Effective immobilization of hexavalent chromium from drinking water by nano-FeOOH coating activated carbon: Adsorption and reduction

Adsorption of Cr(VI) and Cr(III) on layered pipe scales and the effects of disinfectants in drinking water distribution systems

Pipe scales in drinking water distribution systems (DWDS) potentially adsorb chromium (Cr). Meanwhile, the fate of Cr in pipe scales and water could be influenced by the disinfectants used in DWDS since they might influence the valence state of Cr. Therefore, the adsorption of Cr (Cr(VI) and Cr(III)) on pipe scales, the transformation between different valence states, and the effects of disinfectants present in DWDS are important research topics for improving tap water quality but have not yet been sufficiently investigated. This study investigated the properties of layered pipe scales and conducted adsorption kinetic experiments in single and binary Cr(VI) and Cr(III) systems, as well as experiments related to the oxidation and adsorption of Cr(III) under the influence of decaying disinfectants. According to the results, pipe scales exhibited distinct layered structures with varying mechanisms for the adsorption of Cr(VI) and Cr(III). Cr(VI) was adsorbed through surface complexation on the surface and porous core layers, while redox reactions predominantly occurred on the shell-like layer. Furthermore, Cr(III) was adsorbed via surface precipitation on the three-layer pipe scales. Importantly, disinfectants promoted the transformation of Cr(III) to the less readily released Cr(VI) in pipe scales, reducing the Cr exposure risk from the pipe scale phase. Pipe scales also decreased the Cr(VI) concentration in water (almost 0 mg/L), enhancing the safety of DWDS. This study provides theoretical guidance on the safe operation of DWDS.

Simultaneous removal of methylene blue and Cr(VI) in a dual-chamber photocatalytic microbial fuel cell with WO3/MoS2/FTO photocathode

Carbon felt was used as the anode and WO3/MoS2/FTO (fluorine-doped tin oxide) was used as the photocathode in a photocatalytic microbial fuel cell (PMFC). The photoelectric performance of the WO3/MoS2/FTO photocathode and the removal efficiency of methylene blue (MB) and Cr(VI) mixed pollutants were systematically investigated in the cathode chamber. The results showed that after 12 h of light irradiation in the PMFC with WO3/MoS2/FTO as the photocathode, the removal rates of MB and Cr(VI) were 84.56 and 68.11 %, respectively, which were much higher than those using WO3/FTO as a photocathode (55.57 % and 45.26 %, respectively). The corresponding maximum output power was 33.14 mW/m2, which was 1.85 times that of the WO3/FTO photocathode PMFC. These results can be attributed to the fact that WO3 is an n-type semiconductor and MoS2 is a p-type semiconductor. Analysis of trapping experiments showed that the composite of WO3 and MoS2 formed a Z-scheme heterojunction, which improved the separation efficiency of the photoelectric carriers and enhanced the pollutant removal efficiency of the photocathode. PMFCs are a new and environment-friendly technology for removing pollutants thereby providing an experimental basis for future engineering applications.

Chromium immobilization and release by pipe scales in drinking water distribution systems: The impact of anions

Due to its high toxicity, the release of chromium (Cr) by pipe scales poses a serious risk to drinking water quality and human health. This study looked into how Cr immobilized and released by pipe scales. SEM, XRD, and XPS were applied to evaluate the physicochemical characterization of pipe scales. To identify times of immobilization and release and the proper scale to water ratio, the behaviors of Cr were examined in stagnation experiments. Afterward, the common anions in drinking water were designed as nine concentration gradients to explore their species and concentrations impacts on the immobilization and release process of Cr. It is worth mentioning that the pipe scales were classified into block pipe scales, lumpy pipe scales, and powder pipe scales in this experiment. The types of pipe scales were rarely considered as an influencing factor. Results revealed that in contrast to powder pipe scales, block pipe scales and lumpy pipe scales exhibited extremely comparable trends. Specifically, in terms of accumulation capacity, the order from largest to smallest was powder pipe scales, lumpy pipe scales, and block pipe scales. However, the potential of Cr release from block pipe scales was the highest, indicating a high danger of heavy metal re-release in actual circumstances. Findings from this study discovered the turning points of chloride and sulfate concentrations associated with the pipe scales species in the anion-promoted release. These results provide insight into the relationship between pipe scales and Cr in drinking water distribution systems (DWDS).

The Behavior of Polymeric Pipes in Drinking Water Distribution System-Comparison with Other Pipe Materials

The inner walls of the drinking water distribution system (DWDS) are expected to be clean to ensure a safe quality of drinking water. Complex physical, chemical, and biological processes take place when water comes into contact with the pipe surface. This paper describes the impact of leaching different compounds from the water supply pipes into drinking water and subsequent risks. Among these compounds, there are heavy metals. It is necessary to prevent these metals from getting into the DWDS. Those compounds are susceptible to impacting the quality of the water delivered to the population either by leaching dangerous chemicals into water or by enhancing the development of microorganism growth on the pipe surface. The corrosion process of different pipe materials, scale formation mechanisms, and the impact of bacteria formed in corrosion layers are discussed. Water treatment processes and the pipe materials also affect the water composition. Pipe materials act differently in the flowing and stagnation conditions. Moreover, they age differently (e.g., metal-based pipes are subjected to corrosion while polymer-based pipes have a decreased mechanical resistance) and are susceptible to enhanced bacterial film formation. Water distribution pipes are a dynamic environment, therefore, the models that are used must consider the changes that occur over time. Mathematical modeling of the leaching process is complex and includes the description of corrosion development over time, correlated with a model for the biofilm formation and the disinfectants-corrosion products and disinfectants-biofilm interactions. The models used for these processes range from simple longitudinal dispersion models to Monte Carlo simulations and 3D modeling. This review helps to clarify what are the possible sources of compounds responsible for drinking water quality degradation. Additionally, it gives guidance on the measures that are needed to maintain stable and safe drinking water quality.

Schwertmannite and akaganéite for adsorption removals of Cr(VI) from aqueous solutions

Iron hydroxides have received high attention in the treatment of chromium (Cr) polluted wastewater. In this study, the obtained chemical (or biological) pincushion-schwertmannite spheres had a diameter of 2 - 5 μm (0.5-1 μm), and akaganéite rods had a length of 300-500 nm (100-150 nm) at an axial ratio of about 3. The average diameters (μm) of their agglomerated particles in solutions were 20.6-32.5 (only 0.480 for Aka-Chem). Schwertmannites and akaganéites were used to investigate Cr(VI) adsorption behaviors in aqueous solutions by batch experiments, under various reaction times, initial Cr(VI) and adsorbent levels, pH values, temperature and anions of NO₃^-, Cl^-, CO₃^2-, SO₄^2-, and H₂PO₄^-. Adsorption data well fitted to pseudo-second-order rate model (R² = 0.999), and Langmuir (R² = 0.954-0.988) and Freundlich (R² = 0.984-0.996) isothermal models at pH 7.0. Maximum Cr(VI) adsorption capacities were 119/133 for Sch-Chem/Sch-Bio, and 14.6/83.6 for Aka-Chem/Aka-Bio. The H₂PO₄^- than SO₄^2-/CO₃^2- had a stronger effect on Cr(VI) adsorption. Adsorbents with pH_ZPC of near to 4.0 still had a good Cr(VI) removal ability at pH 3.0-8.0. The possible Cr(VI) adsorption mechanisms by FTIR and XPS results for schwertmannite and akaganéite were electrostatic attractions and ion exchanges between hydroxyl (or sulfate) and chromate ions. The Cr(VI) adsorption of optimal schwertmannite and bioakaganéite was a spontaneous, endothermic and random process at the temperatures of 288-318 K. They had a good regeneration ability for Cr(VI) adsorption, and removal ratios could reach to about 80% of original values (60-70% in aqueous solution with 60 mg/L Cr(VI) and pH7.0, and 35-50% in wastewater with 120 mg/L Cr(VI) and about pH4.0), after three cycles. Herein, schwertmannite/bioakaganéite have a promising application in treatment of acidic/neutral wastewater with chromate.

Enhanced removal of aqueous Cr(VI) by the in situ iron loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two step method: Mechanism study

In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO₄/K₂FeO₄ impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K₂Cr₂O₇ on the surface of FeOOH was - 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K₂Cr₂O₇ on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr₂O₇^2- would electrostatically attract to the positively charged AFPAC surface. (2) Cr₂O₇^2- was reduced into Cr₂O₃ by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO₄ and K₂FeO₄. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.

Fate and mechanistic insights into nanoscale zerovalent iron (nZVI) activation of sludge derived biochar reacted with Cr(VI)

While nanoscale zero-valent iron modified biochar (nZVI-BC) have been widely investigated for the removal of heavy metals, the corrosion products of nZVI and their interaction with heavy metals have not been revealed yet. In this paper, nZVI-BC was synthesized and applied for the removal of Cr(VI). Batch experiments indicated that the adsorption of Cr(VI) fit Langmuir isotherm, with the maximum removal capacity at 172.4 mg/g at pH 2.0. SEM-EDS, BET, XRD, FT-IR, Raman and XPS investigation suggested that reduction of Cr(VI) to Cr(III) was the major removal mechanism. pH played an important role on the corrosion of nZVI-BC, at pH 4.5 and 2.0, FeOOH and Fe₃O₄ were detected as the major iron oxide, respectively. Therefore, FeOOH-BC and Fe₃O₄-BC were further prepared and their interaction with Cr were studied. Combining with DFT calculations, it revealed that Fe₃O₄ has higher adsorption capacity and was responsible for the effective removal of Cr(VI) through electrostatic attraction and reduction under acidic conditions. However, Fe₃O₄ will continue to convert to the more stable FeOOH, which is the key to for the subsequent stabilization of the reduced Cr(III). The results showed that the oxide corrosion products of nZVI-BC were subjected to the environment, which will eventually affect the fate and transport of the adsorbed heavy metal.

Fe/C materials prepared by one-step calcination of acidified municipal sludge and their excellent adsorption of Cr(VI)

Biochar derived from municipal sludge can be applied to adsorption. But it usually requires activation and pickling due to the generation of impurities such as metal oxide particles, which is uneconomical. Here, a facile strategy, acidification-one-step calcination, was developed and sludge-based Fe-C materials with good Cr(VI) removal effect were obtained by regulating the amount of hydrochloric acid. The results show that the adsorption capacity of Fe/C-5 (the best sample) for Cr(VI) was 150.84 mg g^-1. According to the Langmuir isotherm and pseudo-second-order kinetic model, the removal of Cr(VI) by Fe/C-5 is spontaneous and endothermic chemisorption process. In addition, Fe/C-5 has good ability to remove Cr(VI) under the interference of coexisting ions, and has good cycle stability. The removal of Cr(VI) by Fe/C-5 is considered to be synergistic process of adsorption and reduction. The Fe atoms were highly dispersed in Fe/C-5 and tightly bonded with C atoms, which not only strengthened the Cr(VI) adsorption by electrostatic attraction, but also activated the C atoms in the biochar material, so that the C atoms can reduce Cr(VI) to Cr(III) under acidic conditions. This may be due to the fact that acid pretreatment converted the iron in municipal sludge in the form of Fe-O/OH to free Fe³⁺ and entered the C lattice during the calcination process. In this work, Fe-C materials with excellent Cr(VI) adsorption capacity were prepared by one-step calcination method, which has important reference significance for the resource utilization of municipal sludge.

Waste tire particles as efficient materials towards hexavalent chromium removal: Characterisation, adsorption behaviour, equilibrium, and kinetic modelling

The growing demand for vehicles is increasing every year, and this paves the way for the environmental problem for the generation of the waste tire (WT) and associated disposal after their end life. On the other direction, effluent from tannery and electroplating industries contented with a higher concentration of chromium, which is a toxic pollutant, induces mutation effects. In this work, the approach of reutilization of waste tires as adsorbent is reported towards the sequestrations of hexavalent chromium (Cr(VI)) from the simulated system. The waste tire sample is activated using orthophosphoric acid (H₃PO₄) and utilized for adsorption studies. Adsorption studies are performed with native tire sample (NTP) and activated tire sample (ATP). About 96.5% of Cr(VI) adsorption efficiency was attained using ATP and at the optimal adsorption solution pH 2.0. The adsorbent performed relatively well in wide a range of initial solution pH tested in the present research. The experimental data analysis of Cr(VI) adsorption onto NTP and ATP revealed the best fit with the kinetic model of pseudo-second-order. Further, the equilibrium data were analysed using two-parameter isotherms, relatively the Langmuir isotherm best represented in both NTP and ATP. ATP demonstrated with higher Cr(VI) removal performance with an adsorption capacity of 102.90 mg/g, according to the Langmuir model. The proposed idea of utilizing waste tire particles as adsorbents in the treatment of Cr(VI) contaminated water offers valuable guidance towards further investigations in the directions of dynamic adsorption and effluent treatment.

Nanocrystalline erdite from iron-rich sludge: Green synthesis, characterization and utilization as an efficient adsorbent of hexavalent chromium

A low-temperature hydrothermal process was developed to synthesize erdite adsorbent from a solid waste sludge contained 10.2% Fe, 6.2% Al and 1.4% Si, alongside 59.5% water content. At 90℃, adding Na₂S and NaOH could convert it into erdite nanorods with a diameter of 80 nm and a length of 100 nm. In the sludge, only Fe oxyhydroxide was involved in the formation of erdite, and the other Al/Si-bearing compounds were dissolved in an alkaline medium. The dissolved Al/Si-bearing compounds were further removed, forming faujasite so that the used medium was purified and then entirely recycled into the next conversion stage. No secondary waste was generated in the pilot-scale conversion, and the adsorption efficiency of the prepared products to wastewater with a high initial Cr(VI) concentration of 1000 mg/L was more than 99.5%. The adsorption data complied with the pseudo-second-order kinetics. During the wastewater treatment, hexavalent chromium anion diffused to erdite surface and replaced OH/SH groups of adjacent structural Fe to form a stable complex ligand. In addition, the redox reaction between hexavalent chromium and the -SH group occurred to generate a trivalent chromium complex on the Fe/S-bearing flocs surface.