The presence of natural arsenic exceeding 10 µg/L in groundwater is nowadays limiting their use as drinking waters in several parts of the world, including Italy. Among arsenic removal technologies, adsorption processes gained the upper hand and granular iron oxides filters (GFH) are the most widely used. In last decades the use of nanoparticles in water treatment has been explored, due to their elevated reactivity (high surface-to-area ratio). Iron nanoparticles have been successfully tested for As removal. Their main advantages are their high selectivity towards As, elevated adsorption capacity in terms of mgAs/gadsorbent, possible filter regeneration and reduction in waste produced by exhausted filters. In this context we tested the ability of green biogenerated iron hydroxides nanoparticles to remove arsenic from water. The strain Klebsiella oxytoca BAS-10 isolated from acid drainage mining area, characteristically produces a ferric hydrogel, consisting of branched heptasaccharide repeating units exopolysaccharide (EPS) with entrapped Fe nanoparticles. The red hydrogel obtained (FeEPS) was used in a dilute form having 2.4-2.5 mg Fe/ml and as dehydrated powder containing 0.3 mg Fe/mg. Batch studies at pH 7-7.5 were performed to assess the dose of FeEPS more suitable for the treatment, the form of the bio-sorbent more effective (hydrogel vs powder), the kinetic of removal process and adsorption isotherms under different level of As(III) and As(V) exposure. Different ratio hydrogel:solution (1:1 up to 1:250) were tested at a level of 1000 µg/L As(V). Solution 1:5 and 1: 25 were selected. The first kinetic studies (6 hrs) with As(V) spiked solution at 5000 µg/L showed 95.1 % and 58.6% removal for ratio 1:5 and 1:25, respectively. The efficiency after 30 min was already 90.1% for 1:5 and 41.8% for 1:10 solution. In the same conditions adsorption isotherms were carried out at 0.25-10 mgAs(V)/L. Maximum calculated Langmuir adsorption capacity were 32.1 and 77.9 mgAs/gFe for ratio 1:5 and 1:25, accordingly. These values are very effective if compared reported values for Fe (hydr-)oxides nanoparticles. Removal efficiency of hydrogel vs powder was tested (2000 µg/L As(V) for 24 hrs) both in ratio 1:5 and 1:20. The powder adsorption showed removal only after 2 hrs contact time with a slow increase up to 24 hrs, caused by minor penetrability of this dried material and consequent less accessibility for As to active adsorption sites. On the other side in this experiment As hydrogel desorption was observed after 2 hrs exposure. The initial high removal could be explained by a favourable As entrapment in the hydrogel structure, forming labile complexes and not really a chemiosorption step. The negatively charged surface for hydrogel at pH>3 (pHpzc=3-3.5) is theoretically unfavourable for arsenate adsorption. Comparison of As(III) and As(V) adsorption isotherms showed As(V) higher affinity for adsorption with saturation after 10 mg/L, on the contrary a sharp increase in As(III) adsorption is noticed at >5 mg/L. Preliminary studies on FeEPS hydrogel showed its good potentialities as "green" bio-material for As removal from contaminated waters. The material seems to be also suitable for As wastewater treatment and As-contaminated soils. Adsorption-desorption curves for hydrogel are needed to properly elucidate As adsorption mechanisms between gel phase and nanoparticles.
Potentialities of biogenerated iron hydroxides nanoparticles in arsenic water treatment
Casentini B;Rossetti S;
2015
Abstract
The presence of natural arsenic exceeding 10 µg/L in groundwater is nowadays limiting their use as drinking waters in several parts of the world, including Italy. Among arsenic removal technologies, adsorption processes gained the upper hand and granular iron oxides filters (GFH) are the most widely used. In last decades the use of nanoparticles in water treatment has been explored, due to their elevated reactivity (high surface-to-area ratio). Iron nanoparticles have been successfully tested for As removal. Their main advantages are their high selectivity towards As, elevated adsorption capacity in terms of mgAs/gadsorbent, possible filter regeneration and reduction in waste produced by exhausted filters. In this context we tested the ability of green biogenerated iron hydroxides nanoparticles to remove arsenic from water. The strain Klebsiella oxytoca BAS-10 isolated from acid drainage mining area, characteristically produces a ferric hydrogel, consisting of branched heptasaccharide repeating units exopolysaccharide (EPS) with entrapped Fe nanoparticles. The red hydrogel obtained (FeEPS) was used in a dilute form having 2.4-2.5 mg Fe/ml and as dehydrated powder containing 0.3 mg Fe/mg. Batch studies at pH 7-7.5 were performed to assess the dose of FeEPS more suitable for the treatment, the form of the bio-sorbent more effective (hydrogel vs powder), the kinetic of removal process and adsorption isotherms under different level of As(III) and As(V) exposure. Different ratio hydrogel:solution (1:1 up to 1:250) were tested at a level of 1000 µg/L As(V). Solution 1:5 and 1: 25 were selected. The first kinetic studies (6 hrs) with As(V) spiked solution at 5000 µg/L showed 95.1 % and 58.6% removal for ratio 1:5 and 1:25, respectively. The efficiency after 30 min was already 90.1% for 1:5 and 41.8% for 1:10 solution. In the same conditions adsorption isotherms were carried out at 0.25-10 mgAs(V)/L. Maximum calculated Langmuir adsorption capacity were 32.1 and 77.9 mgAs/gFe for ratio 1:5 and 1:25, accordingly. These values are very effective if compared reported values for Fe (hydr-)oxides nanoparticles. Removal efficiency of hydrogel vs powder was tested (2000 µg/L As(V) for 24 hrs) both in ratio 1:5 and 1:20. The powder adsorption showed removal only after 2 hrs contact time with a slow increase up to 24 hrs, caused by minor penetrability of this dried material and consequent less accessibility for As to active adsorption sites. On the other side in this experiment As hydrogel desorption was observed after 2 hrs exposure. The initial high removal could be explained by a favourable As entrapment in the hydrogel structure, forming labile complexes and not really a chemiosorption step. The negatively charged surface for hydrogel at pH>3 (pHpzc=3-3.5) is theoretically unfavourable for arsenate adsorption. Comparison of As(III) and As(V) adsorption isotherms showed As(V) higher affinity for adsorption with saturation after 10 mg/L, on the contrary a sharp increase in As(III) adsorption is noticed at >5 mg/L. Preliminary studies on FeEPS hydrogel showed its good potentialities as "green" bio-material for As removal from contaminated waters. The material seems to be also suitable for As wastewater treatment and As-contaminated soils. Adsorption-desorption curves for hydrogel are needed to properly elucidate As adsorption mechanisms between gel phase and nanoparticles.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
