Arsenic pollution of drinking waters across the world is one of the most serious water-related problems due to its well-established consequences on human health even at very low concentrations in the lower µg/L range. Among different well-established options for arsenic remediation, the adsorption onto highly efficient commercial iron oxyhydroxide-based adsorbent such as granular ferric hydroxide (GFH) has proven to be effective and persuasive. However, GFH is a cost-extensive material. During the industrial production of granular fractions of conventional adsorbents, the fine-grained fraction (individual particle size of < 250 µm) is generated as by-product/waste as this fraction of granular adsorbents cannot be applied in fixed-bed adsorption filters because of high clogging potential in filter-bed. In this doctoral thesis, an integrated water process combining the adsorption and submerged microfiltration (MF) unit (abbreviated as SMAHS) was investigated to employ fine-grained iron oxyhydroxides. Air bubbling was applied in the slurry reactor of a SMAHS to introduce shear at the membrane surface for fouling control. Moreover, the powdered-sized fractions (individual particle size of ~ 3 µm) of iron oxyhydroxides were applied to form the pre-deposited dynamic membrane (DM) and the effectiveness of the formed DM was assessed in MF process. n addition to the fine fraction of the GFH, arsenic adsorption on µTMF (fine-grained tetravalent manganese feroxyhyte) was investigated through batch adsorption tests at pH 8 in three different water matrices and different adsorption isotherms were applied. The physical and chemical characteristics of the adsorbents were also fully investigated. The Freundlich isotherm describes the equilibrium isotherm data better than Langmuir isotherm, indicating a heterogeneous nature of the applied adsorbents. The isotherm data shows characteristics of favorable arsenic adsorption onto µGFH and µTMF. Further, adsorption efficiency of applied adsorbents depends strongly on the water quality parameters (pH and water matrix). Arsenic adsorption onto both adsorbents is mostly reversible, with a small proportion of irreversible adsorption. The findings from SMAHS indicate that the arsenic adsorption efficiency is comparable to that found in a fixed-bed adsorption filter packed with conventional adsorbents of the same type, with potential benefits of simultaneous removal of micro-organisms and turbidity. The material cost is estimated to be as low as 0.30 €/m3 of product water when the arsenic concentration in the product water is below the drinking water regulation limit (10 µg/L). The outcomes further suggest that iron oxyhydroxides as forming materials of DMs may be applied in water treatment to achieve arsenic removal rates of greater than 90 % if operating conditions are well controlled. Moreover, arsenic removal rates of the SMAHS and DM can be predicted/modeled using a mathematical model based on a homogenous surface diffusion model (HSDM). In conclusion, it is expected that the new applications of fine-grained iron oxyhydroxides would not only increase the sustainable footprint of the conventional adsorbent production process as the by-product will be utilized but also be efficient solutions for arsenic remediation using the highly efficient low-cost adsorbents in water treatment.
BACKGROUND: This study reports the development of a dynamic membrane (DM) adsorber by the pre‐depositing powdered‐sized fraction of iron oxide‐based adsorptive material on the surface of a microfiltration(MF) membrane. The aim is to use the developed DM adsorber for arsenate (As(V)) remediation from water by a combined mechanism of adsorptive and membrane filtration. The two applied iron oxide‐based adsorptive materials are micro‐sized granular ferric hydroxide and micro‐sized tetravalent manganese feroxyhyte, and are available at an affordable price. RESULTS: The results show that As(V) removal efficiency strongly depends on the physicochemical properties of the depositing material such as specific surface area, isoelectric point, and particle size of the pre‐depositing material. The experimentally determined As(V) removal rates were mathematically modeled using a homogeneous surface diffusion model (HSDM) that incorporates the equilibrium parameters and mass transport coefficients of the adsorption process. The simulations showed that the mathematical model could describe the As(V) removal rates accurately over a broad range of operating conditions. The results further showed that the longer filtration times with very low normalized As(V) permeate concentration (C/Cf = 0.1 for example) can be prolonged by operating DM adsorber at lowermost membrane water flux of 31 L/(m2·h) and large amount of pre‐depositing material on MF membrane surface (Ma= 14 mg/cm2). CONCLUSION: The results presented in this study confirm that use of these inexpensive materials (side‐product of granular iron‐oxide‐based adsorbents) in treating As(V) polluted water would enhance the sustainability of the industrial production process of conventional granular adsorbents by utilizing the wastes created during the process of adsorbent production.
The adsorption of arsenic (V), As(V), on two porous iron oxyhydroxide-based adsorbents, namely, micro-sized tetravalent manganese feroxyhyte (µTMF) and granular ferric hydroxide (µGFH), applied in a submerged microfiltration membrane hybrid system has been investigated and modeled. Batch adsorption tests were carried out to determine adsorption equilibrium and kinetics parameters of As(V) in a bench-scale slurry reactor setup. A mathematical model has been developed to describe the kinetic data as well as to predict the As(V) breakthrough curves in the hybrid system based on the homogeneous surface diffusion model (HSDM) and the corresponding solute mass balance equation. The kinetic parameters describing the mass transfer resistance due to intraparticle surface diffusion (Ds) involved in the HSDM was determined. The fitted Ds values for the smaller (1 - 63 μm) and larger (1 - 250 μm) diameter particles of µGFH and μTMF were estimated to be 1.09 × 10-18 m2/s and 1.53 × 10-16 m2/s, and 2.26 × 10-18 m2/s and 1.01 × 10-16 m2/s, respectively. The estimated values of mass transfer coefficient/ kinetic parameters are then applied in the developed model to predict the As(V) concentration profiles in the effluent of the hybrid membrane system. The predicted results were compared with experimental data for As(V) removal and showed an excellent agreement. After validation at varying adsorbent doses and membrane fluxes, the developed mathematical model was used to predict the influence of different operation conditions on As(V) effluent concentration profile. The model simulations also exhibit that the hybrid system benefits from increasing the amount of adsorbent initially dosed and from decreasing the membrane flux (increasing the contact time).
The small sized powdered ferric oxy-hydroxide, termed Dust Ferric Hydroxide (DFH), was applied in batch adsorption experiments to remove arsenic species from water. The DFH was characterized in terms of zero point charge, zeta potential, surface charge density, particle size and moisture content. Batch adsorption isotherm experiments indicated that the Freundlich model described the isothermal adsorption behavior of arsenic species notably well. The results indicated that the adsorption capacity of DFH in deionized ultrapure water, applying a residual equilibrium concentration of 10 µg/L at the equilibrium pH value of 7.9 ± 0.1, with a contact time of 24 h (i.e., Q10), was 6.9 and 3.5 µg/mg for As(V) and As(III), respectively, whereas the measured adsorption capacity of the conventionally used Granular Ferric Hydroxide (GFH), under similar conditions, was found to be 2.1 and 1.4 µg/mg for As(V) and As(III), respectively. Furthermore, the adsorption of arsenic species onto DFH in a Hamburg tap water matrix, as well as in an NSF challenge water matrix, was found to be significantly lower. The lowest recorded adsorption capacity at the same equilibrium concentration was 3.2 µg As(V)/mg and 1.1 µg As(III)/mg for the NSF water. Batch adsorption kinetics experiments were also conducted to study the impact of a water matrix on the behavior of removal kinetics for As(V) and As(III) species by DFH, and the respective data were best fitted to the second order kinetic model. The outcomes of this study confirm that the small sized iron oxide-based material, being a by-product of the production process of GFH adsorbent, has significant potential to be used for the adsorptive removal of arsenic species from water, especially when this material can be combined with the subsequent application of low-pressure membrane filtration/separation in a hybrid water treatment process.
Arsenic is among the major drinking water contaminants affecting populations in many countries because it causes serious health problems on long-term exposure. Two low-cost micro-sized iron oxyhydroxide-based adsorbents (which are by-products of the industrial production process of granular adsorbents), namely, micro granular ferric hydroxide (μGFH) and micro tetravalent manganese feroxyhyte (μTMF), were applied in batch adsorption kinetic tests and submerged microfiltration membrane adsorption hybrid system (SMAHS) to remove pentavalent arsenic (As(V)) from modeled drinking water. The adsorbents media were characterized in terms of iron content, BET surface area, pore volume, and particle size. The results of adsorption kinetics show that initial adsorption rate of As(V) by μTMF is faster than μGFH. The SMAHS results revealed that hydraulic residence time of As(V) in the slurry reactor plays a critical role. At longer residence time, the achieved adsorption capacities at As(V) permeate concentration of 10 μg/L (WHO guideline value) are 0.95 and 1.04 μg/mg for μGFH and μTMF, respectively. At shorter residence time of ~ 3 h, μTMF was able to treat 1.4 times more volumes of arsenic-polluted water than μGFH under the optimized experimental conditions due to its fast kinetic behavior. The outcomes of this study confirm that micro-sized iron oyxhydroxides, by-products of conventional adsorbent production processes, can successfully be employed in the proposed hybrid water treatment system to achieve drinking water guideline value for arsenic, without considerable fouling of the porous membrane. Graphical abstract