Arup K. SenGupta

Arsenic removal using a polymeric/inorganic hybrid sorbent

A fixed-bed sorption process can be very effective in removing trace concentrations of arsenic from contaminated groundwater provided: the sorbent is very selective toward both As(III) and As(V) species; the influent and treated water do not warrant any additional pre- or post- treatment; pH and composition of the raw water with respect to other electrolytes remain unchanged besides arsenic removal, and the sorbent is durable with excellent attrition resistance properties. In addition, the sorbent should be amenable to efficient regeneration for multiple reuse. This study reports the results of an extensive investigation pertaining to arsenic removal properties of a polymeric/inorganic hybrid sorbent. Each hybrid sorbent particle is essentially a spherical macroporous cation exchanger bead within which agglomerates of nanoscale hydrated Fe oxide (HFO) particles have been uniformly and irreversibly dispersed using a simple chemical-thermal treatment. The new sorbent, referred to as hybrid ion exchanger or HIX, combines excellent mechanical and hydraulic properties of spherical polymeric beads with selective As(III) and As(V) sorption properties of HFO nanoparticles at circum-neutral pH. Comparison of the results of fixed-bed column runs between the new sorbent and the polymeric anion exchanger confirmed that both As(V) and As(III) were removed very selectively with HIX. Equally important, no pH adjustment, pre- or post-treatment was warranted. Besides the absence of arsenic, the treated water composition was identical to that of influent water. HIX was amenable to efficient in situ regeneration with caustic soda and could subsequently be brought into service following a short rinse with carbon dioxide sparged water. During fixed-bed column runs, intraparticle diffusion was identified as the primary rate-limiting step for both As(III) and As(V) sorption. Repeated use of the same HIX particles during various laboratory investigations provided strong evidence that the new sorbent possesses excellent attrition resistance properties and retains its arsenic removal capacity over cycles.

Ultimate removal of phosphate from wastewater using a new class of polymeric ion exchangers

Abstract The presence of trace concentrations of dissolved phosphate is often responsible for causing eutrophication problems in lakes, reservoirs, other confined water bodies and coastal waters. In this regard, both biological and physico–chemical treatment processes have been studied extensively to remove phosphate from contaminated water/wastewater. There, however, remains a major need to identify/develop a viable fixed-bed process which can essentially eliminate phosphate from contaminated water/wastewater. Previous investigators have shown the advantages as well as shortcomings of the fixed-bed process when strong-base anion exchangers, activated alumina and zirconium oxides are used as sorbents. The present study reports the results of a detailed investigation pertaining to selective phosphate removal by a new class of sorbent, referred to as polymeric ligand exchanger (PLE). Laboratory studies show strong evidence that the PLE is very selective toward phosphate, chemically stable, and also amenable to efficient regeneration. Anion exchange accompanied by Lewis acid–base interaction is the underlying reason for PLEs enhanced affinity toward phosphate. In several ways, this new ion exchanger (PLE) overcomes the shortcomings of previously used inorganic and polymeric sorbents.

Polymer supported inorganic nanoparticles: characterization and environmental applications

Abstract Nanoscale Inorganic Particles (NIPs) and their agglomerates offer excellent opportunities conducive to selective removal of a wide array of target compounds from contaminated water bodies. For example, (i) hydrated Fe(III) oxides or HFO particles can selectively sorb dissolved heavy metals like zinc, copper or metalloids like arsenic oxyacids or oxyanions; (ii) Mn(IV) oxides are fairly strong solid phase oxidizing agents; (iii) magnetite (Fe 3 O 4 ) crystals are capable of imparting magnetic activity; (iv) elemental Zn o or Fe o are excellent reducing agents for both inorganic and organic contaminants. Very high surface area to volume ratio of these nanoscale particles offers favorable sorption and/or reaction kinetics. However, applications of NIPs in fixed-bed columns, in-situ reactive barriers and in similar flow-through applications are not possible due to extremely high pressure drops. Also, these NIPs are not durable and lack mechanical strength. Harnessing these inorganic nanoparticles and their aggregates appropriately within polymeric beads offers new opportunities that are amenable to rapid implementation in the area of environmental separation and control. While the NIPs retain their intrinsic sorption/desorption, redox, acid–base or magnetic properties, the robust polymeric support offers excellent mechanical strength, durability and favorable hydraulic properties in the flow-through systems. This paper discusses at length the preparation, characterization and environmental applications of two classes of polymer supported nanoparticles: (i) Hydrated Fe(III) Oxide (HFO) dispersed polymeric exchanger and their As(III), As(V), and heavy metals removal properties; (ii) Magnetically Active Polymeric Particles (MAPPs). The polymer supported nanoparticles are reusable and can be easily reprocessed over many cycles of operation.

Polymer-supported metals and metal oxide nanoparticles: synthesis, characterization, and applications

Metal and metal oxide nanoparticles exhibit unique properties in regard to sorption behaviors, magnetic activity, chemical reduction, ligand sequestration among others. To this end, attempts are being continuously made to take advantage of them in multitude of applications including separation, catalysis, environmental remediation, sensing, biomedical applications and others. However, metal and metal oxide nanoparticles lack chemical stability and mechanical strength. They exhibit extremely high pressure drop or head loss in fixed-bed column operation and are not suitable for any flow-through systems. Also, nanoparticles tend to aggregate; this phenomenon reduces their high surface area to volume ratio and subsequently reduces effectiveness. By appropriately dispersing metal and metal oxide nanoparticles into synthetic and naturally occurring polymers, many of the shortcomings can be overcome without compromising the parent properties of the nanoparticles. Furthermore, the appropriate choice of the polymer host with specific functional groups may even lead to the enhancement of the properties of nanoparticles. The synthesis of hybrid materials involves two broad pathways: dispersing the nanoparticles (i) within pre-formed or commercially available polymers; and (ii) during the polymerization process. This review presents a broad coverage of nanoparticles and polymeric/biopolymeric host materials and the resulting properties of the hybrid composites. In addition, the review discusses the role of the Donnan membrane effect exerted by the host functionalized polymer in harnessing the desirable properties of metal and metal oxide nanoparticles for intended applications.

The Donnan Membrane Principle: Opportunities for Sustainable Engineered Processes and Materials

The Donnan membrane principle can permit many engineered processes and materials to achieve better sustainability.

A new hybrid inorganic sorbent for heavy metals removal

Abstract Iron oxyhydroxides, commonly known as ferrihydrites, are unable to remove dissolved heavy metals at acidic pH, especially below 5.0, due to fierce competition from hydrogen ions. A new hybrid iron-rich inorganic sorbent has been identified and extensively studied in relation to heavy metals removals in fixed-bed processes for influent pH as low as 3.5. Every single particle of this new hybrid sorbent essentially contains ferrihydrite along with a crystalline silicate phase, akermanite, in close proximity (in the order of 100A) to one another. Akermanite has a unique ability to produce hydroxyl ions through incongruent hydrolysis reactions without being washed out from the fixed bed. The simultaneous presence of akermanite and ferrihydrite in a single particle has a synergistic effect on the sorption process: while akermanite helps neutralize aqueous-phase hydrogen ions (thus enhancing sorption capacity of ferrihydrites), neighboring sorption sites in ferrihydrites quickly remove dissolved heavy metals, thus avoiding precipitation. Equally important, the hybrid sorbent can be regenerated with any amine/ammoniacal solution and reused for multiple number of cycles. Some precipitations may occur within the column at relatively high influent concentrations of heavy metals (around 50 mg/l) or due to chromatographic effect. Such precipitates are, however, amenable to removals by conventional backwashing

Abiotic As(III) Oxidation by Hydrated Fe(III) Oxide (HFO) Microparticles in a Plug Flow Columnar Configuration

Scientific evidence pertaining to selective sorption of As(V) species or arsenates and As(III) species or arsenites onto Fe(III) oxide microparticles is widely available in the literature. Practical implications of this phenomenon are well recognized for both natural and engineered systems. The previous investigations to this effect have clearly established that such selective sorption processes are accompanied by formation of inner-sphere complexes between the surface functional groups of Fe(III) oxides and As(V) or As(III) species in question. Although thermodynamically favorable, oxidation of As(III) or arsenite by Fe(III) oxides has not been reported to date. Experimental studies for all such investigations were, however, carried out in continuously stirred batch reactors. The results of the present study confirm that, in a plug flow configuration with stationary Fe(III) oxide microparticles in a column, aqueous-phase As(III) undergoes near-complete conversion to As(V) at neutral to slightly alkaline pH. The stationary phase consists of porous polymeric particles within which submicron hydrated Fe(III) oxide particles have been irreversibly dispersed. As the mobile liquid phase slowly percolates through the stationary column, each As(III) solute progressively binds to a multitude of Fe(III) sorption sites favoring As(III) oxidation. The resulting As(V) and Fe(II) are subsequently sorbed onto iron oxide particles. The plug flow configuration of the system allows thousands of contacts with Fe(III) sorption sites for each As(III) molecule, thus enhancing As(III) oxidation in accordance with law of mass action effect. Also, the absence of Fe(II) in the aqueous phase offers highest possible oxidizing environment near the HFO sorption sites. The reactor configuration approximating plug flow is postulated to be the major contributor for As(III) oxidation by Fe(III) oxide microparticles. This observation may have major ramifications for arsenic-containing groundwater percolating through iron-rich soil. At acidic pH, conversion of As(III) to As(V) is, however, much less pronounced due to unfavorable thermodynamics.

Intraparticle diffusion during selective ion exchange with a macroporous exchanger

Abstract Chlorophenols, quaternary ammonium compounds, benzene and naphthalene sulfonates and benzene carboxylates are examples of environmentally significant synthetic organic compounds which exist as ions in water over a wide range of pH values. This study discusses the sorption kinetics, and more specifically, intraparticle diffusion behaviors of trace concentrations of pentachlorophenate (PCP − ) and other chlorophenates onto a commercially available macroporous polymeric anion exchanger (IRA-900). The anion exchanger is essentially biphasic, i.e. every single exchanger particle contains an enormous number of tiny microgels and an interconnected network of pores. Ion exchange functional groups reside solely within the microgels. For comparison, a gel or microporous anion exchanger is also included in the study. Experimental results reveal a distinctly different type of effect of competing chloride ion concentration on gel and macroporous ion exchangers pertaining to sorption of pentachlorophenate (PCP − ). While the effective intraparticle diffusivity of PCP − for gel-type resin remained unaltered with a change in competing chloride concentration, the same increased significantly for the macroporous exchanger with an increase in chloride concentration. Pore diffusion is considered to be the predominant intraparticle transport mechanism for highly preferred PCP − inside the macroporous exchanger. Under the normal hydrodynamic conditions of a fixed bed column run, intraparticle diffusion was found to be the rate-limiting step. For various chlorophenates, the effective intraparticle diffusivities were inversely correlated to the octanol–water partition coefficients of their parent chlorophenols.

Chromate ion-exchange process at alkaline pH

Abstract The chromate ion-exchange recovery process is always carried out at acidic pH because of its much higher chromate removal capacity at acidic pH as opposed to alkaline pH. However, acidic pH operation always gives rise to early, gradual chromate breakthrough during conventional fixed-bed column runs. Possibility of alkaline pH operation, which gives self-sharpening type chromate breakthrough during column runs, has been studied in detail. A new polystyrene matrix resin with relatively high degree of crosslinking was found to have high chromate selectivity at alkaline pH. This enhanced chromate selectivity is due to the resins increased hydrophobicity. Particularly at low ionic strength of cooling water, alkaline pH operation with this new resin may offer higher chromate selectivity compared to conventional acidic pH operation.

Mitigating arsenic crisis in the developing world: Role of robust, reusable and selective hybrid anion exchanger (HAIX)

In trying to address the public health crisis from the lack of potable water, millions of tube wells have been installed across the world. From these tube wells, natural groundwater contamination from arsenic regularly puts at risk the health of over 100 million people in South and Southeast Asia. Although there have been many research projects, awards and publications, appropriate treatment technology has not been matched to ground level realities and water solutions have not scaled to reach millions of people. For thousands of people from Nepal to India to Cambodia, hybrid anion exchange (HAIX) resins have provided arsenic-safe water for up to nine years. Synthesis of HAIX resins has been commercialized and they are now available globally. Robust, reusable and arsenic-selective, HAIX has been in operation in rural communities over numerous cycles of exhaustion-regeneration. All necessary testing and system maintenance is organized by community-level water staff. Removed arsenic is safely stored in a scientifically and environmentally appropriate manner to prevent future hazards to animals or people. Recent installations have shown the profitability of HAIX-based arsenic treatment, with capital payback periods of only two years in ideal locations. With an appropriate implementation model, HAIX-based treatment can rapidly scale and provide arsenic-safe water to at-risk populations.