Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact

This paper summarizes our studies on metal and metalloid uptake by the date palm, Phoenix dactylifera L., a tree of considerable importance in arid regions. The typical concentrations of 17 elements in the date palm are summarized and compared with existing data in the scientific literature. The role and toxicity of these elements are considered. Issues encountered by us during sample collection, pre-treatment and chemical analysis are described. Future studies are suggested.

Geogenic versus Anthropogenic Metals and Metalloids

Developing a successful strategy for investigating and remediating sites potentially impacted by metals (such as chromium [Cr], copper [Cu], lead [Pb], nickel [Ni], and zinc [Zn]) and metalloids (such as arsenic [As] and antimony [Sb]) can be challenging. These elements occur naturally and geologic materials can be enriched in these elements by natural processes. Conventional environmental investigative methods do not readily support evaluating whether metals and metalloids are geogenic (naturally occurring) or anthropogenic (from human action), or allow differentiating multiple anthropogenic sources. Geochemical methods can potentially determine whether metals and metalloids are geogenic or anthropogenic, and differentiate between possible anthropogenic sources. Conventional geo-chemical methods include whole-rock analysis using x-ray fluorescence (XRF) to yield elemental concentrations;optical petrography and powder x-ray diffraction (XRD) to determine mineral phases present;and electron microprobe (EMP) to confirm both mineral phases present and the distribution of elements within mineral phases and the rock matrix. These methods, with the exception of the EMP, can be performed in the field using portable equipment, allowing for relatively rapid assessment of sites. A case study is presented in which these techniques were successfully utilized to demonstrate, using multiple lines of evidence, that metals and metalloids present in subsurface fractured rock were geogenic and unrelated to recent industrial operations.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids(hereinafter referred to as trace metals)from the soil,some of which are essential but become toxic at high concentrations(e.g.,Cu,Zn,Ni,Co),while others are non-essential and toxic even at relatively low concentrations(e.g.,As,Cd,Cr,Pb,and Hg).Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities.Trace metal contamination can cause toxicity and growth inhibition in plants,as well as accumulation in the edible parts to levels that threatens food safety and human health.Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils.The accumulation of excess trace metals in plants can cause oxidative stress,genotoxicity,programmed cell death,and disturbance in multiple physiological processes.Plants have evolved various strategies to detoxify trace metals through cell-wall binding,complexation,vacuolar sequestration,efflux,and translocation.Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses.In this review,we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity,detoxification,and regulation,as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

Toxic metals and metalloids:Uptake,transport,detoxification,phytoremediation,and crop improvement for safer food

Agricultural soils are under threat of toxic metal/metalloid contamination from anthropogenic activities,leading to excessive accumulation of arsenic(As),cadmium(Cd),lead(Pb),and mercury(Hg)in food crops that poses significant risks to human health.Understanding how these toxic metals and their methylated species are taken up,translocated,and detoxified is prerequisite to developing strategies to limit their accumulation for safer food.Toxic metals are taken up and transported across different cellular compart-ments and plant tissues via various transporters for essential or beneficial nutrients,e.g.As by phosphate and silicon transporters,and Cd by manganese(Mn),zinc(Zn),and iron(Fe)transporters.These transport processes are subjected to interactions with nutrients and the regulation at the transcriptional and post-translational levels.Complexation with thiol-rich compounds,such as phytochelatins,and sequestration in the vacuoles are the common mechanisms for detoxification and for limiting their translocation.A num-ber of genes involved in toxic metal uptake,transport,and detoxification have been identified,offering tar-gets for genetic manipulation via gene editing or transgenic technologies.Natural variations in toxic metal accumulation exist within crop germplasm,and some of the quantitative trait loci underlying these variations have been cloned,paving the way for marker-assisted breeding of low metal accumulation crops.Using plants to extract and remove toxic metals from soil is also possible,but this phytoremediation approach requires metal hyperaccumulation for efficiency.Knowledge gaps and future research needs are also discussed.

Microbial community dynamics during composting of animal manures contaminated with arsenic,copper,and oxytetracycline

Effects of the heavy metal copper(Cu), the metalloid arsenic(As), and the antibiotic oxytetracycline(OTC) on bacterial community structure and diversity during cow and pig manure composting were investigated. Eight treatments were applied, four to each manure type, namely cow manure with:(1) no additives(control),(2) addition of heavy metal and metalloid,(3) addition of OTC and(4) addition of OTC with heavy metal and metalloid;and pig manure with:(5) no additives(control),(6) addition of heavy metal and metalloid,(7) addition of OTC and(8) addition of OTC with heavy metal and metalloid. After 35 days of composting, according to the alpha diversity indices, the combination treatment(OTC with heavy metal and metalloid) in pig manure was less harmful to microbial diversity than the control or heavy metal and metalloid treatments. In cow manure, the treatment with heavy metal and metalloid was the most harmful to the microbial community, followed by the combination and OTC treatments. The OTC and combination treatments had negative effects on the relative abundance of microbes in cow manure composts. The dominant phyla in both manure composts included Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The microbial diversity relative abundance transformation was dependent on the composting time. Redundancy analysis(RDA) revealed that environmental parameters had the most influence on the bacterial communities. In conclusion, the composting process is the most sustainable technology for reducing heavy metal and metalloid impacts and antibiotic contamination in cow and pig manure. The physicochemical property variations in the manures had a significant effect on the microbial community during the composting process. This study provides an improved understanding of bacterial community composition and its changes during the composting process.

Heavy metal: a misused term?

The use of the term"heavy metal"is regularly questioned by the scientific community.Here,we followed the evolution(1970–2020)in the number of published papers including this term in their title.Thus,we can evidence a continuous,albeit sometimes stabilizing,increase especially in environmental journals.After several other warning opinions,we propose that it should be replaced in the scientific literature by terms like"metal","metalloid","trace metal elements"or"potentially toxic element".

Adverse effects induced by chromium Ⅵ, cadmium and arsenic exposure on hypothalamus-pituitary physiology

Environmental contamination with some metalloids and heavy metals(M/HM)raises concern due to well known adverse effects on health.Among these pollutants,chromium VI(Cr VI),cadmium(Cd)and arse-nic(As)are frequently present as a result of natural sources or due to industrial activities.They are able to easily enter the organism and negatively affect many organs and systems.In vivo(exposure to Cr VI,Cd or As through drinking water)and in vitro experiments(primary pituitary cell cultures)were performed in male Wistar rats to address their actions on hypothalamus-pituitary axis.All the M/HM accumulated in hypothalamus and pi-tuitary gland and decreased pituitary cell viability and prolactin release mostly by generation of reactive oxygen species,since it was partially prevented by antioxidant treatment.In the pituitary,they increased lipid peroxy-dation and the expression of several oxidative stress markers.Cell death was mainly due to caspase-dependent apoptosis.Lactotrophs(prolactin-secreting cells)were the most affected pituitary population.Cd-driven cell death was also partially calcium-and calpain-dependent.Parallely,Cd stimulated the production of low levels of nitric oxide which exerted cytoprotective actions.These results showed that these M/HM display deleterious actions in hypothalamic-pituitary physiology by altering hormone release and promoting cell death.

Non-bonding modulations between single atomic cerium and monodispersed selenium sites towards efficient oxygen reduction

Currently,dual atomic catalysts(DACs)with neighboring active sites for oxygen reduction reaction(ORR)still meet lots of challenges in the synthesis,especially the construction of atomic pairs of elements from different blocks of the periodic table.Herein,a“rare earth(Ce)-metalloid(Se)”non-bonding heteronuclear diatomic electrocatalyst has been constructed for ORR by rational coordination and carbon support defect engineering.Encouraging,the optimized Ce-Se diatomic catalysts(Ce-Se DAs/NC)displayed a half-wave potential of 0.886 V vs.reversible hydrogen electrode(RHE)and excellent stability,which surpass those of separate Ce or Se single atoms and most single/dual atomic catalysts ever reported.In addition,a primary zinc-air battery constructed using Ce-Se DAs/NC delivers a higher peak power density(209.2 mW·cm^(−2))and specific capacity(786.4 mAh·gZn^(−1))than state-of-the-art noble metal catalysts Pt/C.Theoretical calculations reveal that the Ce-Se DAs/NC has improved the electroactivity of the Ce-N_(4)region due to the electron transfer towards the nearby Se specific activity(SA)sites.Meanwhile,the more electron-rich Se sites promote the adsorptions of key intermediates,which results in the optimal performances of ORR on Ce-Se DAs/NC.This work provides new perspectives on electronic structure modulations via non-bonded long-range coordination micro-environment engineering in DACs for efficient electrocatalysis.

Bioavailability of Arsenic and Antimony in Terrestrial Ecosystems:A Review

Arsenic (As) and antimony (Sb) are metalloids that belong to group 15 of the periodic table and exhibit toxic properties in the environment. They mostly occur naturally at low concentrations in soil, although these can be significantly elevated in both aquatic and terrestrial food chains as a result of dispersion from anthropogenic sources, e.g ., mining activities. The bioavailability, i.e., the proportion of the contaminant in soil and dust that is available for uptake by plants and other living organisms, presents the greatest risk to terrestrial ecosystems. Various in vivo and in vitro methods have been used to measure As and Sb bioaccessibility in soil and dust. In vivo measurement of bioavailability can be time consuming, expensive, and unethical;thus, in vitro methods are commonly preferred. However, there is considerable uncertainty around the efficacy of in vitro tools used to measure the bioavailable fractions of As and Sb. The results of these methods are dependent on many variables, e.g., soil characteristics, contaminant sources, and chemical composition of in vitro methods. Therefore, substantial variations are observed between in vitro and in vivo results obtained from different test animals and endpoints. In this paper, we review the literature on As and Sb bioavailability in terrestrial ecosystems and current in vivo and in vitro techniques used for assessing bioavailability and bioaccessibility of metalloids. This would reveal research gaps and allow scientists and environmental policy makers to gain a deeper understanding of the potential risks associated with these metalloids in the environment.

Assessment of Methods for Determining Bioavailability of Trace Elements in Soils: A Review

Trace element-contaminated soils(TECSs) are one of the consequences of the past industrial development worldwide. Excessive exposure to trace elements(TEs) represents a permanent threat to ecosystems and humans worldwide owing to the capacity of metal(loid)s to cross the cell membranes of living organisms and of human epithelia, and their interference with cell metabolism.Quantification of TE bioavailability in soils is complicated due to the polyphasic and reactive nature of soil constituents. To unravel critical factors controlling soil TE bioavailability and to quantify the ecological toxicity of TECSs, TEs are pivotal for evaluating excessive exposure or deficiencies and controlling the ecological risks. While current knowledge on TE bioavailability and related cumulative consequences is growing, the lack of an integrated use of this concept still hinders its utilization for a more holistic view of ecosystem vulnerability and risks for human health. Bioavailability is not generally included in models for decision making in the appraisal of TECS remediation options. In this review we describe the methods for determining the TE bioavailability and technological developments, gaps in current knowledge, and research needed to better understand how TE bioavailability can be controlled by sustainable TECS management altering key chemical properties, which would allow policy decisions for environmental protection and risk management.

Integrating geochemical(surface waters, stream sediments)and biological(diatoms) approaches to assess AMD environmental impact in a pyritic mining area： Aljustrel(Alentejo, Portugal)

Aljustrel mines were classified as having high environmental hazard due to their large tailings volume and high metal concentrations in waters and sediments.To assess acid mine drainage impacted systems whose environmental conditions change quickly,the use of biological indicators with short generation time such as diatoms is advantageous.This study combined geochemical and diatom data,whose results were highlighted in 3 groups：Group 1,with low p H（1.9–5.1）and high metal/metalloid（Al,As,Cd,Co,Cu,Fe,Mn,Ni,Pb,Zn;0.65–1032 mg/L）and SO4（405–39124 mg/L）concentrations.An acidophilic species,Pinnularia aljustrelica,was perfectly adapted to the adverse conditions;in contrast,teratological forms of Eunotia exigua were found,showing that metal toxicity affected this species.The low availability of metals/metalloids in sediments of this group indicates that metals/metalloids of the exchangeable fractions had been solubilized,which in fact enables metal/metalloid diatom uptake and consequently the occurrence of teratologies;Group 2,with sites of near neutral p H（5.0–6.8）and intermediate metal/metalloid（0.002–6 mg/L）and SO4（302–2179 mg/L）concentrations;this enabled the existence of typical species of uncontaminated streams（Brachysira neglectissima,Achnanthidium minutissimum）;Group 3,with samples from unimpacted sites,showing low metal/metalloid（0–0.8 mg/L）and SO4（10–315 mg/L）concentrations,high pH（7.0–8.4）and Cl contents（10–2119 mg/L）and the presence of brackish to marine species（Entomoneis paludosa）.For similar conditions of acidity,differences in diversity,abundance and teratologies of diatoms can be explained by the levels of metals/metalloids.

Water extraction kinetics of metals, arsenic and dissolved organic carbon from industrial contaminated poplar leaves

In industrial areas, tree leaves contaminated by metals and metalloids could constitute a secondary source of pollutants. In the present study, water extraction kinetics of inorganic elements （IE： Pb, Zn, Cd, As, Fe and Mn）, dissolved organic carbon, pH and biological activity were studied for industrial contaminated poplar leaves. Moreover, the distribution of the IE through the size fractions of the associated top soil was measured. High quantities ofMn, Zn and As and polysaccharides were released in the solution from the strongly contaminated leaves. The kinetic of release varied with time and metal type. The solution pH decreased while dissolved organic contents increased with time after 30 days. Therefore, these contaminated leaves could constitute a source of more available organic metals and metalloids than the initial inorganic process particles. However, the distribution of the IE through the size fractions of the top soil suggested that a great part of the released IE was adsorbed, reducing in consequence their transfers and bioavailability. It＇s concluded that mobility/boioavailability and speciation of metals and metalloids released from the decomposition of polluted tree leaves depends on soil characteristics, pollutant type and litter composition, with consequences for environmental risk assessment.

Rapid and long-effective removal of broad-spectrum pollutants from aqueous system by ZVI/oxidants

Zero-valent iron(ZVI)technology has recently gained significant interest in the efficient sequestration of a wide variety of contaminants.However,surface passivation of ZVI because of its intrinsic passive layer would lead to the inferior reactivity of ZVI and its lower efficacy in contaminant removal.Therefore,to activate the ZVI surface cheaply,continuously,and efficiently is an important challenge that ZVI technology must overcome before its wide-scale application.To date,several physical and chemical approaches have been extensively applied to increase the reactivity of the ZVI surface toward the elimination of broad-spectrum pollutants.Nevertheless,these techniques have several limitations such as low efficacy,narrow working pH,eco-toxicity,and high installation cost.The objective of this mini-review paper is to identify the critical role of oxygen in determining the reactivity of ZVI toward contaminant removal.Subsequently,the effect of three typical oxidants(H2O2,KMnO4,and NaClO)on broad-spectrum contaminants removal by ZV1 has been documented and discussed.The reaction mechanism and sequestration efficacies of the ZVI/oxidant system were evaluated and reviewed.The technical basis of the ZVI/oxidant approach is based on the half-reaction of the cathodic reduction of the oxidants.The oxidants commonly used in the water treatment industry,i.e.,NaClO,O3,and H2O2,can be served as an ideal coupling electron receptor.With the combination of these oxidants,the surface corrosion of ZVI can be continuously driven.The ZVI/oxidants technology has been compared with other conventional technologies and conclusions have been drawn.

REMEDIATION OF POLLUTED SOILS BY UTILIZING HYDROTHERMALLY TREATED CALCAREOUS FLY ASHES

This paper investigates a treated fly ash to act as a synthetic zeolite to remediate soils polluted with heavy metals and metalloids （As, Pb, Cu, Zn, Fe, Cd and Mn）. Four types of such ＇zeolites＇ were synthesized by hydrothermal treatment of a calcareous fly ash derived from Greek lignite-fired power plants： two with excess of sodium hydroxide in a solid/liquid ratio of 50 g·L^-1, and two with excess of fly ash in a solid/liquid ratio of 100g·L^-1. Soil samples were obtained from a former mining site at Lavrion, Greece. Mobilization and transfer of metals to the retention agents was effected by using HCI aq 1M, with satisfactory results with respect to As, Pb, Cu, Mn and Cd. The great variety of metal complexes in soil was found to be of major importance for the effectiveness of the overall process. The final products were solidified either on their own, or by using additives such as lime and cement.