Influence factors for the oxidation of pyrite by oxygen and birnessite in aqueous systems

The oxidation of exposed pyrite causes acid mine drainage, soil acidification, and the release of toxic metal ions. As the important abiotic oxidants in supergene environments, oxygen and manganese oxides participate in the oxidation of pyrite. In this work, the oxidation processes of natural pyrite by oxygen and birnessite were studied in simulated systems, and the influence of pH, Fe(II) and Cr(III) on the intermediates and redox rate was investigated. SO₄^2 − and elemental S were formed as the major and minor products, respectively, during the oxidation processes. Ferric (hydr) oxides including Fe(OH)₃ and goethite were formed with low degree of crystallinity. Low pH and long-term reaction facilitated the formation of goethite and ferric hydroxide, respectively. The rate of pyrite oxidation by birnessite was enhanced in the presence of air (oxygen), and Fe(II) ions played a key role in the redox process. The addition of Fe(II) ions to the reaction system significantly enhanced the oxidation rate of pyrite; however, the presence of Cr(III) ions remarkably decreased the pyrite oxidation rate in aqueous systems. The introduction of Fe(II) ions to form a Fe(III)/Fe(II) redox couple facilitated the electron transfer and accelerated the oxidation rate of pyrite. The present work suggests that isolation from air and decreasing the concentration of Fe(II) ions in aqueous solutions might be effective strategies to reduce the oxidation rate of pyrite in mining soils.     

Phase transformation of Cr(VI)-adsorbed ferrihydrite in the presence of Mn(II): Fate of Mn(II) and Cr(VI)

Ferrihydrite is an important sink for the toxic heavy metal ions, such as Cr(VI). As ferrihydrite is thermodynamically unstable and gradually transforms into hematite and goethite, the stability of Cr(VI)-adsorbed ferrihydrite is environmentally significant. This study investigated the phase transformation of Cr(VI)-adsorbed ferrihydrite at different pH in the presence of aqueous Mn(II), as well as the fate of Mn(II) and Cr(VI) in the transformation process of ferrihydrite. Among the ferrihydrite transformation products, hematite was dominant, and goethite was minor. The pre-adsorbed Cr(VI) inhibited the conversion of ferrihydrite to goethite at initial pH 3.0, whereas little amount of adsorbed Mn(II) favored the formation of goethite at initial pH 7.0. After the aging process, Cr species in solid phase existed primarily as Cr(III) in the presence of Mn(II) at initial pH 7.0 and 11.0. The aqueous Mn concentration was predominantly unchanged at initial pH 3.0, whereas the aqueous Mn(II) was adsorbed onto ferrihydrite or form Mn(OH)₂ precipitates at initial pH 7.0 and 11.0, promoting the immobilization of Cr(VI). Moreover, the oxidation of Mn(II) occurred at initial pH 7.0 and 11.0, forming Mn(III/IV) (hydr)oxides.     

化学合成及废物基锰氧化物对不同形态砷的去除转化机制

为探究不同类型锰氧化物与As（砷）的交互作用,采用沈阳某饮用水处理厂除锰滤池运行周期结束后产生的废料（滤料和管道沉淀物）和化学合成水羟锰矿、酸性水钠锰矿作为不同类型的锰氧化物,研究化学合成及废物基锰氧化物对不同形态As的去除转化机制.结果表明:锰氧化物的投加量为0.20 g时,管道沉淀物对As（Ⅴ）的吸附量为90 μg,远小于水羟锰矿、酸性水钠锰矿和滤料的191、192、176 μg吸附量,而且管道沉淀物2 h内不能够完全氧化溶液中As（Ⅲ）.四种锰氧化物对As的吸附都更符合准二级动力学方程,说明去除过程是一个快速吸附和慢速吸附相互叠加的过程,包含外部液膜扩散、表面吸附和颗粒内扩散等.水羟锰矿、酸性水钠锰矿和滤料有较好锰氧化物晶型,管道沉淀中SiO₂是主要晶型,滤料晶型和酸性水钠锰矿一致,但是形貌结构却差别很大.四种锰氧化物的Mn平均氧化度较高,分别为3.90、3.89、3.84和3.71.研究显示,滤料作为废物基材料可以替代化学合成的水羟锰矿、酸性水钠锰矿实现对水体中As的去除,达到废物材料的再利用及含As水体修复的协同处置.      

Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis

The effect of crystal structure on photocatalytic activities of manganese oxides and underlying reaction mechanism were investigated.     

Photoelectrochemical performance of birnessite films and photoelectrocatalytic activity toward oxidation of phenol

Birnessite films on fluorine-doped tin oxide(FTO) coated glass were prepared by cathodic reduction of aqueous KMnO_4. The deposited birnessite films were characterized with X-ray diffraction, Raman spectroscopy, scanning electron microscopy and atomic force microscopy.The photoelectrochemical activity of birnessite films was investigated and a remarkable photocurrent in response to visible light was observed in the presence of phenol, resulting from localized manganese d–d transitions. Based on this result, the photoelectrocatalytic oxidation of phenol was investigated. Compared with phenol degradation by the electrochemical oxidation process or photocatalysis separately, a synergetic photoelectrocatalytic degradation effect was observed in the presence of the birnessite film coated FTO electrode.Photoelectrocatalytic degradation ratios were influenced by film thickness and initial phenol concentrations. Phenol degradation with the thinnest birnessite film and initial phenol concentration of 10 mg/L showed the highest efficiency of 91.4% after 8 hr. Meanwhile, the kinetics of phenol removal was fit well by the pseudofirst-order kinetic model.     

Simultaneous removal of Cu(II) and Cr(VI) by Mg–Al–Cl layered double hydroxide and mechanism insight

Mg–Al–Cl layered double hydroxide (Cl-LDH) was prepared to simultaneously remove Cu(II) and Cr(VI) from aqueous solution. The coexisting Cu(II) (20 mg/L) and Cr(VI) (40 mg/L) were completely removed within 30 min by Cl-LDH in a dosage of 2.0 g/L; the removal rate of Cu(II) was accelerated in the presence of Cr(VI). Moreover, compared with the adsorption of single Cu(II) or Cr(VI), the adsorption capacities of Cl-LDH for Cu(II) and Cr(VI) can be improved by 81.05% and 49.56%, respectively, in the case of coexisting Cu(II) (200 mg/L) and Cr(VI) (400 mg/L). The affecting factors (such as solution initial pH, adsorbent dosage, and contact time) have been systematically investigated. Besides, the changes of pH values and the concentrations of Mg²⁺ and Al³⁺ in relevant solutions were monitored. To get the underlying mechanism, the Cl-LDH samples before and after adsorption were thoroughly characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. On the basis of these analyses, a possible mechanism was proposed. The coadsorption process involves anion exchange of Cr(VI) with Cl⁻ in Cl-LDH interlayer, isomorphic substitution of Mg²⁺ with Cu²⁺, formation of Cu₂Cl(OH)₃ precipitation, and the adsorption of Cr(VI) by Cu₂Cl(OH)₃. This work provides a new insight into simultaneous removal of heavy metal cations and anions from wastewater by Cl-LDH.     

Synthesis, characterization and application of Lagerstroemia speciosa embedded magnetic nanoparticle for Cr(VI) adsorption from aqueous solution

Lagerstroemia speciosa bark (LB) embedded magnetic nanoparticles were prepared by co-precipitation of Fe²⁺ and Fe³⁺ salt solution with ammonia and LB for Cr(VI) removal from aqueous solution. The native LB, magnetic nanoparticle (MNP), L. speciosa embedded magnetic nanoparticle (MNPLB) and Cr(VI) adsorbed MNPLB particles were characterized by SEM–EDX, TEM, BET-surface area, FT-IR, XRD and TGA methods. TEM analysis confirmed nearly spherical shape of MNP with an average diameter of 8.76 nm and the surface modification did not result in the phase change of MNP as established by XRD analysis, while led to the formation of secondary particles of MNPLB with diameter of 18.54 nm. Characterization results revealed covalent binding between the hydroxyl group of MNP and carboxyl group of LB particles and further confirmed its physico-chemical nature favorable for Cr(VI) adsorption. The Cr(VI) adsorption on to MNPLB particle as an adsorbent was tested under different contact time, initial Cr(VI) concentration, adsorbent dose, initial pH, temperature and agitation speed. The results of the equilibrium and kinetics of adsorption were well described by Langmuir isotherm and pseudo-second-order model, respectively. The thermodynamic parameters suggest spontaneous and endothermic nature of Cr(VI) adsorption onto MNPLB. The maximum adsorption capacity for MNPLB was calculated to be 434.78 mg/g and these particles even after Cr(VI) adsorption were collected effortlessly from the aqueous solution by a magnet. The desorption of Cr(VI)-adsorbed MNPLB was found to be more than 93.72% with spent MNPLB depicting eleven successive adsorption–desorption cycles.     

Synthesis of a novel illite@carbon nanocomposite adsorbent for removal of Cr(VI) from wastewater

A novel illite@carbon (I@C) nanocomposite adsorbent has been synthesized via a facile hydrothermal carbonization process (HTC) using glucose as carbonaceous source and illite as the carrier. The morphology, microstructure and surface properties of the prepared nanocomposite adsorbent were analyzed by FESEM, TGA, XRD, FT-IR and Zeta potential measurements. Batch experiments were carried out on the adsorption of Cr(VI) to determine the adsorption properties of the composite. The adsorption of Cr(VI) onto the I@C nanocomposite was well described by the pseudo-second-order kinetic model and Langmuir isotherm. Compared with the illite and carbon material (SC) separately, the prepared I@C nanocomposite adsorbent exhibited enhanced adsorption performance for Cr(VI) with a maximum adsorption capacity of 149.25 mg/g, which was higher than that of most reported adsorbents. In addition, the adsorption process was spontaneous and endothermic based on the adsorption thermodynamics study. The adsorption of Cr(VI) by I@C was highly pH-dependent and the optimum adsorption occurred at pH 2.0. The Zeta potential analysis results indicated that the electrostatic interactions between anionic Cr(VI) and the positively charged surface of the adsorbent might be critical to the adsorption mechanism. This study demonstrated that the I@C nanocomposite should be a promising candidate for a low-cost, environmental friendly and highly efficient adsorbent for the removal of toxic Cr(VI) from wastewater.     

掺钴水钠锰矿对铅的吸附及对砷的氧化

为了研究过渡金属离子掺杂对锰氧化物作为环境友好材料物理化学性质的影响,在浓盐酸还原高锰酸钾制备水钠锰矿过程中添加氯化钴,在常压回流条件下一步合成了三维纳米微球状水钠锰矿,考察了其铅吸附和砷氧化能力.应用粉晶X射线衍射、化学分析、N2物理吸附、场发射扫描电镜（FE-SEM）、X射线光电子能谱（XPS）等手段表征产物晶体结...     

铁氧化物改性黏土对Cr(Ⅵ)的吸附性能研究

利用铁氧化物对黏土进行了包覆改性,采用静态吸附法对该改性黏土吸附Cr(Ⅵ)的特性进行了研究,并考虑了反应时间、pH、浓度、温度对吸附的影响。实验结果表明,整个吸附过程基本在3h内完成;改性黏土对Cr(Ⅵ)的吸附量随溶液初始pH的增大而明显减小;随着溶液初始浓度和温度的增大而增大。同时,在初始pH为3.0的条件下,研究了改性黏土吸附Cr(Ⅵ)的动力学和热力学特性,结果表明,改性黏土对Cr(Ⅵ)吸附能较好地符合准二级动力学方程和Langmuir等温式。由Langmuir等温式得出,在293K、初始pH为3.0条件下的单层饱和吸附量为12.91 mg•g-1。确定了改性黏土吸附Cr(Ⅵ)的热力学参数,表明该吸附过程是一个吸热的自发过程。与原土的对比实验表明,改性黏土对Cr(Ⅵ)的吸附能力大大增强。     

Removal of hexavalent chromium from contaminated waters by ultrasound-assisted aqueous solution ball milling

Batch mode experiments were conducted to study the removal of hexavalent chromium(Cr(Ⅵ)) from aqueous solutions using ultrasound-assisted aqueous solution ball milling.The results show that the reduction rate of Cr(Ⅵ) by ultrasound-assisted aqueous solution ball milling was significantly faster than that by ball milling or ultrasound treatment alone,and an initial Cr(Ⅵ) concentration of 166 mg/L could be decreased to 0.35 mg/L at 120 min.The decisive factors, including initial concentration of Cr(Ⅵ), p H value, ultrasonic frequency and filling gas, were studied. It was found that the optimal ultrasonic frequency for ultrasound-assisted aqueous solution ball milling device was 20 k Hz, and the rate of Cr(Ⅵ)reduction as a function of filling gas followed the order: Ar air N_2 O_2. Samples were characterized by X-ray diffraction, fluorescence measurements, atomic absorption and the diphenylcarbazide colorimetric method. The Cr(Ⅵ) transformed into a precipitate that could be removed from the contaminated water, after which the water could be reused.     

Al(III)对糖浆溶液中有机还原物质还原六价铬的影响

通过批实验研究了Al(III)对糖浆溶液化学还原六价铬反应的影响,揭示了不同条件下Al(III)对六价铬还原反应动力学的影响.结果表明:Al(III)能够促进糖浆溶液还原六价铬反应进行;其作用机制是Al(III)与糖浆溶液中有机还原物质及Cr(VI)反应形成三者的络合物,降低糖浆中多酚等有机还原物质还原Cr(VI)的反应活化能,提高六价铬还原反应速率.Al(III)存在时,该六价铬还原反应符合准一级动力学反应; pH 2.0,2.5,3.0,3.5时,添加Al(III)的实验组中六价铬反应速率常数比对应的空白对照组中反应速率常数分别增加了0.0251,0.0139,0.0058, 0.0048h-1.添加Al(III)前后反应体系中六价铬还原的反应活化能(Eа)分别为66.38,62.80kJ/mol.当糖浆浓度不足时,Al(III)能够提高糖浆溶液还原六价铬的反应去除率.     

水钠锰矿的Mn（Ⅲ）分布及Na4P2O7溶液处理对其铅吸附性能的影响

以深入理解水钠锰矿结构中低价锰离子的含量及分布对其铅吸附性能的影响为目的,通过不同pH（pH为2、4、5）的焦磷酸钠溶液络合浸提酸性水钠锰矿结构中的Mn（Ⅱ）和Mn（Ⅲ）,研究了浸提锰离子的形态、含量、矿物的锰平均氧化度和Pb2＋的吸附量及其伴随Mn2＋、H＋释放量的变化.结果表明,锰平均氧化度为3.670的酸性水钠锰...     

Arsenite oxidation by three types of manganese oxides

Oxidation of As（Ⅲ） by three types of manganese oxides and the effects ofpH, ion strength and tartaric acid on the oxidation were investigated by means of chemical analysis, equilibrium redox, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. Three synthesized Mn oxide minerals, bimessite, cryptomelane, and hausmannite, which widely occur in soil and sediments, could actively oxidize As（Ⅲ） to As（Ⅴ）. However, their ability in As（Ⅲ）-oxidation varied greatly depending on their structure, composition and surface properties. Tunnel structured cryptomelane exhibited the highest ability of As （Ⅲ） oxidation, followed by the layer structured birnessite and the lower oxide hausmannite. The maximum amount of As （Ⅴ） produced by the oxidation was in the order （mmol/kg） of cryptomelane （824.2） 〉 bimessite （480.4） 〉 hausmannite （117.9）, As pH increased from the very low value（pH 2.5）, the amount of As（Ⅲ） oxidized by the tested Mn oxides was firstly decreased, then negatively peaked in pH 3.0 6.5, and eventually increased remarkably. Oxidation of As（Ⅲ） by the Mn oxides had a buffering effects on the pH variation in the solution. It is proposed that the oxidative reaction processes between As （Ⅲ） and biruessite（or cryptomelane） are as follows： （1） at lower pH condition： （MnO2）x＋ H3AsO3 ＋ 0.5H^＋=0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋Mn〉^2＋ （MnO2）x-1 ＋ H2O; （2） at higher pH condition： （MnO2）x ＋ H3AsO3 = 0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋ 1.5H^＋ ＋ （MnO2）x-1. MnO. With increase of ion strength, the As（Ⅲ） oxidized by bimessite and cryptomelane decreased and was negatively correlated with ion strength. However, ion strength had little influence on As （Ⅲ） oxidation by the hausmarmite. The presence of tartaric acid promoted oxidation of As（Ⅲ） by birnessite. As for cryptomelane and hansmannite, the same effect was observed when the concentration of tartaric acid was below 4 mmol/L, otherwise the oxidized As（Ⅲ） decreased. These findings are of great significance in improving our understanding of As geochemical cycling and controlling As contamination.     

Adsorption and desorption of Cu on paddy soil aggregates pretreated with different levels of phosphate

Interactions between anions and cations are important for understanding the behaviors of chemical pollutants and their potential risks in the environment. Here we prepared soil aggregates of a yellow paddy soil from the Taihu Lake region, and investigated the effects of phosphate (P) pretreatment on adsorption-desorption of Cu²⁺ of soil aggregates, free iron oxyhydrates-removed soil aggregates, goethite, and kaolinite with batch adsorption method. The results showed that Cu²⁺ adsorption was reduced on the aggregates pretreated with low concentrations of P, and promoted with high concentrations of P, showing a V-shaped change. Compared with the untreated aggregates, the adsorption capacity of Cu²⁺ was reduced when P application rates were lower than 260, 220, 130 and 110 mg/kg for coarse, clay, silt and fine sand fractions, respectively. On the contrary, the adsorption capacity of Cu²⁺ was higher on P-pretreated soil aggregates than on the control ones when P application rates were greater than those values. However, the desorption of Cu²⁺ was enhanced at low levels of P, but suppressed at high levels of P, displaying an inverted V-shaped change over P adsorption. The Cu²⁺ adsorption by the aggregate particles with and without P pretreatments was well described by the Freundlich equation. Similar results were obtained on P-pretreated goethite. However, such P effects on Cu²⁺ adsorption-desorption were not observed on kaolinite and free iron oxyhydrates-removed soil aggregates. The present results indicate that goethite is one of the main soil substances responsible for the P-induced promotion and inhibition of Cu²⁺ adsorption.     

Mn(Ⅲ)在水钠锰矿氧化Cr(Ⅲ)反应中的作用

环境中的氧化锰矿物是可氧化Cr(Ⅲ)的唯一天然无机氧化剂,氧化锰矿物与Cr(Ⅲ)相互作用的反应速率与机制备受关注.本研究以水钠锰矿为对象,采用批量动力学方法研究了其结构中Mn(Ⅲ)在氧化Cr(Ⅲ)反应中的作用及动力学特点.结果表明,水钠锰矿氧化Cr(Ⅲ)符合准一级动力学方程,表观速率常数Kobs为0.0313 min-1,以Na4P2O7预处理水钠锰矿可络合出结构中的部分Mn(Ⅲ),使其Mn的平均氧化度升高.当处理Na4P2O7浓度为10、20、50 mmol/L时,其Mn氧化度由3.50升高至3.63、3.73和3.78,处理后的水钠锰矿对Cr(Ⅲ)的平衡氧化率增加,但初始氧化速率变化并不明显,其相应表观速率常数分别为0.0351、0.0325和0.0309 min-1.水钠锰矿氧化Cr(Ⅲ)的反应历程中,Mn(Ⅳ)→Mn(Ⅲ)的电子转移过程生成的Mn(Ⅲ)显著影响水钠锰矿氧化Cr(Ⅲ)的速率.当新生成Mn(Ⅲ)被Na4P2O7络合后,反应速率降低45%~88%,且水钠锰矿的氧化度较低时,结构中Mn(Ⅲ)含量高,反应中被络合的新生态Mn(Ⅲ)多,反应速率降低幅度相对较大.因此,反应新生成的Mn(Ⅲ)具有较高的反应活性和较快的电子转移速率,而Mn(Ⅳ)→Mn(Ⅲ)的电子转移速率较慢,可能为水钠锰矿氧化Cr(Ⅲ)的反应速率控制步骤.     

硫酸盐还原颗粒污泥对溶液中Cr(Ⅵ)去除特性及总Cr平衡吸附研究

利用小型EGSB反应器培养的硫酸盐还原颗粒污泥进行Cr(Ⅵ)去除实验,考察硫酸盐还原颗粒污泥对Cr(Ⅵ)的去除特性及对总Cr的平衡吸附并进行吸附等温式拟合.结果表明,颗粒污泥对Cr(Ⅵ)的去除是物理、化学及生物过程共同作用的结果,主要包括了还原及吸附作用.颗粒污泥对Cr(Ⅵ)的去除作用与环境因素、颗粒污泥的化学组成和结构的完整程度等密切相关.对Cr(Ⅵ)的去除速率随颗粒污泥投加量及Cr(Ⅵ)初始浓度的增加而提高;提高振荡速度和温度均可提高Cr(Ⅵ)的去除效率及对总Cr的吸附速率,但当振荡速度达到150r·min-1时或温度达到40℃时颗粒污泥会出现结构的离散并降低总Cr的平衡吸附量;pH值越低颗粒污泥对Cr(Ⅵ)的去除效率就越高,但颗粒污泥表面的硫化物在pH值为酸性时会转化成H2S气体逸出,并因此影响对总Cr的吸附效率.颗粒污泥对总Cr的最大吸附量为6.84mg·g-1,其对总Cr的吸附过程符合Langmuir吸附等温式.     

On-line batch production of ferrate with an chemical method and its potential application for greywater recycling with Al(III) salt

Ferrate(VI) salt is an oxidant and coagulant for water and wastewater treatment. It is considered as a possible alternative method in greywater treatment. However, challenges have existed in putting ferrate(VI) technology into full-scale practice in water and wastewater treatment due to the instability of ferrate solution and high production cost of solid ferrate products. This study demonstrated a new approach of greywater treatment with on-line batch production of Fe(VI) to which Fe(III) salt was oxidized at a weak acidity solution. A series of experiments were conducted to investigate the effect of Fe(VI) on light greywater (total organic carbon (TOC) = 19.5 mg/L) and dark greywater (TOC = 55 mg/L) treatment under different conditions with varying pH and Fe(VI) doses. In addition, the combination use of Fe(VI) and Al(III) salts was proved to be more efficient than using the Fe(VI) salts alone at greywater recycling. The optimum dosage of Fe(VI)/Al(III) salts was 25/25 mg/L for light greywater, 90/60 mg/L for dark greywater, respectively. The TOC values of both light greywater and dark greywater were reduced to less than 3 mg/L with the dosages. The cost for treating greywater was 0.06–0.2 $/ton at ferrate(VI) dosage of 25–90 mg/L and 0.008–0.024 $/ton at AlCl₃ dosage of 25–60 mg/L. The full operating cost needs further assessment before the Fe(VI)/Al(III) technology could be implemented in greywater treatment.     

Preparation and evaluation of aminopropyl-functionalized manganese-loaded SBA-15 for copper removal from aqueous solution

A novel material, aminopropyl-functionalized manganese-loaded SBA-15 (NH₂-Mn-SBA-15), was synthesized by bonding 3-aminopropyl trimethoxysilane (APTMS) onto manganese-loaded SBA-15 (Mn-SBA-15) and used as a Cu^2 + adsorbent in aqueous solution. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction spectra (XRD), N₂ adsorption/desorption isotherms, high resolution field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the NH₂-Mn-SBA-15. The ordered mesoporous structure of SBA-15 was remained after modification. The manganese oxides were mainly loaded on the internal surface of the pore channels while the aminopropyl groups were mainly anchored on the external surface of SBA-15. The adsorption of Cu^2 + on NH₂-Mn-SBA-15 was fitted well by the Langmuir equation and the maximum adsorption capacity of NH₂-Mn-SBA-15 for Cu^2 + was over two times higher than that of Mn-SBA-15 under the same conditions. The Elovich equation gave a good fit for the adsorption process of Cu^2 + by NH₂-Mn-SBA-15 and Mn-SBA-15. Both the loaded manganese oxides and the anchored aminopropyl groups were found to contribute to the uptake of Cu^2 +. The NH₂-Mn-SBA-15 showed high selectivity for copper ions. Consecutive adsorption–desorption experiments showed that the NH₂-Mn-SBA-15 could be regenerated by acid treatment without altering its properties.