in details and employed to evaluate the combustion rate and efficiency.It was found that CeO_(2)significantly enhanced the combustion rate and efficiency,which was respectively 2 and 7 times that of non-catalytic case.An interesting phenomenon was observed that the catalytic performance of CeO_(2) in polyethylene low-temperature combustion was significantly improved by the 7-day storage in the room environment or water treatment.XPS analysis confirmed the co-existence of Ce^(3+) and Ce^(4+) in CeO_(2),and the 7-day storage and water treatment promoted the amount of Ce^(3+),which facilitated the formation of the oxygen vacancies.That may be the reason why CeO_(2) exhibited excellent catalytic performance in polyethylene low-temperature combustion.

Recent Advances on MOF Derivatives for Non-Noble Metal Oxygen Electrocatalysts in Zinc-Air Batteries

Oxygen electrocatalysts are of great importance for the air electrode in zinc-air batteries(ZABs).Owing to the high specific surface area,controllable pore size and unsaturated metal active sites,metal-organic frameworks(MOFs)derivatives have been widely studied as oxygen electrocatalysts in ZABs.To date,many strategies have been developed to generate efficient oxygen electrocatalysts from MOFs for improving the performance of ZABs.In this review,the latest progress of the MOF-derived non-noble metal-oxygen electrocatalysts in ZABs is reviewed.The performance of these MOF-derived catalysts toward oxygen reduction,and oxygen evolution reactions is discussed based on the categories of metal-free carbon materials,single-atom catalysts,metal cluster/carbon composites and metal compound/carbon composites.Moreover,we provide a comprehensive overview on the design strategies of various MOF-derived non-noble metal-oxygen electrocatalysts and their structure-performance relationship.Finally,the challenges and perspectives are provided for further advancing the MOF-derived oxygen electrocatalysts in ZABs.

Efficient Visible Light Hydrogen Evolution Catalyst Composed of Non-noble Metal Nitride (Ni_(3)N) Cocatalyst and Zn_(0.5)Cd_(0.5)S Solid Solution

In recent years,many effective photocatalysts have been developed to solve the problem of environmental pollution and clean energy shortage.In this paper,non-noble metal cocatalyst Ni_(3)N nanoparticles supported Zn_(0.5)Cd_(0.5)S(ZCS)nanorods(Ni_(3)N/ZCS)composites were successfully synthesized by ultrasonic method.The hydrogen production efficiencies of the photocatalysts were tested under visible light,which was found that when the loading of Ni_(3)N was 2%of the mass of ZCS,and the Ni_(3)N/ZCS composite had the best hydrogen evolution performance,which could reach 70.3 mmol·h^(-1)·g^(-1).In addition,the quantum efficiency under 420 nm monochromatic light irradiation was 27.2%.Through different characterization analyses,such as X-ray diffraction(XRD),scanning electron microscopy(SEM),and UV-Vis diffuse reflectance spectra(DRS),a possible photocatalytic mechanism was proposed,providing some reference values for non-precious metals as cocatalysts.

Non-noble metal-based catalysts for acetylene semihydrogenation:from thermocatalysis to sustainable catalysis

Selective semihydrogenation of acetylene in raw olefin streams to ethylene is a key industrial reaction to produce polymer-grade feeds for the manufacture of corresponding polymers.The currently used process in industry is the thermocatalytic acetylene semihydrogenation with pressurized hydrogen and Pd-based catalysts at relatively high temperatures.The high cost of Pd urgently desires the design of non-noble metal-based catalysts.However,non-noble metal-based catalysts commonly require much higher reaction temperatures than Pd-based catalysts because of their poor intrinsic activity.Therefore,aiming at increasing economic efficiency and sustainability,various strategies are explored for developing non-noble metal-based catalysts for thermocatalytic and green acetylene semihydrogenation processes.In this review,we systematically summarize the recent advances in catalytic technology from thermocatalysis to sustainable alternatives,as well as corresponding regulation strategies for designing high-performance non-noble metal-based catalysts.The crucial factors affecting catalytic performance are discussed,and the fundamental structure-performance correlation of catalysts is outlined.Meanwhile,we emphasize current challenging issues and future perspectives for acetylene semihydrogenation.This review will not only promote the rapid exploration of non-noble metal-based catalysts for acetylene semihydrogenation,but also advance the development of sustainable processes like electrocatalysis and photocatalysis.

A review of non-noble metal-based electrocatalysts for CO_(2)electroreduction

The excessive emission of CO_(2) has caused many environmental issues and is severely threatening the eco-system.CO_(2) electroreduction reaction(CO_(2) RR) that driven by sustainable power is an ideal route for realizing the net reduction of CO_(2) and carbon recycle.Developing efficient electrocatalysts with low cost and high performance is critical for the wide applications of CO_(2) RR electrolysis.Among the various explored CO_(2) RR catalysts,non-noble metal(NNM)-based nanomaterials have drawn increasing attentions due to the remarkable performance and low cost.In this mini-review,the recent advances of NNM-based CO_(2) RR catalysts are summarized,and the catalysts are classified based on their corresponding reduction products.The preparation strategies for engineering the electrocatalysts are introduced,and the relevant CO_(2) RR mechanisms are discussed in detail.Finally,the current challenges in CO_(2) RR research are presented,and some perspectives are proposed for the future development of CO_(2) RR technology.This mini-review introduces the recent advances and frontiers of NNM-based CO_(2) RR catalysts,which should shed light on the further exploration of efficient CO_(2) RR electrocatalysts.

Recent advances in carbonized non-noble metal–organic frameworks for electrochemical catalyst of oxygen reduction reaction

The non-noble metal oxygen reduction reaction(ORR) catalysts prepared by carbonization of metal–organic framework(MOF) have attracted more and more attentions in the fields of fuel cells and metal-air batteries due to their unique intrinsic advantages such as high catalytic activity, low price, simple synthesis and good adaptability. Different from the study of traditional high active noble metal catalysts, this review systematically summarizes recent developments on non-noble metal(Fe,Co, Cu, Ni, Mn and Mo) ORR catalysts prepared by various MOFs carbonization in different metal centers. The effects of synthesis strategies and pyrolysis conditions on the catalyst properties are discussed. Meanwhile, the key parameters of catalytic performances(including active sites, dispersed state and specific surface area) are discussed and the prospect is presented. It is expected that this review will provide effective guidance for future studies on carbonized non-noble MOFs for ORR electrochemical catalyst.

Comparative Studies of Non-noble Metal Modified Mesoporous M-Ni-CaO-ZrO2 （M=Fe, Co, Cu） Catalysts for Simulated Biogas Dry Reforming

The present work is aimed to improve the performance of Ni-based catalysts for biogas dry reforming by adding a second non-noble metal （Fe, Co, Cu） into a previously studied mesoporous Ni-CaO-ZrO2 nanocomposite. Biogas was simulated with equivalent methane and carbon dioxide for the dry reforming reaction. X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS）, N2 adsorption, temperature-programmed reduction （TPR）, thermogravi- metric analysis （TGA）, and transmission electron microscopy （TEM） measurements were taken to characterize the structural and textual properties of the bimetallic catalysts as well as the accumulated carbon deposition. The addition of Fe leads to a less ordering growth of mesopores of Fe-Ni-CaO-ZrO2 sample, and the existence of Cu results in a relatively larger portion of free NiO in Cu-Ni-CaO-ZrO2. Compared with Fe and Cu, the presence of Co could efficiently form a beneficial dual metal effect and enhance the strong metal support interaction between Ni and CaO-ZrO2, thus enhancing the activity and stability of the catalyst in biogas dry reforming reaction.

Non-noble metal-based bifunctional electrocatalysts for hydrogen production

Hydrogen is a promising candidate for clean and sustainable energy resources to substitute fossil fuels to mitigate global environmental issues.Electrochemical hydrogen production has been regarded as a viable and promising strategy.The overall water splitting is currently the predominant electrochemical hydrogen production method,which could be driven by renewable energy to achieve sustainable production.However,the current challenges are the intrinsically sluggish and energy-intensive oxygen evolution reduction(OER)at the anode and the expensive noble metal-based catalysts for overall water splitting,which limit the practical applications.Extensive efforts have been made to develop bifunctional non-noble metal-based electrocatalysts to boost hydrogen production efficiency and lower the cost.Meanwhile,alternative oxidation reactions with faster kinetics and less energy requirement than OER are being explored as the anodic reaction to couple with the hydrogen evolution reaction for energy-saving hydrogen production.In this review,the non-noble metal-based bifunctional electrocatalysts for overall water splitting,as well as other novel energy-saving hydrogen productions have been introduced and summarized.Current challenges and outlooks are commented on at the end of the article.

A novel synthesis of highly active and highly stable non-noble-nickel-modified persulfated Al_(2)O_(3)@ZrO_(2) core-shell catalysts for n-pentane isomerization

The non-noble metal modified sulfated zirconia was found easy to deactivate.Herein,highly active and highly stable non-noble core-shell Ni-S_(2)O_(8)^(2−)/Al_(2)O_(3)@ZrO_(2) catalysts(Ni-SA@Z-x,x=Al content in wt%)have been successfully prepared and investigated for n-pentane isomerization.The results showed that the core-shell Ni-SA@Z-30 provided a sustained high isopentane yield(63.1%)with little or no deactivation within 5000 min at a mild reaction pressure of 2.0 MPa,which can be attributed to the following factors:(i)carbon deposition was greatly suppressed by the large pore size and huge pore volume;(ii)the loss of sulfur entities was suppressed because the small and highly dispersed tetragonal ZrO_(2) particles can bond with the S species strongly;(iii)strong Brønsted acidity can be maintained well after the isomerization.The pore structures and acid nature of the core-shell Ni-SA@Z-x are entirely different from those of the normal structure Ni-S_(2)O_(8)^(2−)/ZrO_(2)-Al_(2)O_(3),even though the Al content and the compositions of the individual components are the same.The Al_(2)O_(3)cores endow the catalysts with high internal surface area and high mechanical strength.Meanwhile,the ZrO_(2) shell,which consists of more and smaller tetragonal ZrO_(2) particles because of the large surface area of the Al_(2)O_(3)core,promotes the formation of more stable sulfur species and stronger binding sites.

Recent Progress in Carbon-based Materials of Non-Noble Metal Catalysts for ORR in Acidic Environment

Proton exchange membrane fuel cell(PEMFC)has important implications for the success of clean transportation in the future.One of the key factors affecting the cost and performance of PEMFC is the cathode electrocatalyst for the oxygen reduction reaction(ORR)to overcome sluggish kinetics and instability in an acidic environment.As an essential component of the electrocatalyst,the support material largely determines the activity,mass transfer,charge transfer,and durability of the electrocatalyst.Thereby,the support material plays a critical role in the overall performance of the electrocatalyst.Carbonbased materials are widely used as electrocatalyst supports because of their high porosity,conductivity,chemical stability,and tunable morphology.Recently,some new carbon-based materials with excellent structure have been introduced,such as carbon nanotubes,carbon nanowires,graphene,metal-organic framework(MOF)-derived carbon,and biomass-derived carbon materials.Combined with a variety of strategies,such as controllable construction of porous structures and surface defects,proper doping heteroatoms,the ingenious design of model electrocatalysts,and predictive theoretical calculation,a new reliable path was provided for further improving the performance of electrocatalysts and exploring the catalytic mechanism.Based on the topic of carbon-based materials for ORR in acidic medium,this review summarizes the up-to-date progress and breakthroughs,highlights the factors affecting the catalytic activity and stability of ORR electrocatalysts in acids,and discusses their future application and development.

Monodispersed Pt nanoparticles on reduced graphene oxide by a non-noble metal sacrificial approach for hydrolytic dehydrogenation of ammonia borane

Downsizing noble metal nanoparticles,such as Pt,is an essential goal for many catalytic reactions.A non-noble metal sacrificial approach was used to immobilize monodispersed Pt nanoparticles （NPs） with a mean size of 1.2 nm on reduced graphene oxide （RGO）.ZnO co-precipitated with Pt NPs and subsequently sacrificed by acid etching impedes the diffusion of Pt atoms onto the primary Pt particles and also their aggregation during the reduction of precursors.The resulting ultrafine Pt nanoparticles exhibit high activity （a turnover frequency of 284 min-1 at 298 K） in the hydrolytic dehydrogenation of ammonia borane.The non-noble metal sacrificial approach is demonstrated as a general approach to synthesize well-dispersed noble metal NPs for catalysis.

Nanoscale visualization of hot carrier generation and transfer at non-noble metal and oxide interface

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers(HCs).However,due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics,HC-based devices have low efficiencies.In the present work,we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption(TA)measurements.The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer,as also confirmed by conductive atomic force microscopy.In addition,the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer,passivation of the surface states by the top TiN layer,and smaller barrier height of the interfaces.Furthermore,electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoO_(x)film at the nanoscale.The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale,which indicate efficient electron transfer from TiN to MoO_(x).Thus,our simple and effective approach can facilitate HC collection under white light,thereby achieving high conversion efficiency for optoelectronic devices.

Templated synthesis of transition metal phosphide electrocatalysts for oxygen and hydrogen evolution reactions

Transition metal phosphides(TMPs)have been regarded as alternative hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)catalysts owing to their comparable activity to those of noble metal-based catalysts.TMPs have been produced in various morphologies,including hollow and porous nanostructures,which are features deemed desirable for electrocatalytic materials.Templated synthesis routes are often responsible for such morphologies.This paper reviews the latest advances and existing challenges in the synthesis of TMP-based OER and HER catalysts through templated methods.A comprehensive review of the structure-property-performance of TMP-based HER and OER catalysts prepared using different templates is presented.The discussion proceeds according to application,first by HER and further divided among the types of templates used-from hard templates,sacrificial templates,and soft templates to the emerging dynamic hydrogen bubble template.OER catalysts are then reviewed and grouped according to their morphology.Finally,prospective research directions for the synthesis of hollow and porous TMP-based catalysts,such as improvements on both activity and stability of TMPs,design of environmentally benign templates and processes,and analysis of the reaction mechanism through advanced material characterization techniques and theoretical calculations,are suggested.

Oxygen functionalization-assisted anionic exchange toward unique construction of flower-like transition metal chalcogenide embedded carbon fabric for ultra-long life flexible energy storage and conversion

The metal-organic framework(MOF)derived Ni–Co–C–N composite alloys(NiCCZ)were“embedded”inside the carbon cloth(CC)strands as opposed to the popular idea of growing them upward to realize ultrastable energy storage and conversion application.The NiCCZ was then oxygen functionalized,facilitating the next step of stoichiometric sulfur anion diffusion during hydrothermal sulfurization,generating a flower-like metal hydroxysulfide structure(NiCCZOS)with strong partial implantation inside CC.Thus obtained NiCCZOS shows an excellent capacity when tested as a supercapacitor electrode in a three-electrode configuration.Moreover,when paired with the biomass-derived nitrogen-rich activated carbon,the asymmetric supercapacitor device shows almost 100%capacity retention even after 45,000 charge–discharge cycles with remarkable energy density(59.4 Wh kg^(-1)/263.8μWh cm^(–2))owing to a uniquely designed cathode.Furthermore,the same electrode performed as an excellent bifunctional water-splitting electrocatalyst with an overpotential of 271 mV for oxygen evolution reaction(OER)and 168.4 mV for hydrogen evolution reaction(HER)at 10 mA cm−2 current density along with 30 h of unhinged chronopotentiometric stability performance for both HER and OER.Hence,a unique metal chalcogenide composite electrode/substrate configuration has been proposed as a highly stable electrode material for flexible energy storage and conversion applications.

Regulating zinc ion transport behavior and solvated structure towards stable aqueous Zn metal batteries

Aqueous Zn metal batteries(AZMBs)with intrinsic safety,high energy density and low cost have been regarded as promising electrochemical energy storage devices.However,the parasitic reaction on metallic Zn anode and the incompatibility between electrode and electrolytes lead to the deterioration of electrochemical performance of AZMBs during the cycling.The critical point to achieve the stable cycling of AZMBs is to properly regulate the zinc ion solvated structure and transfer behavior between metallic Zn anode and electrolyte.In recent years,numerous achievements have been made to resolve the formation of Zn dendrite and interface incompatible issues faced by AZMBs via optimizing the sheath structure and transport capability of zinc ions at electrode-electrolyte interface.In this review,the challenges for metallic Zn anode and electrode-electrolyte interface in AZMBs including dendrite formation and interface characteristics are presented.Following the influences of different strategies involving designing advanced electrode structu re,artificial solid electrolyte interphase(SEI)on Zn anode and electrolyte engineering to regulate zinc ion solvated sheath structure and transport behavior are summarized and discussed.Finally,the perspectives for the future development of design strategies for dendrite-free Zn metal anode and long lifespan AZMBs are also given.

From Liquid to Solid‑State Lithium Metal Batteries:Fundamental Issues and Recent Developments

The widespread adoption of lithium-ion batteries has been driven by the proliferation of portable electronic devices and electric vehicles,which have increasingly stringent energy density requirements.Lithium metal batteries(LMBs),with their ultralow reduction potential and high theoretical capacity,are widely regarded as the most promising technical pathway for achieving high energy density batteries.In this review,we provide a comprehensive overview of fundamental issues related to high reactivity and migrated interfaces in LMBs.Furthermore,we propose improved strategies involving interface engineering,3D current collector design,electrolyte optimization,separator modification,application of alloyed anodes,and external field regulation to address these challenges.The utilization of solid-state electrolytes can significantly enhance the safety of LMBs and represents the only viable approach for advancing them.This review also encompasses the variation in fundamental issues and design strategies for the transition from liquid to solid electrolytes.Particularly noteworthy is that the introduction of SSEs will exacerbate differences in electrochemical and mechanical properties at the interface,leading to increased interface inhomogeneity—a critical factor contributing to failure in all-solidstate lithium metal batteries.Based on recent research works,this perspective highlights the current status of research on developing high-performance LMBs.

From Topological Nodal-Line Semimetals to Quantum Spin Hall Insulators in Tetragonal SnX Monolayers(X=F,Cl,Br,I)

Two-dimensional(2D)topological materials have recently garnered significant interest due to their profound physical properties and promising applications for future quantum nanoelectronics.Achieving various topological states within one type of materials is,however,seldom reported.Based on first-principles calculations and tightbinding models,we investigate topological electronic states in a novel family of 2D halogenated tetragonal stanene(T-SnX,X=F,Cl,Br,I).All the four monolayers are found to be unusual topological nodal-line semimetals(NLSs),protected by a glide mirror symmetry.When spin-orbit coupling(SOC)is turned on,T-SnF and TSnCl are still ascertained as topological NLSs due to the remaining band inversion,primarily composed of Sn pxy orbitals,while T-Sn Br and T-SnI become quantum spin Hall insulators.The phase transition is ascribed to moving up in energy of Sn s orbitals and increasing of SOC strengths.The topology origin in the materials is uniformly rationalized through elementary band representations.The robust and diverse topological states found in the 2D T-SnX monolayers position them as an excellent material platform for development of innovative topological electronics.

Towards advanced zinc anodes by interfacial modification strategies for efficient aqueous zinc metal batteries

Developing sustainable and clean energy sources(e.g.,solar,wind,and tide energy)is essential to achieve the goal of carbon neutrality.Due to the discontinuous and inco nsistent nature of common clean energy sources,high-performance energy storage technologies are a critical part of achieving this target.Aqueous zinc metal batteries(AZMBs)with inherent safety,low cost,and competitive performance are regarded as one of the promising candidates for grid-scale energy storage.However,zinc metal anodes(ZMAs)with irreversible problems of dendrite growth,hydrogen evolution reaction,self-corrosio n,and other side reactions have seriously hindered the development and commercialization of AZMBs.An increasing number of researchers are focusing on the stability of ZMAs,so assessing the effectiveness of existing research strategies is critical to the development of AZMBs.This review aims to provide a comprehensive overview of the fundamentals and challenges of AZMBs.Resea rch strategies for interfacial modification of ZMAs are systematically presented.The features of artificial interfacial coating and in-situ interfacial coating of ZMAs are compared and discussed in detail,as well as the effect of modified interfacial ZMA on the full-battery performance.Finally,perspectives are provided on the problems and challenges of ZMAs.This review is expected to offer a constructive reference for the further development and commercialization of AZMBs.

Evaluation of Four Anthropogenic Activity Impacts on Heavy Metal Quality of the Kumba River in the South West Region of Cameroon

Anthropogenic activities have contributed to pollution of water bodies through deposition of diverse pollutants amongst which are heavy metals. These pollutants, which at times are above the maximum concentration levels recommended, are detrimental to the quality of the water, soil and crops (plant) with subsequent human health risks. The objective of the work was to evaluate the impacts of human-based activities on the heavy metal properties of surface water with focus on the Kumba River basin. Field observations, interviews, field measurements and laboratory analyses of different water samples enabled us to collect the different data. The results show four main human-based activities within the river basin (agriculture, livestock production, domestic waste disposal and carwash activities) that pollute surface water. Approximately 20.61 tons of nitrogen and phosphorus from agricultural activities, 156.48 tons of animal wastes, 2517.5 tons of domestic wastes and 1.52 tons of detergent from carwash activities were deposited into the river each year. A highly significant difference at 1% was observed between the upstream and downstream heavy metal loads in four of the five heavy metals tested except for copper that was not significant. Lead concentrations were highest in all the activities with an average of 2.4 mg∙L−1 representing 57.81%, followed by zinc with 1.596 mg∙L−1 (38.45%) and manganese with 0.155 mg∙L−1 (3.74%) for the different anthropogenic activities thus indicating that these activities highly lead to pollution of the Kumba River water. The level of zinc and manganese was significantly influenced at ρ 005 by anthropogenic activities though generally the variations were in the order: carwash (3.196 mg∙L−1) < domestic waste disposal (3.347 mg∙L−1) < agriculture (4.172 mg∙L−1) < livestock (4.886 mg∙L−1) respectively and leading to a total of 14.04 tons of heavy metal pollutants deposited each day.