Precipitation Of Compounds Research Articles

Recovering phosphorus as struvite from the concentrated solution produced by the alternating aerobic/anaerobic biofilm system

The recovery of phosphorus is so far limited to recovering phosphorus as struvite or vivianite from the supernatant of digester, but this process is not economically viable. In this study, a biofilm reactor was operated to directly enrich phosphorus from sewage to investigate whether phosphorus harvest affected the phosphorus uptake/release of phosphorus accumulating organisms (PAOs) and evaluated the potential of phosphorus recovery from concentrated liquid. The results showed that phosphorus was quickly released in the new recirculated solution by PAOs with saturated poly-phosphate after phosphorus harvest, which promoted the synthesis or degradation of polyhydroxyalkanoate (PHA) and glycogen in PAOs, restoring its ability to absorb phosphorus. Although the results of crystallization experiments demonstrated that there were traces of K-struvite and Na-struvite (<0.1 wt%) in precipitate, the simultaneous precipitation of compounds could not significantly affect the purity of products. Using the concentrated solution with a phosphorus concentration of 168.5 ± 2.7 mg·L−1 as phosphorus source for struvite precipitation achieved phosphorus recovery efficiency of 74.5–95.8% and obtained struvite crystals with a size of 83.5–114.3 µm, whereas the quantity of fine products (<10 µm) was rised due to an increase in saturation. Especially, the highest phosphorus recovery efficiency (95.8 ± 3.2%) was obtained at pH= 8.5 and Mg: P = 1.5. Moreover, life cycle assessment (LCA) analysis indicated that the CO2 equivalent emission of struvite production was 0.64–1.01 kg CO2 equivalent/kg struvite, and the phosphorus recovery efficiency and environmental impact could be balanced at pH= 8.5 and Mg: P = 1.5:1.

Effects of four cost-effective amendments on the synchronous stabilization of As, Zn, Cu and Cd in contaminated mixture of residue and soil from an arsenic smelting site

The effects of four cost-effective amendments including flue gas desulfurization gypsum (FGDG), red soil (RS), biofuel ash (BA) and limestone (LS) on the synchronous stabilization of As, Zn, Cu, Cd in acid mixture of residue and soil (MRS) from an abandoned As smelting site were investigated in this work. The change of environmental pH, EC, CEC and heavy metal fractions also have been examined. The four amendments stabilized the heavy metals in the order of BA>LS>FGDG≈RS. The correlation analysis indicated that the change of pH dominated the stabilization of heavy metals. And the application of BA and LS could efficiently increase the environmental pH. The treatment of 3% BA and LS immobilized As, Zn, Cu, Cd by 62.03%, 97.86%, 99.81%, 90.39% and 38.49%, 99.96%, 99.99%, 99.62% respectively, following an obvious decreasing of F1+F2 for As and B1 for Zn, Cu. But no significant transformation of speciation distribution appeared for Cd. The stabilization of Zn, Cu, Cd was achieved through their precipitation as carbonate and hydroxide in the high pH, as well as forming Me-bearing silicate (Me=Zn, Cu, Cd) with the amorphous silicate in BA. And As was immobilized by transforming unstable As into the precipitation of Ca-As compounds due to the alkaline circumstance and released Ca2+ from BA and LS. The results demonstrated that the BA and LS were cost-effective stabilizers for As, Zn, Cu, Cd in acid MRS.

Exploring the Interface of Porous Cathode/Bipolar Membrane for Mitigation of Inorganic Precipitates in Direct Seawater Electrolysis.

Direct seawater electrolysis utilizes natural seawater as the electrolyte. Hydroxide ions generated from the hydrogen evolution reaction at the cathode induce the precipitation of inorganic compounds, which block the active sites of the catalysts, leading to high cell voltage. To mitigate inorganic scaling, herein, an optimized interface between a porous electrode and a bipolar membrane (BPM, as a separator) was suggested in zero‐gap seawater electrolyzers. Despite the formation of inorganic deposits at the front side (facing bulk seawater) of the porous cathode due to the water reduction reaction, the back side facing the cation exchange layer of the BPM remained free from thick inorganic deposits. This was ascribed to the locally acidic environment generated by proton flux from water dissociation at the BPM, enabling stable hydrogen production via the proton reduction at low overpotential. This asymmetric hydrogen evolution reaction at the porous cathode led to a considerably lower cell voltage and higher stability than that achieved with the mesh electrode. Moreover, precipitation at the front side of the porous cathode was further mitigated through acidification of the seawater by introducing an open area of the BPM that was not in contact with the porous cathode, allowing free protons that were not involved in the electron transfer reaction to diffuse out into the bulk seawater. These findings may provide critical guidance for the investigation of interfacial phenomena for the complete mitigation of inorganic scaling in the direct electrolytic splitting of seawater.

Quantitative analysis of rare earth elements in Mg–Zn–RE(Ce, Y, Gd)–Zr alloy

The solid solution amount of rare earth elements and the mass fraction of compounds were measured by low-temperature phase separation technology, the existing forms of rare earth elements in ZK-RE(Ce, Y, Gd) alloy were quantitatively studied, and the difference of solid solution ability of rare earth elements and the factors affecting the number of rare earth compounds were analyzed. The experimental results show that the solid solution amount of rare earth elements mainly depends on their addition amount, Zn and RE inhibit each other’s solid solution amount, and the order of solid solution amount (wt%) of rare earth elements with the same addition amount is GD > Y > Ce. Rare earth elements are mostly distributed in Mg–Zn–RE compounds, and the increase of rare earth content is helpful to form more Mg–Zn–RE compounds. With the same content of elements, the order of mass fraction of Mg–Zn–RE compounds is ZK-Ce system > ZK-Y system > ZK-Gd system. The order of solid solution ability of rare earth elements is Gd > Y > Ce, and the amount of solid solution directly affects the initial element content of compound precipitation in the remaining melt. The influence of Zn/Ce ratio in the residual melt of ZK-Ce alloy is shown in the number of rare earth compounds, while the Zn/Y(Gd) ratio in the residual melt of ZK-RE(Y, Gd) alloy determines the type and number of rare earth compounds.

Nanocompound-induced anti-softening mechanisms: Application to CuCr alloys

A nanocompound-induced anti-softening mechanism is proposed for preparing high strength and high conductivity CuCr alloys with favorable softening resistance. The mechanism is to introduce nanocompound at the Cu/Cr interface to suppress the coarsening of Cr precipitate. To verify the mechanism, Co and Ti are added to the CuCr alloy. CoTi compounds precipitate around the Cr precipitates after aging treatment. The coarsening of Cr and CoTi phases are both suppressed thanks to the low diffusion rates of Co and Ti atoms in Cu. Additionally, Cr and CoTi precipitates also inhibit the recrystallization. As a result, the CuCrCoTi alloy displays a higher softening temperature (>600 °C) than CuCr alloys.

Influence of Heat Treatment on the Microstructure and Wear Properties of Arc-Sprayed FeCrAl/Al Coating

Intermetallic compounds formed during heat treatment of alloy coatings affect the coating structure and properties. In order to determine the phase changes and coating performance, FeCrAl (Fe Bal., Cr 26 wt.%, Al 6 wt.%) and Al (99.9 wt.%) coating was sprayed onto low carbon steel substrates and subsequently heat-treated at different temperatures. The effects of heat treatment on the microstructure, phase composition, tensile bonding strength, microhardness, and wear properties of the coatings were analyzed. The as-sprayed coating had a dense, layered structure with an average porosity of 3.6%. The microhardness of the as-sprayed coating was comprised of hard FeCrAl splats and ductile Al splats with an average microhardness value of 494 HV0.1. The coating at 300 °C had the highest tensile strength of 37.5 MPa. At 500 °C, FeAl intermetallic compounds formed at the phase boundaries due to the diffusion of elements. The coating microhardness and wear resistance were affected by the uniform coating structure and the precipitation of FeAl intermetallic compounds. Compared with the annealed coatings, the as-sprayed coating had the lowest wear rate.

Rising risks of compound extreme heat‐precipitation events in China

AbstractWhile the changes of extreme weather and climate events have been well investigated, the change of compound events (i.e., combinations of multiple weather/climate extremes and/or hazards), which severely affect the biophysical and human systems, remains poorly understood. Here, we examine the decadal trends of subsequently (or “preconditioned”) compound extreme heat‐precipitation events (i.e., extreme precipitation events preceded by an extreme heat) across China during 1961–2016. We find that, on average, around one‐quarter of summer precipitation extremes over China (especially western China) are preceded by an extreme heat event. In most areas of China, the fraction of the compound precipitation events preceded by heat extremes exhibits significant increases since the 1960s, with a national mean increasing tendency of 2.51%·decade−1. Furthermore, the rising trends in the fractional contribution of hot weather to extreme precipitation events over most parts of China have accelerated in more recent decades.

Large-Scale Environments of Successive Atmospheric River Events Leading to Compound Precipitation Extremes in California

Abstract Successive atmospheric river (AR) events—known as AR families—can result in prolonged and elevated hydrological impacts relative to single ARs due to the lack of recovery time between periods of precipitation. Despite the outsized societal impacts that often stem from AR families, the large-scale environments and mechanisms associated with these compound events remain poorly understood. In this work, a new reanalysis-based 39-yr catalog of 248 AR family events affecting California between 1981 and 2019 is introduced. Nearly all (94%) of the interannual variability in AR frequency is driven by AR family versus single events. Using k-means clustering on the 500-hPa geopotential height field, six distinct clusters of large-scale patterns associated with AR families are identified. Two clusters are of particular interest due to their strong relationship with phases of El Niño–Southern Oscillation (ENSO). One of these clusters is characterized by a strong ridge in the Bering Sea and Rossby wave propagation, most frequently occurs during La Niña and neutral ENSO years, and is associated with the highest cluster-average precipitation across California. The other cluster, characterized by a zonal elongation of lower geopotential heights across the Pacific basin and an extended North Pacific jet, most frequently occurs during El Niño years and is associated with lower cluster-average precipitation across California but with a longer duration. In contrast, single AR events do not show obvious clustering of spatial patterns. This difference suggests that the potential predictability of AR families may be enhanced relative to single AR events, especially on subseasonal to seasonal time scales.

4D synchrotron X-ray tomographic study of the influence of transverse magnetic field on iron intermetallic compounds precipitation behavior during solidification of Al–Si–Fe alloy

Hard and brittle iron intermetallic compounds (IMCs) will be formed during solidification due to the existence of iron element in Aluminum alloy. These IMCs will cause cracks or fractures in the material, which can deteriorate the mechanical properties. Synchrotron X-ray tomography were used to study the real time formation and growth dynamics of IMCs in Al-10 wt.%Si-1 wt.%Fe alloy during solidification under transverse magnetic field (TMF). The results indicated that the precipitation behavior of IMCs was significantly affected by 0.07 T TMF. The IMCs were observed to form only on the periphery of the sample under TMF, comparing with random distribution without TMF. In addition, the growth dynamics of IMCs were also affected by TMF. When solidified without TMF, an explosive nucleation and growth of IMCs was presented at the intermediate solidification stage, where a large number of fine IMCs precipitated at the entire regions of the sample. In contrast, the IMCs located at the periphery areas were observed to grow steadily under TMF. The growth rate and formation mechanism of IMCs were also quantified and revealed, showing difference with and without TMF. The tip growth rate was relatively stable when solidified with TMF, while an accelerating growth stage was presented when TMF is absent. The discrepancy in precipitation behavior of IMCs could be best explained as the convection of the melt resulted from the applied TMF. The TMF was demonstrated to generate a directional flow with a velocity of ∼3.3 mm/s. The flow was supposed to drive the Fe solute from the center to the edge of the sample, providing enough Fe solute for IMCs to nucleate and grow on the surface of the sample. In addition, due to the influence of TMF induced flow on the distribution of Fe solute and temperature, the morphology and tip growth rate of IMCs were also affected. The result presented in current study provides the possibility to regulate the precipitation behavior of Fe-rich IMCs in Al alloys. The imposed TMF also shows the potential application in Fe removal by driving the Fe solute to the surface of sample in Al alloys.

Условия кристаллизации слитков высокопрочных алюминиевых сплавов с повышенным содержанием циркония

The structure of model alloys of the aluminum-zirconium system and industrial 1973-type alloy with an increased content of zirconium after rapid cooling to temperatures of the liquid-solid region and isothermal holding at these temperatures has been studied. It is shown that during primary crystallization of intermetallic precipitates with low crystallization rate, it is advisable to use the model of decomposition of supersaturated solid solutions. It was determined that formation of Al3Zr particles leads to zirconium decrease in the solid solution in center and grain boundary. Under certain crystallization conditions, there is an incubation period for the zirconium intermetallic compounds precipitation. It is shown that the incubation period for aluminum with 0.25 % zirconium is more than 2 minutes, and for 0.5 % zirconium — less than 0.5 minutes. The value of the dendritic parameter of the 1973-type alloy with an increased zirconium content during two-stage crystallization corresponds to the known regularity of the change in the dendritic parameter depending on the cooling rate. Casting of alloy ingots of 1960-type alloy with 0.27 % Zr by the two-stage cooling with the use of a water-cooling tray has been tested. This made possible to produce ingots with 84 mm diameter without the formation of primary intermetallic compounds. The mechanical properties of forgings from such ingots showed an increase in ductility by 20 % while maintaining the strength characteristics.

Stabilizing Fe-based nanocrystalline alloys via a high-entropy strategy

Fe-based nanocrystalline alloys with unique dual-nano-phase structure and superior magnetic softness have aroused tremendous interest, yet they generally suffer from the harsh annealing process due to the poor thermal stability. In this study, a high-entropy strategy was proposed to enhance the soft-magnetic property and nanostructure stability of Fe-based nanocrystalline alloys by adding P and C elements in the FeCuSiBPC alloy system. This alloying approach by complicating the composition will greatly increase the mixing entropy, which significantly increase the frequency factor for the formation of α-Fe grains and activation energy for the formation of compounds, which on one hand enhance the competition and soft-impingement effects due to the increased number density of α-Fe grains, and on the other hand hinder the precipitation of compounds in the intergranular amorphous interphase due to the improved crystallization resistance. These both contribute to a thermodynamically and kinetically stable dual-nano-phase structure with fine α-Fe grains embedded in amorphous matrix. Following this strategy of nanostructure stabilization by tuning the compositional complexity, FeCuSiBPC alloy with enlarged processing window and enhanced soft-magnetic properties were successfully obtained. This high-entropy strategy can be applied in a verity of alloy systems to develop high performance nanocrystalline alloys with excellent thermal stability suitable for large-scale industrial processing.

The non-isothermal crystallization kinetics and mechanism of FeGaGeBCu alloy

In this paper, Fe76Ga5Ge5B13Cu1 ribbon was prepared by melt spinning method and its thermal stability and non-isothermal crystallization kinetics were analyzed. It is found that the primary crystallization phase is α-Fe(Ga,Ge), following by the precipitation of Fe-B compounds. Besides, Kissinger and Ozawa methods were utilized to calculate the initial activation energy Ex and apparent activation energy Ea. The results show that Ex1 > Ea1, which is due to that the diffusion of Ga and Ge is difficult during the nucleation process but left a channel for the subsequent diffusion of atoms. In addition, Avrami index n was calculated by Johnson-Mehl-Avrami-Kolmogorov (JMAK) model and the crystallization mechanism of Fe76Ga5Ge5B13Cu1 alloy was analyzed. For the alloy with grains or clusters embedded in the amorphous matrix, it is concluded that the annealing condition with a higher heating rate is more conducive to obtain homogeneous nanocrystalline and improve soft magnetic properties.

Corrosion Resistance and Surface Bioactivity of Ti6Al4V Alloy after Finish Turning under Ecological Cutting Conditions

The influence of cooling conditions and surface topography after finish turning of Ti6Al4V titanium alloy on corrosion resistance and surface bioactivity was analyzed. The samples were machined under dry and minimum quantity lubrication (MQL) conditions to obtain different surface roughness. The surface topographies of the processed samples were assessed and measured using an optical profilometer. The produced samples were subjected to electrochemical impedance spectroscopy (EIS) and corrosion potential tests (Ecorr) in the presence of simulated body fluid (SBF). The surface bioactivity of the samples was assessed on the basis of images from scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis. The inspection of the surfaces of samples after turning under dry and MQL conditions revealed unevenly distributed precipitation of hydroxyapatite compounds (Ca/P) with a molar ratio in the range of 1.73–1.97. Regardless of the cutting conditions and surface roughness, the highest values of Ecorr ~0 mV were recorded on day 7 of immersion in the SBF solution. The impedance characteristics showed that, compared to the MQL conditions, surfaces machined under dry conditions were characterized by greater resistance and the presence of a passive layer on the processed surface. The main novelty of the paper is the study of the effect of ecological machining conditions, namely, dry and MQL cutting on the corrosion resistance and surface bioactivity of Ti6Al4V titanium alloy after finish turning. The obtained research results have practical significance. They can be used by engineers during the development of technological processes for medical devices made of Ti6Al4V alloy to obtain favorable functional properties of these devices.

Processing optimization, microstructure, mechanical properties and nanoprecipitation behavior of 18Ni300 maraging steel in selective laser melting

Thanks to the application needs of conformal cooling channels in mold field, selective laser melting (SLM) has become a key technique for fabricating novel mold with high cooling efficiency. This work systematically investigates the action mechanism of SLM processing parameters on densification of 18Ni300 maraging steel, and corresponding phase identification, microstructural evolution, nanoprecipitation behaviors and mechanical properties of as-built samples with optimal relative density (RD) before and after solution-aged treatment. Response surface method and analysis of variance are employed to analyze the significance levels of effects for SLM processing parameters on RD. The optimal processing parameter set and resultant RD of 99.45% as well as corresponding processing windows for SLMed 18Ni300 sample can be obtained. It is found that as-built sample consists of major martensite α phase and scarce austenite γ phase. Solution-aged treatment is in favor of reversion of martensite to austenite and precipitation of intermetallic compounds. Both as-built and solution-aged samples exhibit no obvious texture fiber, but there are different grain sizes and orientations. It is worthy noted that the mechanical properties (ultimate tensile strength of ∼1245 MPa) of SLMed 18Ni300 sample are comparable to that of corresponding wrought sample.

Conditions and processes of precipitation of iron compounds upon discharge of high-mineralized artesian water from artesian borehole R-30, Staro Oryahovo, Bulgaria.

The artesian borehole R-30-Staro Oryahovo with total depth of 1740m gives waters of chlorine-sodium type with a mineralization of 31g/L from formations of Paleogene age. These waters contain high concentrations of iron and because of this, intense precipitation of iron compounds takes place at and around the wellhead. To clarify the ongoing processes, samples of precipitates and overflow water were taken. It was shown that the predominant form of iron in the water before reaching the surface is Fe2+-85-90% and FeCl+-4-11%. The calculated saturation index for a number of Fe2+-containing minerals showed that hydroxides and sulfates of Fe2+ are the possible candidates for sources of iron in the water. The reductive dissolution of wide spread Fe3+-containing minerals (hematite, goethite) was assumed as a possible alternative process releasing Fe2+ into water. At the surface, due to the contacts with the atmospheric oxygen, the environment is sharply changed to oxidizing one thus forming a geochemical barrier and causing mass precipitation of Fe3+ oxides. Simultaneous Fe2+ oxidation, hydrolysis of Fe3+ and co-precipitation with silica and other components are assumed as the major processes causing the formation of low-crystalline Si-containing ferrihydrite or ferrihydrite-like phase. After formation, the precipitated gels due to the processes of aging and continued interaction with the outflowing water have suffered the further change including the formation of goethite. It was shown that due to the intensive precipitation processes, the iron migration to the surface is limited to a very small area and does not affect adjacent agricultural territories with Fe.

Experimental Study on Early Age Characteristics of Lime-GGBS-Treated Gypseous Clays under Wet–Dry Cycles

Gypseous soils are capable of presenting ground construction challenges to civil and geotechnical engineers due to their unpredictable deformation characteristics. These undesirable responses are sometimes caused by environmental changes in moisture content due to temperature variations, fluctuation of underground water table, surface water, and gypsum content. Hence, the adoption of effective and economical means of stabilising gypseous soils is imperative. This study’s focus is on the early age strength and microstructural characteristics of gypseous soils treated with lime and GGBS. Treated and untreated gypseous soils with 5%, 15%, and 25% gypsum content were subjected to wet–dry cycles while investigating unconfined compressive strength (UCS), water absorption, pH, microstructural changes, and swell. The analysis of the results shows that at zero cycle, the UCS of the untreated gypseous soils increases from 0.62 to 0.79 MPa and swell decreases from 69 to 23%, respectively, as gypsum content increases. However, upon subjection to wet–dry cycles, the UCS reduced from 0.16 to 0.08 MPa at the end of the sixth cycle due to dissolution of gypsum within the soil pores which reduced the strength. The result also shows that gypsum content increases water absorption and reduces the pH of the untreated gypseous soils because of the neutral pH of gypsum. Furthermore, lime-GGBS-treated gypseous soils maintained a higher pH after six wet–dry cycles compared to untreated gypseous soils due to the high pH of lime and the increase in calcium content which improved bonding. In addition, microstructural analysis using SEM indicated early age precipitation of cementitious compounds (CSH) for increasing strength of lime-GGBS-treated gypseous soils compared to untreated gypseous soils.

Insight into the inhibitory mechanism of inorganic ligands on the adsorption of tetracycline onto hematite

Inorganic ligands, ubiquitous in the natural environment, can interact with iron oxide minerals. To date, our knowledge regarding the effects of inorganic ligands on the adsorption properties of antibiotics onto iron oxides is still limited. In this work, the influences of different inorganic ligands (chosen iodate, silicate, and phosphate as the model ligands) on the adsorption of tetracycline (TC) onto hematite were examined. Adsorption isotherms indicated that inorganic ligands inhibited TC adsorption. The observations were attributed to the increase of electrostatic repulsion between anionic species (i.e., TC–) and negatively charged hematite particles as well as the competition between TC– species and inorganic ligand anions for the adsorption sites on hematite surfaces. Interestingly, the inhibitory effects of the three inorganic ligands were in the order of phosphate > silicate > iodate; the trend was stemmed from their differences in the binding affinities to hematite and the molecular size. When the background solutions contained divalent cations (e.g., Ca2+), surface precipitation of Ca-inorganic ligand compounds on hematite was another important mechanism for the inhibitory effects. Furthermore, adsorption of TC onto hematite with or without inorganic ligands was strongly affected by solution pH, which was due to the combination of the amphoteric behavior of TC and highly pH-dependent surface charges of the hematite mineral. Current results highlight the critical roles of ubiquitous inorganic ligands in revealing the fate of tetracycline antibiotics in natural systems.

Study on mechanical behavior of Cu-bearing antibacterial titanium alloy implant

Cu-bearing titanium alloy has been proved to have excellent antibacterial properties, which can effectively reduce the incidence of infection caused by implantation. On the other hand, the addition of Cu in the titanium alloy can also significantly improve the strength of the alloy due to the dispersed precipitation of Ti2Cu compounds in matrix, which will enhance the biomechanical safety of Cu-bearing antibacterial titanium alloys. In this study, a Ti6Al4V5Cu antibacterial titanium alloy and the ordinary Ti6Al4V titanium alloy were used to make a variety of orthopedic trauma repair implants, including bone pin, bone screw and bone plate. Through the internationally accepted test methods, the mechanical properties such as tension, torsion and bending of the implants were studied, and the difference between the implants made of two materials was analyzed. The results showed that the tensile strength of Ti6Al4V5Cu pin was 25% higher than that of Ti6Al4V pin, the torsion strength of the screw was increased up to 89%, the static bending load of the plate was increased by 67-89%, and the maximum loading force of the plate after 1 million cycles of four point dynamic bending was increased by 41-91%. The above improvements should be attributed to the strengthening effect of Cu in the antibacterial titanium alloy, indicating that the Cu-bearing antibacterial titanium alloy implant possesses both advantages of reducing the bacterial infection and improving the biomechanical safety, and a broad clinical application prospects.

Application of a hybrid material formed by layered zinc hydroxide chloride modified with spiropyran in the adsorption of Ca2+ from water

The excess of metal ions in water is associated with drawbacks in industrial processes due to the precipitation of insoluble compounds in pipes and cooling systems. Therefore, investigating alternatives for the removal of ions is especially important and can be achieved with the use of adsorbing materials. In this paper, a novel hybrid material formed by zinc hydroxy chloride (ZHC) functionalized with molecules of the spiropyran class is presented as an alternative for the removal of calcium ions from water. Characterization techniques, including XRD, FTIR, and EDS allowed to identify the spiropyran molecules covering the surface of ZHC in the merocyanine form, forming active sites for calcium chemisorption. The material, described as ZHC/MC24, showed good adsorption capacities that reached 19 mg g−1 at pH 8.0. ZHC/MC24 presented a negatively charged surface, according to zeta potential analyses. Shifting the pH to alkaline values increased the negative potential, which reflected in greater calcium adsorptions. The isotherm investigation showed that the adsorption occurs in a monolayer process, and a first-order adsorption model was found to describe the process. Control tests performed with the ZHC without spiropyran showed low adsorption capacities, inferior to 2 mg g−1, evidencing the importance of the functionalization with spiropyran to the proposed application.

Silver leaching from jarosite-type compounds using cyanide and non-cyanide lixiviants: A kinetic approach

Precipitation of jarosite-type compounds (jarosites) is widely used in hydrometallurgical processes to control metal impurities in solution. During this process, economically valuable elements like silver may co-precipitate. Due to the toxicity of cyanide, used for silver leaching, alternative complexing agents are needed. These agents should have low human and environmental toxicity and efficiently extract silver from ores. In this study, thiosulfate (S2O32−) and thiocyanate (SCN−) were used as alternatives to cyanide in silver extraction from jarosites. The silver extraction and leaching rates obtained with these leaching agents were compared with those obtained with cyanide. Sodium jarosite containing silver was synthesized by the hydrothermal method and characterized by different analytical techniques. Experimental data obtained from batch leaching experiments were fitted with the chemical reaction-controlled shrinking core kinetic model based on studies reported in the literature for similar systems. The effects of pH, temperature, concentration, and the type of leaching agent on the leaching rates were studied. It was found that, under the same temperature and pH conditions, the best extraction efficiency of silver with S2O32− and cyanide were 99.6 % and 97.2%, respectively, but there was a noticeable higher consumption of reagent with S2O32−. The best silver extraction with SCN− was only 35.4 %. However, the leaching rates and the kinetic parameters obtained were very similar, indicating that the kinetics of the jarosite dissolution and the silver complexation could be similar, independent of the complexing agent used. The global kinetic models obtained for extraction of silver from jarosites are useful for evaluating the viability of new complexing agents.