Effective Elimination of Hazardous Chromium (VI) Using Periodic Elements and Contemporary Adsorption Methods by Using Magnesium Ferrite Nanoparticle: A Review

A well-known hazardous metal and top contaminant in wastewater is hexavalent chromium. The two forms of most commonly found chromium are chromate ( CrO 4 2− ) and dichromate ( Cr 2 O 7 2− ). Leather tanning, cooling tower blow-down, plating, electroplating, rinse water sources, anodizing baths etc. are the main sources of Cr (VI) contamination. The Cr (VI) is not only non-biodegradable in the environment but also carcinogenic to living population. It is still difficult to treat Cr contaminated waste water effectively, safely, eco-friendly, and economically. As a result, many techniques have been used to treat Cr (VI)-polluted wastewater, including adsorption, chemical precipitation, coagulation, ion-exchange, and filtration. Among these practices, the most practical method is adsorption for the removal of Cr (VI) from aqueous solutions, which has gained widespread acceptance due to the ease of use and affordability of the equipment and adsorbent. It has been revealed that Fe-based adsorbents’ oxides and hydroxides have high adsorptive potential to lower Cr (VI) content below the advised threshold. Fe-based adsorbents were also discovered to be relatively cheap and toxic-free in Cr (VI) treatment. Fe-based adsorbents are commonly utilized in industry. It has been discovered that nanoparticles of Fe-, Ti-, and Cu-based adsorbents have a better capacity to remove Cr (VI). Cr (VI) was effectively removed from contaminated water using mixed element-based adsorbents (Fe-Mn, Fe-Ti, Fe-Cu, Fe-Zr, Fe-Cu-Y, Fe-Mg, etc.). Initial findings suggest that Cr (VI) removal from wastewater may be accomplished by using magnesium ferrite nanomaterials as an efficient adsorbent.

Green-synthesized, biochar-supported nZVI from mango kernel residue for aqueous hexavalent chromium removal: Performance, mechanism and regeneration

A biochar-supported green nZVI(G-nZVI@MKB)composite was synthesized using mango kernel waste with“dual identity”as reductant and biomass of biochar.The G-nZVI@MKB with a Fe/C mass ratio of 2.0(G-nZVI@MKB2)was determined as the most favorable composite for hexavalent chromium(Cr(VI))removal.Distinct influencing parameters were discussed,and 99.0%of Cr(VI)removal occurred within 360 min under these optimized parameters.Pseudo-second order kinetic model and intra-particle diffusion model well depicted Cr(VI)removal process.The XRD,FTIR,SEM,and XPS analyses verified the key roles of G-nZVI and functional groups,as well as the primary removal mechanisms involving electrostatic attraction,reduction,and complexation.G-nZVI@MKB2 exhibited good stability and reusability with only a 16.4%decline in Cr(VI)removal after five cycles.This study offered evidence that mango kernel could be recycled as a beneficial resource to synthesize green nZVI-loaded biochar composite for efficient Cr(VI)elimination from water.

Enhanced visible-light-driven reduction of hexavalent chromium under neutral conditions using quasi-MOF photocatalysts via thiocyanate modulation

In order to address the issue that the photocatalytic reduction of hexavalent chromium(Cr(VI))is often limited by the inefficient utilization of electrons in photocatalysts,a quasi-MOF photocatalyst using thiocyanate(-SCN)was developed as a modulator to enhance the charge transfer properties of ZIF-L-based photocatalysts.The incorporation of-SCN introduced structural defects,which improved visible light absorption and the reduction ability of photogenerated electrons.-SCN significantly adjusted the electronic properties and established a stable electron release pathway,serving as active sites for reduction.The optimized quasi-MOF demonstrated a Cr(VI)reduction rate of 94.8%in neutral potassium thiocyanate solution under visible light without a hole scavenger.The reaction rate constant is 2.8 times that of the photocatalyst without defect modulation.This study offers a promising strategy for developing highly efficient photocatalysts for environmental remediation.

Hexavalent Chromium Cr (VI) Removal from Water by Mango Kernel Powder

Metal trace elements (MTE) are among the most harmful micropollutants of natural waters. Eliminating them helps improve the quality and safety of drinking water and protect human health. In this work, we used mango kernel powder (MKP) as bioadsorbent material for removal of Cr (VI) from water. Uv-visible spectroscopy was used to monitor and quantify Cr (VI) during processing using the Beer-Lambert formula. Some parameters such as pH, mango powder, mass and contact time were optimized to determine adsorption capacity and chromium removal rate. Adsorption kinetics, equilibrium, isotherms and thermodynamic parameters such as ΔG˚, ΔH˚, and ΔS˚, as well as FTIR were studied to better understand the Cr (VI) removal process by MKP. The adsorption capacity reached 94.87 mg/g, for an optimal contact time of 30 min at 298 K. The obtained results are in accordance with a pseudo-second order Freundlich adsorption isotherm model. Finally FTIR was used to monitor the evolution of absorption bands, while Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to evaluate surface properties and morphology of the adsorbent.

Migration and transfer of chromium in soil-vegetable system and associated health risks in vicinity of ferro-alloy manufactory

Study was carried out to analyze the distribution and migration patterns,soil-to-plant transfer and potential health risks of chromium in soil-vegetable system in areas near a ferro-alloy manufactory in Hunan province.The results show that soils near sewer outlet,sewer channel and in control area are averaged 2 239.5,995.33 and 104.9 mg/kg,respectively.The total Cr has a relative accumulation in soil depth of 200-400 mm near the sewer outlet,mainly enriches in the surface layer（0-200 mm） near the sewer channel and decreases gradually in unpolluted soils.The differential concentration level of enrichment between layers is little.The results also indicate that the three vegetables of celery,lettuce and Chinese cabbage are able to convert the potentially toxic Cr（Ⅵ） species into the non-toxic Cr（Ⅲ） species,and the chromium contents in the edible parts of the vegetables are averaged 11.95 mg/kg.The transfer factors of the three vegetables follow the order：Chinese cabbage lettuce celery.The estimated total daily intake of chromium substantially exceeds the dietary allowable value,which may pose health risks to local population.

Behaviors of vanadium and chromium in coal-based direct reduction of high-chromium vanadium-bearing titanomagnetite concentrates followed by magnetic separation

The reduction behaviors of FeO·V2O3 and FeO·Cr2O3 during coal-based direct reduction have a decisive impact on the efficient utilization of high-chromium vanadium-bearing titanomagnetite concentrates. The effects of molar ratio of C to Fe n(C)/n(Fe) and temperature on the behaviors of vanadium and chromium during direct reduction and magnetic separation were investigated. The reduced samples were characterized by X-ray diffraction(XRD), scanning election microscopy(SEM) and energy dispersive spectrometry(EDS) techniques. Experimental results indicate that the recoveries of vanadium and chromium rapidly increase from 10.0% and 9.6% to 45.3% and 74.3%, respectively, as the n(C)/n(Fe) increases from 0.8 to 1.4. At n(C)/n(Fe) of 0.8, the recoveries of vanadium and chromium are always lower than 10.0% in the whole temperature range of 1100-1250 °C. However, at n(C)/n(Fe) of 1.2, the recoveries of vanadium and chromium considerably increase from 17.8% and 33.8% to 42.4% and 76.0%, respectively, as the temperature increases from 1100 °C to 1250 °C. At n(C)/n(Fe) lower than 0.8, most of the FeO·V2O3 and FeO·Cr2O3 are not reduced to carbides because of the lack of carbonaceous reductants, and the temperature has little effect on the reduction behaviors of FeO·V2O3 and FeO·Cr2O3, resulting in very low recoveries of vanadium and chromium during magnetic separation. However, at higher n(C)/n(Fe), the reduction rates of FeO·V2O3 and FeO·Cr2O3 increase significatly because of the excess amount of carbonaceous reductants. Moreover, higher temperatures largely induce the reduction of FeO·V2O3 and FeO·Cr2O3 to carbides. The newly formed carbides are then dissolved in the γ(FCC) phase, and recovered accompanied with the metallic iron during magnetic separation.

海藻酸钠基凝胶吸附材料对Cr(VI)吸附性能研究

海藻酸钠(SA)是一种天然生物质材料,其来源广、价格低,聚乙烯亚胺是一种含有氨基(-NH_(2))的水溶性高分子聚合物,螯合重金属离子能力强,众多研究者将二者结合其制备成吸附剂去除水溶液中的金属离子。本研究以海藻酸钠为基体,加入碳酸钙和聚乙烯亚胺,以戊二醛为交联剂,经冷冻干燥后形成吸附较好的海藻酸钠-聚乙烯亚胺吸附材料。采用吸附实验研究了材料对水中Cr(VI)的吸附性能,并研究初始浓度、溶液pH、吸附时间、吸附剂用量等对吸附效果的影响。实验结果表明:在Cr(VI)溶液初始浓度为15 mg/L、吸附剂用量0.4 g、吸附时间200 min,吸附温度为313 K时,Cr(VI)去除率达90%以上。

Response of soil catalase activity to chromium contamination

The impact of chromium（Ⅲ） and （Ⅵ） forms on soil catalase activity was presented. The Orthic Podzol, Haplic Phaeozem and Mollic Gleysol from different depths were used in the experiment. The soil samples were amended with solution of Cr（Ⅲ） using CrCl3, and with Cr（Ⅵ） using K2Cr2O7 in the concentration range from 0 to 20 mg/kg, whereas the samples without the addition of chromium served as control. Catalase activity was assayed by one of the commonly used spectrophotometric methods. As it was demonstrated in the experiment, both Cr（Ⅲ） and Cr（Ⅵ） have an ability to reduce soil catalase activity. A chromium dosage of 20 mg/kg caused the inhibition of catalase activity and the corresponding contamination levels ranged from 75% to 92% for Cr（Ⅲ） and 68% to 76% for Cr（Ⅵ）, with relation to the control. Catalase activity reached maximum in the soil material from surface layers （0-25 cm）, typically characterized by the highest content of organic matter creating favorable conditions for microorganisms.

A study on the adsorption of chromium on laterite from Guizhou Province,China

The adsorption behaviors of Cr(VI) on laterite from Guizhou Province were studied in this paper, and the adsorption mechanism was discussed as well. Results showed that different mineral compositions in the laterite would cause differences in the capacity of laterite to absorb Cr(VI). Gibbsite, iron oxide minerals and non-crystalloids are the main contributors to enhancing the capacity of laterite to absorb Cr(VI). The pH of the solution is an important factor affecting the adsorption of Cr(VI) on laterite. Acidic environment (pH=2-5) is favorable to the adsorption of Cr(VI). The amount of adsorbed Cr(VI) decreases with increasing pH of the solution. With increasing initial concentrations of Cr(VI), the amount of adsorbed Cr(VI) increases first, and then decreases. The optimal adsorption concentration of Cr(VI) on laterite is 250 μg/mL. The adsorption of Cr(VI) on laterite is a rapid process, about 80% Cr(VI) will be adsorbed within 2 hours. And the adsorption of Cr(VI) on kaolinite is a slow process.

Effect of Cr2O3 addition on oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets with simulated coke oven gas

The oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets(CVTP)with Cr2O3 addition were studied,and the reduction swelling index(RSI)and compressive strength(CS)of the reduced CVTP with simulated coke oven gas(COG)injection were investigated.The results showed that the CS of the CVTP decreases and the porosity of the CVTP increases with increasing amount of Cr2O3 added.The Cr2O3 mainly exists in the form of(Cr,Fe)2O3 solid solution in the CVTP and as Fe-Cr in the reduced CVTP.The CS of the reduced CVTP increases and the RSI of the reduced CVTP decreases with increasing amount of Cr2O3 added.The limited aggregation and diffusion of metallic iron contribute to the formation of dense lamellar crystals,which leads to the slight decrease for reduction swelling behavior of reduced CVTP.This work provides a theoretical and technical basis for the utilization of CVTP and other Cr-bearing ores such as chromite with COG recycling technology.

Effects of chromium on the corrosion and electrochemical behaviors of ultra high strength steels

The effects of chromium on the corrosion and the electrochemical behaviors of ultra high strength steels were studied by the salt spray test and electrochemical methods. The results show that ultra high strength steels remain martensite structures and have anodic dissolution characteristic with an increase of chromium content. There is no typical passive region on the polarization curves of an ultra high strength stainless steel, AerMet 100 steel, and 300M steel. However, chromium improves the corrosion resistance of the stainless steel remarkably. It has the slowest corrosion rate in the salt spray test, one order of magnitude less than that of AerMet 100 and 300M steels. With the increase of chromium content, the polarization resistance becomes larger, the corrosion potential shifts towards the positive direction with a value of 545 mV, and the corrosion current density decreases in electrochemical measures in 3.5wt% NaCl solutions. Because of the higher content of chromium, the ultra high strength stainless steel has a better corrosion resistance than AerMet 100 and 300M steels.

Sintering behaviors and consolidation mechanism of high-chromium vanadium and titanium magnetite fines

To achieve high efficiency utilization of high-chromium vanadium-titanium magnetite （V-Ti-Cr） fines, an investigation of V Ti42r fines was conducted using a sinter pot. The chemical composition, particle parameters, and granulation of V-Ti-Cr mixtures were analyzed, and the effects of sintering parameters on the sintering behaviors were investigated. The results indicated that the optimum quicklime dosage, mixture moisture, wetting time, and granulation time for V-Ti-Cr fines are 5wt%, 7.5wt%, 10 min, and 5-8 min, respectively. Meanwhile, the vertical sintering speed, yield, tumbler strength, and productivity gains were shown to be 21.28 mm/min, 60.50wt% , 58.26wt%, and 1.36 t·m^-2·h^-1, respectively. Furthermore, the consolidation mechanism of V-Ti-Cr fines was clarified, revealing that the consolidation of a V-Ti-Cr sinter requires an approximately 14vo1% calcium ferrite liquid-state, an approximately 15vo1% silicate liq- uid-state, a solid-state reaction, and the recrystallization of magnetite. Compared to an ordinary sinter, calcium ferrite content in a V-Ti-Cr sinter is lower, while the perovskite content is higher, possibly resulting in unsatisfactory sinter outcomes.

Alleviation of Chromium Toxicity by Silicon Addition in Rice Plants

The alleviatory effect of silicon （Si） on chromium （Cr） toxicity to rice plants was investigated using a hydroponic experiment with two Cr levels （0 and 100μmol L-1）,three Si levels （0,1.25,and 2.5 mmol L-1） and two rice genotypes,differing in grain Cr accumulation （Dan K5,high accumulation and Xiushui 113,low accumulation）.The results showed that 100μmol L-1 Cr treatment caused a marked reduction of seedling height,dry biomass,soluble protein content,and root antioxidant enzyme activity,whereas significantly increased Cr concentration and TBARS （thiobarbituric acid reactive substances） content.However,the reductions of seedling height,dry biomass,and soluble content were greatly alleviated due to Si addition to the culture solution.Compared with the plants treated with Cr alone,Si addition markedly reduced Cr uptake and translocation in rice plants.No significant differences were observed between the two Si treatments （1.25 and 2.5 mmol L-1） in shoot Cr concentration and Cr translocation factor.Under the treatment of 100μmol L-1 Cr＋2.5 mmol L-1 Si,higher root Cr concentration but lower shoot Cr concentration and Cr translocation factor were observed in Dan K5 than those in Xiushui 113,indicating that the beneficial effect of Si on inhibiting Cr translocation was more pronounced in Dan K5 than in Xiushui 113.Si addition also alleviated the reduction of antioxidative enzymes （superoxide dismutase （SOD） and ascorbate peroxidase （APX） in leaves;catalase （CAT） and APX in roots） and the increase of TBARS content in the Cr-stressed plants.Furthermore,the beneficial effects of Si on activities of antioxidative enzymes under Cr stress were genotype-dependent.The highest activities of SOD,POD （guaiacol peroxidase）,CAT,and APX in leaves occurred in the treatment of 100μmol L-1 Cr＋2.5 mmol L-1 Si for Xiushui 113 and in the treatment of 100μmol L-1 Cr＋1.25 mmol L-1 Si for Dan K5.The beneficial effect of Si on alleviating oxidative stress was much more pronounced in Dan K5 than in Xiushui 113.It may be concluded that Si alleviates Cr toxicity mainly through inhibiting the uptake and translocation of Cr and enhancing the capacity of defense against oxidative stress induced by Cr toxicity.

EFFECT OF CHROMIUM AND BORON ON ENVIRONMENTAL EMBRITTLEMENT OF FORGED Fe_3Al

The effects of addition of chromium or boron on room temperature tensile properties,fracture behavior and susceptibility to test environments (air vs. vacuuwi of forged Fe3Al have been investigated. The results indicated that both chromium and boron result in increasing room temperuture ductility and fracture strength of the Fe3Al alloy whether tested in air or in vacuum. The susceptibility to test envimnment was described with the embrittlement index I: I=(δv-δA)/δv. The embrittlement indexes, for Fe-30Al, Fe-30Al-4Cr and Fe-30Al-0. 13B (at. %), are 24%, 37% and 29%,respectively. Scanning electron microscope examination of fracture surface revealed that the fracture mode of the three alloys remains unchanged, and all of them exhibited a transgranular cleavage fracture mode when tested in vacuum or air.

Oxidative stress related enzymes in response to chromium(VI) toxicity in Oxya chinensis (Orthoptera: Acridoidae)

The toxic effects of Cr（Vl） on antioxidant enzymes of Oxya chinensis（Orthoptera： Acridoidae） were determined. Changes in the activities of the antioxidant enzymes superoxide dismutase（SOD）, catalase（CAT）, and guaiacol peroxidase（GPx） were measured in O. chinensis insects injected with Cr（VI）. Fifth-nymphs of O. chinensis insects were injected with Cr（VI） with different concentrations （0, 75, 150, 225, 300, 375, 450 mg/kg of body weight）. The results showed that Cr（VI） led to the change of SOD, CAT, and GPx activities at different concentrations, which revealed that： （1） The oxidative stress of SOD increased with the increase of Cr （VI） concentration. （2） With the increase of Cr （VI） concentrations, CAT activities for females increased at lower concentrations, but decreased at higher concentration range, which indicated that antioxidant system of O. chinensis was not influenced by the presence of Cr （VI）. A very similar response to Cr（VI） effect for males indicated that Cr（VI） concentrations were not high enough to damage O. chinensis in terms of CAT. （3） The GPx activity for females increased in all treatments, which revealed that the damage power of Cr（VI） was increased with the increase of Cr（Vi） concentrations in terms of GPx, but the effect was not so remarkable. There was not a consistent trend of GPx activities for males in all treatments of Cr（VI）. Cr（VI）-induced changes in antioxidant enzymes were different for SOD, CAT and GPx, of which the tendency was that activities generally changed with increase of concentrations of Cr（VI） suggesting SOD, CAT, and GPx could serve as indices of oxidative stress to some extent.

Corrosion Behaviors of P110 Steel and Chromium Coating in CO_2-saturated Simulated Oilfield Brine

The protective chromium coating was prepared on P110 steel by employing pack cementation.The corrosion behaviors of P110 steel and the obtained coating in CO2-saturated simulated oilfield brine were studied by static complete immersion tests and electrochemical measurements.The corrosion attacks of the samples were determined by mass loss,corroded surface morphologies,corrosion products,and results of electrochemical measurements.The experimental results showed that the coating was uniform,continuous and compact.The chromium coating was slightly corroded,and the mass loss and corrosion rate of the coating were far lower than those of P110 steel.Chromium coating has higher self-corroding potential and lower corrosion current density than P110 steel in accordance with the electrochemical tests results.Taken as a whole,chromizing treatment has significantly improved the corrosion resistance of P110 steel.

应用型背景下标志与VI设计课程实施课程思政的路径研究

全面推进课程思政建设是落实立德树人根本任务的战略举措。为适应新时代对课程思政高质量发展的要求,该文基于高校“三全育人”工作需求,在应用型背景下,探索课程思政元素有机融入艺术设计类专业课程的路径。针对标志与VI设计课程,首先根据神经语言程序学者罗伯特·迪尔茨关于人的认知的六个层次理论,设计课程思政元素体系,进而通过详细解读和挖掘六种层次描述课程思政元素体系的设计思路,最后提出从五个讲授部分和三个实践部分,在课程实施过程中融入课程思政元素的具体策略和路径,以期促进艺术设计类专业教学与课程思政有机融合,充分发挥课程思政在高校艺术设计类专业课程中的育人作用,为标志与VI设计课程的思政教学设计与教学改革提供参考。

Catalysis of Dissolved and Adsorbed Iron in Soil Suspension for Chromium(VI) Reduction by Sulfide

The kinetics of Cr（Ⅵ） reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe（Ⅱ） concentrations was examined using batch anaeroblc experimental systems at constant temperature. The results showed that the reaction rate of Cr（Ⅵ） reduction was in the order of red soil 〈 yellow-brown soil 〈 chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr（Ⅵ） reduction. The reaction involved in the Cr（Ⅵ） reduction by Fe（Ⅱ） to produce Fe（ⅡI）, which was reduced to Fe（Ⅱ） again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr（Ⅵ） was reduced. The catalysis occurred because the one-step reduction of Cr（Ⅵ） by sulfide was slower than the two-step process consisting of rapid Cr（Ⅵ） reduction by Fe（Ⅱ） followed by Fe（Ⅲ） reduction by sulfide. In essence, Fe（Ⅱ）/Fe（Ⅲ） species shuttle electrons from sulfide to Cr（Ⅵ）, facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe（Ⅱ）-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr（Ⅵ） concentration. The plots of In e[Cr（Ⅵ）] versus reaction time were linear up to approximately 70% of Cr（Ⅵ） reduction, suggesting a first-order reaction kinetics with respect to Cr（Ⅵ）. Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr（Ⅵ） reduction at a later stage of the reaction, resulting in deviation from linearity for the In c[Cr（Ⅵ）] versus time plots.

Chromium does not belong in the diabetes treatment arsenal: Current evidence and future perspectives

Chromium is considered to have positive effects on insulin sensitivity and is marketed as an adjunctive therapy for inducing glucose tolerance in cases of insulin resistance("the glucose tolerance factor"). Case reports on patients who received prolonged parenteral nutrition indeed showed that the absence of trivalent chromium caused insulin resistance and diabetes. However, whether patients with type 2 diabetes can develop a clinically relevant chromium deficiency is unclear. This review summarizes the available evidence regarding the potential effectiveness of chromium supplementation on glycemic control(Hemoglobin A1c levels) in patients with type 2 diabetes. No studies investigating the longterm safety of chromium in humans were found. All clinical trials that have been performed had a relative short follow-up period. None of the trials investigated whether the patients had risk factors for chromium deficiency.The evidence from randomized trials in patients with type 2 diabetes demonstrated that chromium supplementation does not effectively improve glycemic control. The meta-analyses showed that chromium supplementation did not improve fasting plasma glucose levels. Moreover, there were no clinically relevant chromium effects on body weight in individuals with or without diabetes. Future studies should focus on reliable methods to estimate chromium status to identify patients at risk for pathological alterations in their metabolism associated with chromium deficiency. Given the present data, there is no evidence that supports advising patients with type 2 diabetes to take chromium supplements.

Catalysis of Manganese(II) on Chromium(VI) Reduction by Citrate

The catalysis of manganese（Ⅱ） （Mn^2＋） on chromium（Ⅵ） （Cr^6＋） reduction by citrate was studied through batch experiments with the concentration of citrate greatly in excess of Cr^6＋ at 25 ℃ and in pH ranges of 4.0 go 5.0. Results showed that at pH 4.5 within 22 h direct reduction of Cr^6＋ by citrate was not observed, bug for the same time when Mn^2＋ （50 to 200 μmol L^-1） was added, nearly all Cr^6＋ was reduced, with the higher initial Mn^2＋ concentration having faster Cr^6＋ reduction. In the initial stage of the reaction, the Cr^6＋ reduction could be described with a pseudo-first-order kinetics equation. In the lager stage of the reaction, plots of lnc（Cr^6＋） versus t, where c（Cr^6＋） is the Cr^6＋ concentration in the reaction and t is the reaction time, deviated from the initial linear trend. The deviations suggested that the pseudo-first-order kinetics did not apply go the whole experimental period and that some reaction intermediates could have greatly accelerated Cr^6＋ reduction by citrate. The catalysis of the intermediates increased with the reaction time and gradually reached stability. Then, the plot of lnc（Cr^6＋） versus t in the presence of Mn^2＋ was linear again, with the rate constant increasing by 102 times compared with the absence of Mn^2＋. Complexation between Mn^2＋ and citrate was likely a prerequisite for the catalysis of Mn^2＋ on the reaction. Additional experiments showed that introducing eghylenediaminegegraacegic acid （EDTA） into the reaction system strongly suppressed the catalysis of Mn^2＋.