Adsorption Performance And Mechanism Of Arsenic From Water Using Iron/Iron-Oxide Amended Composite Adsorbents

Arsenic pollution in drinking water sources has become a global issue in recent years. The drinking of arsenic contaminated water threatens to over 200 million people in both developing and developed countries. Arsenic can exist in both organic and inorganic forms in the environment. The inorganic forms of arsenic (arsenite or As(III), and arsenate or As(V)) are considered in this study because the available literature reveal that the amount of organic arsenic in water sources is insignificant. Epidemiological studies prove that the massive exposure to arsenic causes serious and chronic health problems including skin cancer, hyper pigmentation, chronic lungs disease, liver fibrosis, and so on. Therefore, the removal of inorganic arsenic from potable water is central to public health safety. Using conventional adsorption process, arsenic removal from contaminated water has drawn considerable attention of researchers in recent years. So far, several types of adsorbing materials have been evaluated, but little attention was paid to rich sources of locally available adsorbents and to their feasible synthesis methods. In the present study, abundantly.available honeycomb briquette cinders (HBC) and Kans grass (Saccharum spontaneum) were employed as main adsorbent carriers after necessary amendment with iron/iron-oxide. The iron-coated HBC (referred as Fe-HBC), calcined HBC/Fe3O4 composite (referred as magnetic honeycomb briquette cinders (MHBC)), and magnetic biochar synthesis from Kans grass (referred as magnetic Kans grass biochar (MKGB)) were synthesized by chemical co-precipitation. Consequently, five extensive experiments were performed using the synthesized adsorbents; HBC, Fe-HBC, MHBC and MKGB in batch and column-based systematic studies for the selective removal of As(Ⅲ) or As(V) or both. Under different environmental conditions, batch and column experiments reveal the performance of arsenic removal by synthesized adsorbents and elucidate possibly the underlying adsorption mechanisms and kinetics.(1) The application of honeycomb briquette cinders by the low-cost mechanical granulation was integrated with surface amendment technology to prepare adsorbents (HBC and Fe-HBC) for the adsorptive removal of As(V) from aqueous solutions. Detailed characterizations were performed using FTIR, XRD, EDX and SEM techniques. Operating parameters including initial As(V) concentration, pH, contact time, adsorbent dose, iron leaching and the effects of competing ions on As(V) removal were evaluated. Results demonstrated that with the same adsorbent dose, nearly 75% As(V) removal was achieved using Fe-HBC as compared to 15% using HBC. The higher amount of As(V) (961.5 μg g-1) was adsorbed using Fe-HBC at pH 7.5 in 14 h contact time. Langmuir, Freundlich and Temkin isotherm models were used to analyze the adsorption data, whereas Langmuir model was found to best represent the data with a correlation co-efficient (R2=0.999). Thus, As(V) sorption on Fe-HBC surface suggested monolayer sorption by forming inner-sphere surface complexes and tabular crystal structures appeared on adsorbent surface after As(V) adsorption, as also revealed by SEM images. Moreover, the dimensionless parameter (RL) value was calculated to be about 0.118 that indicated the process was favorable and spontaneous. The influences of competing ions on As(V) removal followed the order:PO43-> HCO3-> F-> C1-. The profound inhibition effects of PO43-demonstrated its higher affinity towards iron(oxy)hydroxide. Findings from this study suggested that Fe-HBC was more effective than HBC and the reason could be the availability of more exchangeable surface sites.(2) The scaled-down sand filter can be used to treat contaminated water at small scale for household application in scattered settlements and the effective adsorbents further enhance its removal performance. The feasibility of HBC and Fe-HBC in saturated sand filter (SSF) for As(V) and As(Ⅲ) removal was investigated in the second experiment of this study. For this purpose, two filters; i.e. SSF(a) for As(V) and SSF(b) for As(Ⅲ) were designed and the removal experiments were performed at 23 ± 2.0℃ temperature in 24-days long treatment. Experimental parameters including dissolved oxygen (DO), pH, iron leaching and the effects of co-occurring ions on arsenic removal were investigated. Sampling times (ST) (5-120 min) during treatment days (TD) 1,6,12,18 and 24 were purposively decided to assess filters’ performance from tap water with the spiked of 200 μg L-1 of As(V) and As(Ⅲ) into SSF(a) and SSF(b), respectively. Results demonstrated that the removal efficiencies of arsenic in SSF(a) and SSF(b) were> 95% and> 85% till TD-12 which then decreased to 78% and 60%, respectively, on TD-24. Lower arsenic removal in SSF(b) could be the result of As(Ⅲ) slower oxidation at the studied influent pH under oxic conditions. Decreased effluent DO and increased iron leaching were measured in both the filters from TD-1 to TD-24. Minor variations of effluent pH and co-occurring ions (F-, NO3- and SO42-) were recorded, but no significant effect on arsenic removal was measured in the reported ranges. Performance of NaOH-regenerated filter-beds was evaluated and both the filters removed nearly 95% of arsenic in three-cycle treatments. This study suggested an in situ treatment; by the oxidation of As(Ⅲ) into As(Ⅴ) in the presence of DO and hydroxyl radical on adsorbent surfaces, followed by subsequent adsorption by Fe-As complexation. SSF containing cost-effective HBC and Fe-HBC could be an effective in situ treatment method for low concentration arsenic remediation to produce safe drinking water.(3) Calcination process has been recognized to enhance adsorbent surface efficiency. The prepared MHBC was calcined at 1000 ℃ under different environments (air and nitrogen) and reported in the third experiment. MHBC and the resultant calcination products (MHBC(A) and MHBC(N)) were investigated for the adsorptive removal of As(Ⅲ) and As(Ⅴ) from aqueous solutions as a function of solution’s pH, contact time, temperature, arsenic concentration and phosphate anions. Results demonstrated that at higher calcination temperature under nitrogen flow, MHBC converted into a new iron silicate product (fayalite, Fe2SiO4) via phase transformation as confirmed by characterizations analyses (FTIR, XRD and HT-XRD). In aqueous medium, ligand exchange between arsenic and the effective sorbent site (FeOOH) was established from the release of hydroxyl group. The removal of As(Ⅲ) and As(Ⅴ) remained stable in a wider pH range (4-10) using MHBC(N). Additionally, adsorption data fitted well to pseudo-second-order (R2> 0.995) rather than pseudo-first-order kinetics model. The adsorption of As(Ⅲ) and As(Ⅴ) onto adsorbent composites increased with an increase in temperature. Phosphate concentration (0.01 mmol L-1 or higher) demonstrated to strongly inhibit As(Ⅲ) and As(Ⅴ) removal through the mechanism of competitive adsorption. This study suggested that the adsorbent (MHBC(N)) prepared by selective calcination process could be useful to improve the adsorbent efficiency for enhanced arsenic removal from contaminated water bodies.(4) With the decrease in water adhesion properties of calcined adsorbents, an attempt has been made to employ MHBC, MHBC(A) and MHBC(N) in fixed-bed column for As(Ⅲ) removal. In the fourth experiment, the adsorption experiments of calcined products were investigated in column-based treatment. Characterizations revealed that the calcination at 1000 ℃ under nitrogen flow has significantly increased the adsorbent particles size; favored phase transformation and improved saturation magnetization (>20 Am2 kg-1). Additionally, the new iron silicate phase in aqueous medium generated highly reactive iron oxide species, which effectively adsorbed As(Ⅲ) from the influent water by ligand exchanges. In contrast, calcination under dynamic air drastically reduced the saturation magnetization (<1 Am2 kg-1) and assisted to form segregated magnetite, quartz and hematite, as revealed in XRD patterns. The breakthrough curve of each column was compared with Thomas model parameters and the adsorption performance followed the order:MHBC(N)> MHBC> MHBC(A). The maximum solid phase concentration (qT) was found to be about 56.07 mg g-1 for MHBC(N). The column beds could be successfully regenerated using 10% NaOH solution. This study suggested that the optimum calcination process for magnetic adsorbents development can more efficiently remove As(Ⅲ) from contaminated water using column-based treatment.(5) Magnetic biochar is increasingly known as a multi-functional material for pollutant remediation and the selection of appropriate synthesis method further increases the removal efficiency of contaminated matters from water. Last but not the least, the effects of synthesis methods on the fabrication of Kans grass straw/biochar from Kans grass with Fe3+/Fe2+ by chemical co-precipitation and subsequently pyrolyzing at 500 ℃ for 2 and 4 h were studied in detail, and compared their As(Ⅲ,Ⅴ) adsorption potentials under different operating conditions. Magnetic biochars (MKGB3 and MKGB4) prepared from KGS revealed higher As(Ⅲ,Ⅴ) adsorption efficiency and saturation magnetization (45.7 Am2 kg-1) than that of KGB (MKGB1 and MKGB2). Moreover, thermogravimetric analysis (TGA) demonstrated three stages of decomposition and MKGB3 and MKGB4 generated higher residual mass (> 60%) at stage 3 (1000 ℃) due to greater Fe3O4 contents in biochar matrix and turned to be thermally more stable. As(Ⅲ) and As(Ⅴ) adsorption equilibrium data well fitted the Langmuir model and followed the order:MKGB4> MKGB3> MKGB2> MKGB1. The maximum As(Ⅲ) and As(Ⅴ) adsorption capacities were about 2.0 mg g-1 and 3.1 mg g-1, respectively. Lower pH was found beneficial for As(Ⅴ) adsorption and the maximum As(Ⅲ) adsorption was recorded at pH 8. Arsenic adsorption efficiency increased with increase in aqueous temperature revealing an endothermic process. The data best fitted the pseudo-second-order (R2> 0.99) rather than pseudo-first-order kinetics model indicative of more complex mechanism. The adsorption of As(Ⅲ) and As(Ⅴ) was found to decrease with increase in ionic strength of competing ions and PO43- was found to strongly inhibit arsenic adsorption. Highest desorption was achieved at pH 13.5 using NaOH. This study suggested that the magnetic composite adsorbent has higher adsorption performance for arsenic and the selection of proper adsorbent synthesis method has a great influence on the adsorption ability of the adsorbents.