Molecular level picture of the interplay between pH and phosphate binding at the goethite-water interface.

The soil pH plays a substantial role in controlling phosphorus (P) adsorption and mobilization. These processes are strongly affected by the phosphate interaction strength with P-fixing soil minerals such as goethite. The target of the current contribution is to draw a molecular level picture of the interplay between pH and phosphate binding at the goethite-water interface via a joint experimental-theoretical approach. Periodic density functional theory (DFT) calculations were carried out to provide a molecular level understanding of the pH dependence of P adsorption. To validate the modeling approach, adsorption experiments of phosphate at goethite were performed in the pH range of 4-12. There was agreement between experiments and simulations in the description of the adsorption behavior by two pH-dependent successive stages. The adsorption increases along the pH change from 4 to 8. A further increase of pH leads to a decrease of adsorption. By comparing with literature data it is concluded that the first stage will be observed only if there is no significant change of the surface charge at low pH. Moreover, the molecular modeling results point to the abundance of the monodentate (M) binding motif at both extremely low and high pH ranges. Otherwise, the bidentate (B) one is predominant along the intermediate pH range. These observations could resolve the existing debate about the assignment of phosphate-goethite binding motifs. Furthermore, the results point to a decrease of pH upon phosphate sorption due to an induced acidification of soil solution. The present joint experimental-theoretical approach provides a better understanding and description of the existing phosphate sorption experiments and highlights new findings at the atomistic/molecular scale.

Impact of mining on the floristic association of gold mined sites in Southwest Nigeria.

BACKGROUND: Occurrences in land use, human activities and climate change have both direct and indirect influences on the environment. Of interest for this study is mining; a common activity in developing countries such as Nigeria which is endowed with over 34 solid minerals. The gold mining sites in the Southwest region of the country is predominantly by Artisanal and Small-Scale Mining (ASM). Though the benefits are known, its induced consequences are enormous. To understand its extent of floristic diversity, identification of functional plants and plant species surviving on the mined sites (despite its characterized mining and alteration level); this study compared the floristic composition of an abandoned mining site (Site 1), an active mining site (Site 2) and an undisturbed vegetation sites (Control) of similar vegetation zone. RESULTS: A total of 54, 28 and 37 species belonging to 31, 20 and 23 families were found on Site 1, Site 2 and the control site, respectively. It shows that the floristic composition of all the sites has been altered due to its past intense agricultural colonization and human activities, but severe on Site 1 and 2 due to mining. Lots of the identified species are functional species and stand as ecological indicators. Species such as Acanthus montanus and Icacina trichantha found on the Control sites are native and significance but species such as Capsicum frutescens and Crassocephalum crepidioides on Site 2 are due to human inference while most species on Site 1 shows both original and altered floristic composition (e.g. Adenia venenata and Grewia flavescens). CONCLUSIONS: Apart from the on-going farming activities, ASM activities such as pollution, deforestation and exposure of the forest soils to direct sunlight has greatly stressed and disturbed the floristic composition, species richness, life form patterns, of the mined sites as well as introduction of non-native plant species. It is therefore necessary to develop effective approaches and policies to curb these illegal ASM activities, empower the community (especially youths), stabilize the economy and establish sustainable development strategies with adequate reclamation measures.

Degradation of ferrocyanide by natural isolated bacteria.

This study aims at investigating the iron cyanide (CN) degradation potential of two natural bacterial isolates with the purpose of their application in iron CN phytoremediation. The strains were isolated from contaminated soil and incubated over 4 months with 50 mg L-1 CN (as ferrocyanide) as the sole iron and nitrogen source. Unlike previous reports, the study provides control for bacterial growth, biotic and abiotic CN losses. Bacterial growth, CN, ammonium, and nitrate concentrations were monitored regularly. Both strains grew less rapid with iron CN compared with the positive control. However, the growth was diauxic. The CN concentration in the media decreased with 20% and 25% respectively, while that in the sterile controls remained stable. Ammonium was detected in the media of both strains implying that a fraction of the initially applied ferrocyanide has been converted. The nitrogen lost from the system evened out with that in the cells at the end of the experiments. These results showed that the investigated strains were undoubtedly able to grow on iron CN as an alternative nitrogen source, but contrary to some previous findings, the iron CN utilization is much slower and takes place only after complete exhaustion of the cellular nitrogen reserves.

Cool Farm Tool Water: A global on-line tool to assess water use in crop production.

The agricultural sector accounts for 70% of all water consumption and poses great pressure on ground water resources. Therefore, evaluating agricultural water consumption is highly important as it allows supply chain actors to identify practices which are associated with unsustainable water use, which risk depleting current water resources and impacting future production. However, these assessments are often not feasible for crop producers as data, models and experiments are required in order to conduct them. This work introduces a new on-line agricultural water use assessment tool that provides the water footprint and irrigation requirements at field scale based on an enhanced FAO56 approach combined with a global climate, crop and soil databases. This has been included in the Cool Farm Tool - an online tool which already provides metrics for greenhouse gas emissions and biodiversity impacts and therefore allows for a more holistic assessment of environmental sustainability in farming and agricultural supply chains. The model is tested against field scale and state level water footprint data providing good results. The tool provides a practical, reliable way to assess agricultural water use, and offers a means to engage growers and stakeholders in identifying efficient water management practices.

Infrared spectroscopic characterization of phosphate binding at the goethite-water interface.

The interaction between phosphates and soil mineral surfaces, such as Fe- and Al-(oxyhydr)oxides, plays a crucial role in the immobilization of P and thus its availability for plants. The reactions of phosphates with Fe-hydroxides and especially goethite have been studied extensively. But a molecular-level picture of the phosphate binding mechanisms at the goethite-water interface is still lacking. Therefore, in the current contribution we have explored the molecular binding mechanisms for the adsorbed phosphate at the goethite-water interface by performing sorption kinetics experiments for orthophosphate and characterizing the adsorbed species by FT-IR spectroscopy. In parallel, periodic DFT calculations have been performed to explore the interaction mechanisms and to assign the IR spectra for monodentate (M) and bidentate (B) orthophosphate complexes at two different goethite surface planes (010 and 100) in the presence of water. In general, our interaction energy results give evidence that the mono-protonated B phosphate complex is favored to be formed at the goethite-water interface, although the M motif could exist as a minor fraction. Moreover, it was found that water plays an important role in controlling the phosphate adsorption process at the goethite surfaces. The interfacial water molecules form H-bonds (HBs) with the phosphate as well as with the goethite surface atoms. Furthermore, some water molecules form covalent bonds with goethite's Fe atoms while others dissociate at the surface to protons and hydroxyl groups. The present theoretical assignment of IR spectra introduces a benchmark for characterizing experimental IR data for the adsorbed KH2PO4 species at the goethite-water interface. In particular, the IR spectra of the mono-protonated (2O + 1Fe) B complex at the 010 goethite surface plane and the M complex at the 100 goethite surface plane were found to be consistent with the experimental data. In order to explore the role of different abundances of surface planes and binding motifs, IR spectra obtained from weighted averages have been analyzed. The results confirmed the conclusions drawn from interaction energy calculations.

Detoxification of ferrocyanide in asoil-plant system.

The detoxification of iron cyanide in a soil-plant system was investigated to assess the total cyanide extracted from contaminated soil and allocated in the leaf tissue of willow trees (Salix caprea). They were grown in soil containing up to 1000 mg/kg dry weight (dw) of cyanide (CN), added as 15N-labeled potassium ferrocyanide and prepared with a new method for synthesis of labeled iron cyanides. CN content and 15N enrichment were monitored weekly over the exposure in leaf tissue of different age. The 15N enrichment in the young and old leaf tissue reached up to 15.197‰ and 9063‰, respectively; it increased significantly over the exposure and with increasing exposure concentrations (p < 0.05). Although the CN accumulation in the old leaf tissue was higher, compared to the young leaf tissue (p < 0.05), the 15N enrichment in the two tissue types did not differ statistically. This indicates a non-uniform CN accumulation but a uniform 15N allocation throughout the leaf mass. Significant differences were detected between the measured CN content and the C15N content, calculated from the 15N enrichment (p < 0.05), revealing a significant CN fraction within the leaf tissue, which could not be detected as ionic CN. The application of labeled iron CN clearly shows that CN is detoxified during uptake by the willows. However, these results do not exclude other detoxification pathways, not related to the trees. Still, they are strongly indicative of the central role the trees played in CN removal and detoxification under the experimental conditions.

Impact of crystalline and amorphous iron- and aluminum hydroxides on mechanisms of phosphate adsorption and desorption.

Fourier-transform infrared (FT-IR) spectroscopic experiments were carried out during phosphate adsorption on highly crystalline gibbsite, poorly crystalline 2-line-ferrihydrite and amorphous iron-aluminum-hydroxide mixtures in the molar ratio 1:0, 10:1, 5:1, 1:1, 1:5, 1:10 and 0:1. The OH stretching vibrational bands were utilized to analyze changes in structural and surface OH groups during adsorption, because the position of characteristic PO vibrational bands can shift depending on reaction conditions, pH or adsorbed phosphate content. Adsorption and desorption kinetics were studied at pH6 and different initial phosphate concentrations to achieve varying phosphate coverage on the mineral surfaces. For gibbsite the formation of AlHPO4 and Al2HPO4 can be assumed, while for ferrihydrite, a FeHPO4 or Fe2PO4 complex and the precipitation of FePO4 with longer equilibration time were proposed. Fe2HPO4 or a Fe2PO4 surface complex was deduced for Fe-hydroxides, an AlH2PO4 surface complex was identified for Al-hydroxide, and both displayed either hydrogen bonds to neighboring hydroxyl groups or hydrogen bonds to outer-sphere complexes. Fe:Al-hydroxide mixtures with high Al ratios showed a low phosphate desorption rate, while ferrihydrite and the Fe:Al-hydroxide mixtures with high Fe ratios had almost negligible desorption rates. It was concluded that within the weakly associated amorphous FeO(OH) materials, FePO4 precipitated, which was bound by outer-sphere hydrogen bonds. With high Al ratios, desorption increased, which indicated weaker phosphate binding of both inner-sphere and outer-sphere complexes and hence, either no or minor quantities of precipitate. Ferrihydrite showed a more rigid structure and a lower extent of precipitation compared to amorphous Fe-hydroxide.

Uptake of ferrocyanide in willow and poplar trees in a long term greenhouse experiment.

Phytoremediation of sites contaminated with iron cyanides can be performed using poplar and willow trees. Poplar and willow trees were grown in potting substrate spiked with ferrocyanide concentrations of up to 2,000 mg kg(-1) for 4 and 8 weeks respectively. Soil solution and leaf tissue of different age were sampled for total cyanide analysis every week. Chlorophyll content in the leaves was determined to quantify cyanide toxicity. Results showed that cyanide in the soil solution of spiked soils differed between treatments and on weekly basis and ranged from 0.5 to 1,200 mg l(-1). The maximum cyanide content in willow and poplar leaves was 518 mg kg(-1) fresh weight (FW) and 148 mg kg(-1) FW respectively. Cyanide accumulated in the leaves increased linearly with increasing cyanide concentration in the soil solution. On the long term, significantly more cyanide was accumulated in old leaf tissue than in young tissue. Chlorophyll content in poplar decreased linearly with increasing cyanide in the soil solution and in leaf tissue, and over time. The inhibitory concentration (IC50) value for poplars after 4 weeks of exposure was 173 mg l(-1) and for willow after 8 weeks of exposure-768 mg l(-1). Results show that willows tolerate much more cyanide and over a longer period than poplars, making them very appropriate for remediating sites highly contaminated with iron cyanides.