Henrik K. Hansen

Water splitting at ion-exchange membranes and potential differences in soil during electrodialytic soil remediation

The optimum current for electrodialytic soil remediation occurs when the limiting current of the anion-exchange membrane is exceeded while that for the cation-exchange membrane is not. At this current, an acidic front will pass through the soil from the anion-exchange membrane towards the cathode, and the polluting heavy metals will be mobilized in the acidic environment. At the same time no production of base will occur from the cation-exchange membrane. A basic environment causes precipitation of hydroxides in the soil next to the cation-exchange membrane, and this will give an increase in voltage drop in the system and furthermore hinder the transport of the heavy metals out of the soil. When the acidic front passes through the soil, the voltage drop will decrease, and the end of the remediation can be predicted by the decrease in voltage to a very low level between the working electrodes.

Electrodialytic remediation of soils polluted with Cu, Cr, Hg, Pb and Zn

Electrodialytic remediation of heavy metal polluted soil is a method which combines the technique of electrodialysis with the electromigration of ions in the polluted soil. Results from laboratory-scale remediation experiments of soil samples from three real contaminated sites with different heavy metals are presented. In the three cases it was possible to mobilize and reduce the amount of the pollutants in the soil. The pollutants were (1) copper and chromium, (2) mercury and (3) copper, lead and zinc. For a loamy sand polluted with copper and chromium it was possible to decontaminate the soil to an extent lower than the recommended critical values for metal concentration in soil. Parameters that were identified as important for the efficiency of the electrodialytic remediation method were pH in the soil, lime content and speciation of the heavy metal.

Speciation and mobility of cadmium in straw and wood combustion fly ash.

Two fly ashes from biomass combustion have been analysed regarding cadmium speciation and mobility. A fly ash from straw combustion contained 10 mg Cd/kg dry matter, and around 50% of the cadmium was leachable in water. The possible main speciation of cadmium in this fly ash was CdCl2. When adding this fly ash to agricultural soil a threat for groundwater contamination and plant uptake is existing. A fly ash from wood chip combustion had 28.6 mg Cd/kg dry matter. In this fly ash, the cadmium was bound more heavily, with only small amounts of cadmium leached in mild extractants. A possible speciation of cadmium in this fly ash was as oxide or as CdSiO3. Long-term effects and accumulation of cadmium could be a problem when adding this fly ash to agricultural or forest soils.

Interference subspace rejection: a framework for multiuser detection in wideband CDMA

We present a unifying framework for a new class of receivers that employ linearly-constrained interference cancellation (IC). The associated multiuser detectors operate in various modes and options ranging in performance from that of IC detectors to that of linear receivers, yet provide more attractive performance/complexity tradeoffs. They exploit both space and time diversities as well as the array-processing capabilities of multiple antennas and carry out simultaneous channel and timing estimation, signal combining and interference rejection. Additionally, they can operate on both links and in multiple mixed-rate traffic scenarios. The improved performance can be translated to increased utilization of wideband code division multiple access networks, particularly at high data rates.

Removal of Cu, Pb and Zn in an applied electric field in calcareous and non-calcareous soils

The pH dependency of the removal of Cu, Zn and Pb by electrodialytic soil remediation from different industrially polluted soils was examined. From 18 experiments performed with five different soils, it was found that the order of mobilization due to a pH decrease was Zn>Cu>Pb. It was found, too, that each of the elements was removed at higher soil pH in calcareous soils (about 12% carbonates) than in soils with a carbonate content of less than 3.7%. In soils rich in carbonates, precipitation of heavy metal carbonates is an important retention mechanism and the heavy metal carbonates are dissolved at higher pH values than the pH at which heavy metals are desorbed in non-calcareous soils. Thus, the relation between the soil pH and the mobility of the heavy metal in the electric field is not only dependent on the heavy metal in focus, but also on the fraction of the heavy metal precipitated as carbonates.

Adsorption of copper onto agriculture waste materials.

Mining activities generate large amounts of wastewater, which contain heavy metals of elevated concentration according to legislative threshold values. Therefore, treatment is necessary, and there has been a recent focus on more environmentally friendly methods. One of these is biosorption, where heavy metals are adsorbed from the wastewater via materials of biological origin. In this work copper sorption capacity, kinetics and isotherms of different low-cost residual agricultural materials was studied. Seven different materials were investigated: peanut shells, nut shells, plum seeds, eucalyptus bark, olive pips, peach stones, and pine sawdust. The best sorption results were obtained in acidic pH for olive pips, peach stones and pine sawdust. Furthermore, it was observed that at higher pH, a longer duration of time is required before equilibrium is established. In general, the better biosorbents were found to be peach stones and pine sawdust with a sorption capacity at acidic pH around 10-15 mgCu g(-1) biosorbent. In addition it was found, that the Ho and McKay second order model described the sorption kinetics very satisfactorily. Both Langmuir and Freundlich models described the equilibrium sorption isotherms well for the biosorbents studied - with the last model being slightly better.

Electrodialytic Removal of Heavy Metals from Different Solid Waste Products

A variety of heavy metal polluted waste products must be handled today. Electrochemical methods have been developed for remediation of polluted soil. One of the methods is the electrodialytic remediation method that is based on electromigration of heavy metal ions and ionic species within the soil matrix, and a separation of the soil and the process solutions, where the heavy metals are concentrated, with ion exchange membranes. For remediation of some soils, such as calcareous soils, it is necessary to add an enhancement solution. It was shown in a laboratory experiment that ammonium citrate could be used when removing Cu and Cr from a soil with 25% carbonates. The final concentrations of the elements were below the target values after the remediation. A question of whether the electrodialytic remediation method can be used for other waste products arose. Preliminary experiments showed that the method could be used for removal of different heavy metals from impregnated wood waste, fly ash from straw combustion, and fly ash from municipal solid waste incineration. The best result was obtained with the wood waste where more than 80% of each of the polluting elements Cu, Cr and As was removed in a 7-day experiment in which oxalic acid was used as enhancement solution. From the straw ash, 66% of the Cd was removed, but 64% of the fly ash dry mass dissolved during the treatment. In this actual experiment, no enhancement solution was used but that will be necessary to avoid dissolution of the ash to such a high extent. For the fly ash from waste incineration, ammonium citrate was tested as enhancement solution and in 14 days 62% Cd, 53% Cu, 6% Pb, and 31% Zn were removed. The preliminary results were thus promising for developing the electrodialytic method to other products than soil, although more research is needed especially in finding the best enhancement solutions for each product.

Utilization of electromigration in civil and environmental engineering—Processes, transport rates and matrix changes

Electromigration (movement of ions in an applied electric field) is utilized for supply or extraction of ions from various porous materials within both civil and environmental engineering. In civil engineering, most research has been conducted on the removal of chlorides from concrete to hinder reinforcement corrosion while in environmental engineering remediation of heavy metal polluted soil is the issue most studied. Never the less, experiments have been conducted with utilization for several other materials and purposes within both engineering fields. Even though there are many topics of common interest in the use of electromigration for the two fields, there is no tradition for collaboration. The present paper is a review with the aim of pointing out areas of shared interest. Focus is laid on the purposes of the different processes, transport rates of various ions in different materials and on changes in the matrix itself. Desorption and dissolution of the target elements into ionic form is a key issue to most of the processes, and can be the limiting step. The removal rate is generally below 1 cm day− 1, but it can be much less than 1 mm day− 1 when desorption is slow and insufficient. Matrix changes occurs under the action of the applied electric field and it includes both physico-chemical and hydrological changes. Some of the solid phases is weathered and new can be formed. Increased fundamental understanding of the effects and side effects, when applying the electric field to a porous material, can lead to improvement of the known technologies and possibly to new applications.

Electrochemical Analysis of Ion-Exchange Membranes with Respect to a Possible Use in Electrodialytic Decontamination of Soil Polluted with Heavy Metals †

Abstract Transport numbers in different metal chloride solutions were estimated using the emf method for two ion-exchange membranes: Ionics CR67 HMR412 (cation-exchange membrane) and Ionics AR204 SXRA 7639 (anion-exchange membrane). The cation-exchange membrane was found to work nearly ideally for NaCl and CaCl2 solutions even at high concentrations, whereas deviation from ideality was seen for ZnCl2 and CuCl2 solutions. The anion-exchange membrane showed transport numbers for the anion around 0.95 for NaCl, CaCl2 and ZnCl2 solutions for the concentration range investigated. Electrodialytic desalting experiments taken as a simplified simulation of the electrokinetic decontamination method showed that it was possible to remove all ions in the simulated soil volume, with a sharp increase in the potential difference over the soil volume as a result, and that it was possible to control the metal content in the different solutions in the electrodialytic decontamination method. †Dedicated to J⊘rgen Birger Jense...

Removal of Arsenic from Wastewaters by Airlift Electrocoagulation. Part 1: Batch Reactor Experiments

Abstract Arsenic removal from wastewater is a key problem for copper smelters. Conventional methods either prove to be complicated, expensive, or not sufficiently effective. This work shows the results of electrocoagulation (EC) in aqueous solutions containing arsenic in a newly designed and constructed cylindrical continuous airlift reactor. The residence time distribution measurements showed that the reactor behaved as an ideal continuous stirred tank reactor (CSTR) with perfect mixing. Ten EC experiments were carried out in the continuous airlift reactor with sacrificial iron electrodes. The variables were: initial As(V) concentration, liquid flow rate, and electric current density. The results showed that the airlift EC process could reduce an initial As concentration from 1000 mg L−1 to 220 mg L−1 – corresponding to a reduction of 78%. In addition, a 100 mg L−1 solution was reduced by 88%. The Fe-to-As (mol/mol) ratio, when EC was working properly, was in the range of 1.3–1.5, which is very promising for the future development of the reactor. The arsenic removal is proportional with the electric current, the electric charge and the CSTR residence time. On the other hand, when the flow rate is increased, the arsenic removal decreases.