Sanjoy Kumar Maji

Arsenic removal from aqueous solutions by adsorption on laterite soil

Laterite soil was used as an adsorbent for arsenic removal from contaminated groundwater. Effects of pH, adsorbent dose, adsorbent size, contact time, initial arsenic concentration and presence of interfering species on arsenic removal were found out. Laterite soil was found to be very effective for arsenic adsorption. It was found that 4 h contact time was sufficient for ∼ 98% and ∼95% removal from the contaminated water samples at an adsorbent dose of 10 g/L and 20 g/L for As(III) and As(V) respectively at an initial concentration level of 0.5 mg/L at a pH of 5.7 ± 0.2. Although there was no significant interference from Cl−, NO3 −, SO4 − 2, Ca2 +and Fe2 +/3 + on arsenite removal but its removal was little affected due to the presence of HPO4 − 2 and SiO3 − 2. Arsenate removal efficiency, however, was decreased to a large extent in presence of HPO4 − 2 and SiO3 − 2. The other ions Cl−, NO3 −, SO4 − 2, Ca2 + and Fe2 +/3 + had no significant interference on arsenate removal. The common organic contaminants such as 2,4-dichlorophenoxyacetic acid (2,4- D), atrazine, endosulfan and humic acid had no effect on As(III) removal, but they (excepting 2,4-D) cause decrease in the removal efficiency for As(V). Both Langmuir and Freundlich adsorption isotherm models fitted well and the maximum adsorption capacity was estimated to be 1.384 mg/g and 0.04 mg/g for arsenite and arsenate respectively. The real arsenic contaminated groundwater was also tested and it was found that laterite soil is very effective for arsenic removal from real groundwater sample, and up to ∼ 99% removal could be achieved under normal condition. The advantage of the material is that the pH of the raw water did not change after arsenic removal, and iron was not leached.

Surfactant-modified alumina: an efficient adsorbent for malachite green removal from water environment.

Surface of alumina was modified with sodium dodecyl sulfate (SDS), an anionic surfactant. The surfactant-modified alumina (SMA) was characterized by FTIR and thermal analysis. The SMA was then used for the removal of malachite green (MG; Basic Green 4), a well-known toxic cationic dye from aqueous environment. The removal of MG takes place in the micellar structure formed on alumina surface, and the process is called adsolubilization. All the studies were carried out in batch mode. The kinetic studies showed that 1 h contact time was sufficient to attain equilibrium. SMA was very efficient to remove MG up to 99% under optimum conditions. The concentration range of MG was 20–100 mg/L. The isotherm studies showed that it follows Langmuir model better than the Freundlich model. The maximum adsorption capacity was 185 mg/g. The effects of various parameters such as pH, presence of interfering ions (Cl−, NO3 −, H2PO4 −, SO4 2−, Fe2 +, Ca2 +) and organics (pesticides such as 2,4-dichlorophenoxyacetic acid, atrazine, endosulfan, and humic acid) are evaluated. It was observed that H2PO4 −, Fe2 +, endosulfan, and humic acid have maximum interference. Desorption of MG from exhausted SMA using acetone, and its reuse was studied. The regenerated adsorbent shows ∼ 80% efficiency on the removal of MG. The usability of SMA for the removal of MG from real wastewater was also examined. The kinetic equilibrium was attained within 1 h and the removal could be achieved up to ∼ 95% at a dose of 20 g/L. The adsorption followed Freundlich isotherm model better than the Langmuir model.

Sorption kinetics of arsenic on laterite soil in aqueous medium

The efficiency of a locally available laterite soil in removing both arsenite and arsenate from aqueous medium by adsorption was evaluated. It was observed that in batch experiment conducted at 0.5 mg/L initial concentration of arsenic, laterite soil could remove up to 98% of arsenite and 95% of arsenate under optimized conditions. The kinetic profiles under various conditions were developed. Both arsenite and arsenate removal followed pseudo—second order reaction kinetic model. Pore and film diffusion coefficients were determined from the half-time equation and film diffusion appeared to be the rate-limiting. This was further supported by multiple interruption tests.

Modeling and fixed bed column adsorption of As(V) on laterite soil

Laterite soil, an abundant locally available natural adsorbent, has been evaluated for As(V) removal from aqueous solutions in column mode operation. The column studies were conducted using columns of 10, 20, 30 cm bed depth with 2 cm internal diameter. Initial As(V) concentration was 0.5 mg/L and flow rate was 7.75 mL/min. Bohart and Adams sorption model was employed for the determination of different parameters like height of exchange zone, adsorption rate, time required for exchange zone to move, and the adsorption capacity. Effect of flow rate and initial concentration was studied. The adsorption capacity of the laterite soil for 0.5 mg/L of As(V) was found to be 62.32 mg/L, and the adsorption rate constant was 1.0911 L/mg h for the minimum bed depth of 8.47 cm. The column was designed by the BDST model. Freundlich isotherm model was used to compare the theoretical and experimental breakthrough profile in the dynamic process. The bed saturation obtained was 36–80%. Regeneration of the exhausted column was possible with 1M NaOH.

Adsorption Based Technologies for Arsenic Removal from Aqueous Environment: A Review

In the present paper we have focussed on some different adsorbents for arsenic removal from aqueous media. Adsorbents were divided categorically in two different classes viz., synthetic adsorbents and natural adsorbents. Some of the synthetic adsorbents are found to be good for As(III) and some are good for As(V) removal. Similar results were also obtained for the natural adsorbents although removal efficiency depends on a number of factors. The efficiency of the adsorbents, merits and demerits were discussed in detail. Finally, we have discussed on some patented technologies and some potential filters.

Behaviour of fixed-bed column for the adsorption of malachite green on surfactant-modified alumina.

Prepared surfactant-modified alumina (SMA) was used to remove malachite green (MG) from aqueous media. At a dose of 10 g/L, SMA removed ∼ 99% MG (initial concentration 100 mg/L). The adsorption capacity (Q max) of SMA was 185 mg/g as calculated from Langmuir isotherm. In a fixed-bed column study, using the MG-spiked distilled water, the column design parameters were evaluated by Logit model at a bed depth of 10 cm. The adsorption rate constant (K) and adsorption capacity (N o ) was obtained as 0.002636 L/(mg h) and 76283.16 mg/L for the minimum bed depth 3.33 cm in the 1st cycle. Acetone was used for desorption of MG from SMA. In batch regeneration study, regenerated SMA could remove only ∼ 80% of MG under the same experimental conditions. In column regeneration study, the efficiency of the regenerated bed decreased and the values obtained as, K = 0.007931 L/mg h and N o = 12341.08 mg/L for the minimum bed depth of 6.83 cm. Column study was conducted with the real MG bearing wastewater (MG concentration was 396.54 mg/L) under the same experimental condition. The value of adsorption rate constant (K) and adsorption capacity (N o ) was obtained as 0.0008786 L/(mg h) and 197939.02 mg/L, respectively for the minimum bed depth 5.92 cm.

Galvanic replacement of As(0) nanoparticles by Au(III) for nanogold fabrication and SERS application

A galvanic replacement reaction between As(0) nanoparticles and Au(III) ions has been reported for the first time. Initially the stable yellow-brown As(0) nanoparticles were prepared by the borohydride reduction of an arsenite solution. The characterization of the As(0) particles was discussed in a recent report. In the present work these As(0) nanoparticles were exploited to fabricate gold nanoparticles (AuNPs). The as-obtained red colored gold sol showed a λmax at 540 nm and the size of the AuNPs were 62 ± 7 nm as observed from TEM analyses. It was interesting to note that the size of the AuNPs was comparable to that of the As(0) nanoparticles, which could be a sign of galvanic replacement in the absence of any stabilizer. The particles were spherical with a hollow core. The AuNPs were characterized by SEM, TEM, XRD, DLS and UV-visible spectroscopy. FTIR and Raman analysis indicated that during the galvanic replacement reaction As(0) was oxidized to arsenate, which stabilized the AuNPs through adsorption and H-bonding. Thus a stable assembly of AuNPs was obtained in the absence of any external stabilizer. The potential of such an assembly was further exploited for SERS detection of Rhodamine 6G, 4-mercaptopyridine and 4-aminothiophenol.