Simultaneous removal of As, Cd, Cr, Cu, Ni and Zn from stormwater using high-efficiency industrial sorbents: Effect of pH, contact time and humic acid.

The effect of contact time, solution pH, and the presence of humic acid (HA) on the combined removal of As, Cd, Cr, Cu, Ni and Zn is investigated in batch tests using alumina, granulated activated carbon (GAC), and bauxsol coated sand (BCS) as sorbents. It is found that the equilibrium time for Cd, Cu, Ni and Zn is about 4h, while no clear equilibrium is observed for As and Cr. It is also found that increasing the pH until pH~8 enhanced Cd, Cu, Ni and Zn removal, but increasing the pH above this point had no major effect. In the cases of As and Cr, higher pH values (i.e. >7) decreased their removal. The presence of both 20 and 100mg/L HA suppressed the heavy metal removal except for Cr, and the suppression was higher at the higher HA concentration. Geochemical simulations suggest that this is due to the formation of dissolved HA-metal complexes preventing effective metal sorption. In the case of Cr, the presence of HA increased the removal when using alumina or BCS, while hindering the removal when using GAC. The findings show that the pH-value of the stormwater to be treated must be in the range of 6-7 in order to achieve removal of the full spectrum of metals. The results also show that natural organic matter may severely influence the removal efficiency, such that, for most metals the removal was reduced to the half, while for Cr it was increased to the double for alumina and BCS. Consequently, a properly working filter set up may not work properly anymore when receiving high loads of natural organic acids during the pollen season in spring or during defoliation in autumn and early winter, and during mixing of runoff with snowmelt having a low pH.

Simultaneous removal of As, Cd, Cr, Cu, Ni and Zn from stormwater: experimental comparison of 11 different sorbents.

The potential of using alumina, activated bauxsol-coated sand (ABCS), bark, bauxsol-coated sand (BCS), fly ash (FA), granulated activated carbon (GAC), granulated ferric hydroxide (GFH), iron oxide-coated sand (IOCS), natural zeolite (NZ), sand, and spinel (MgAl(2)O(4)) as sorbents for removing heavy metals from stormwater are investigated in the present study. The ability of the sorbents to remove a mixture of As, Cd, Cr, Cu, Ni and Zn from synthetic stormwater samples were evaluated in batch tests at a starting pH of 6.5. The metal speciation and saturation data is obtained using the PHREEQ-C geochemical model and used to elucidate the sorption data. It is found that BCS, FA, and spinel have significantly higher affinity towards heavy metals mainly present as cationic or non-charged species (i.e. Cd, Cu, Ni and Zn) compared to those present as anionic species (i.e. As and Cr). However, IOCS, NZ and sand have higher affinity towards As and Cr, while alumina has equally high affinity to all tested heavy metals. The Freundlich isotherm model is found to fit the data in many cases, but ill fitted results are also observed, especially for FA, BCS and GAC, possibly due to leaching of some metals from the sorbents (i.e. for FA) and oversaturated conditions making precipitation the dominant removal mechanism over sorption in batches with high heavy metal concentrations and pH. Calculated sorption constants (i.e. K(d)) are used to compare the overall heavy metal removal efficiency of the sorbents, which in a decreasing order are found to be: alumina, BCS, GFH, FA, GAC, spinel, ABCS, IOCS, NZ, bark, and sand. These findings are significant for future development of secondary filters for removal of dissolved heavy metals from stormwater runoff under realistic competitive conditions in terms of initial heavy metal concentrations, pH and ionic strength.

Arsenate removal from water using sand--red mud columns.

This study describes experiments in which sorption filters, filled with chemically modified red mud (Bauxsol) or activated Bauxsol (AB) coated sand, are used to remove As(V) (arsenate) from water. Bauxsol-coated sand (BCS) and AB-coated sand (ABCS) are prepared by mixing Bauxsol or AB with wet sand and drying. Samples of the BCS and ABCS are also used in batch experiments to obtain isotherm data. The observed adsorption data fit the Langmuir model well, with adsorption maxima of 3.32 and 1.64 mgg(-1) at pH values of 4.5 and 7.1, respectively for BCS; and of 2.14 mgg(-1) for ABCS at a pH of 7.1. Test results show that higher arsenate adsorption capacities can be achieved for both BCS and ABCS when using the columns compared to results for batch experiments; the difference is greater for BCS. Additional batch tests, carried out for 21 days using BCS to explain the observed discrepancy, show that the equilibrium time previously used in batch experiments was too short because adsorption continued for at least 21 days and reached 87% after 21 days compared to only 35% obtained after 4h. Fixed bed column tests, used to investigate the effects of flow rate and initial arsenate concentration indicate that the process is sensitive to both parameters, with lower flow rates (longer effective residence times in the columns) and initial arsenate concentrations providing better column performance. An examination of the combined effect of potential competing anions (i.e. silicate, phosphate, sulphate and bicarbonate) on the column performance showed that the presence of these anions in tap water slightly decreases arsenate removal. Each breakthrough curve is compared to the Thomas model, and it is found that the model may be applied to estimate the arsenate sorption capacity in columns filled with BCS and ABCS. The data obtained from both batch and column studies indicate that BCS and ABCS filtration could be effectively used to remove arsenate from water, with the latter being more efficient.

Adsorption of arsenic from water using activated neutralized red mud.

In this paper activated seawater-neutralized red mud, herein referred to as activated Bauxsol (AB), is used as a novel adsorbent for removing inorganic arsenic (As) from water. The adsorption of As onto AB is studied as a function of contact time, particle size, pH, initial As concentration, AB dosage, and temperature. Kinetic data indicate that the process pseudoequilibrates in 3 and 6 h for As(V) (arsenate) and As(III) (arsenite), respectively, and follows a pseudo-first-order rate expression. Within the range tested, the optimal pH for As(V) adsorption is 4.5, and close to 100% removal can be achieved irrespective of the initial As(V) concentration. Desorption of As(V) is greatest at pH 11.6 where a maximum of 40% can be achieved. In contrast, the optimum pH for As(III) removal is 8.5, and the removal efficiency changes with the initial As(III) concentration. The adsorption data fit the Langmuir isotherm and its linearized form well, with thermodynamic data indicating the spontaneous and endothermic nature of the process. The FITEQL (V.4) and PHREEQC (V.2) computer programs are used to predict As(V) adsorption at various pH values (based on diffuse double layer models). The modeling results fit the experimental results very well and indicate that surface complexation modeling is useful in describing the complex AB surface during the adsorption process. This study shows that As(III) needs to be oxidized to As(V) for a favorable removal using AB and that AB can be a very efficient unconventional adsorbent for removing As(V) from water.

Increasing the arsenate adsorption capacity of neutralized red mud (Bauxsol).

The possibility of increasing the arsenate adsorption capacity of seawater-neutralized red mud (Bauxsol) through acid treatment, combined acid and heat treatment, and the addition of ferric sulfate (Fe(2)(SO(4))(3).7H(2)O) or aluminum sulfate (Al(2)(SO(4))(3). 18H(2)O) is investigated. The results show that acid treatment alone, as well as in combination with heat treatment increases the removal efficiency, with the combination providing the best removal. Adding ferric sulfate or aluminum sulfate, however, suppress the removal. The results also show that activated Bauxsol (AB) produced using combined acid and heat treatment can remove roughly 100% arsenate (at pH 4.5) with or without competing anions (i.e., phosphate, bicarbonate, and sulfate) when the initial arsenate concentration is < or = 2 mgl(-1). Furthermore, it is found that the adsorption process using AB is not accompanied by the release of unwanted contaminants, and TCLP results indicate that the spent AB is not hazardous. It is believed that the AB produced here has good potential as an alternative adsorbent to conventional methods for removing arsenate from water.