Electrodialytic removal of Cd from biomass combustion fly ash suspensions

Abstract

h i g h l i g h t sElectrodialytic remediation could be used to remove Cd from biomass fly ash suspensions. Cd limiting concentrations in the fly ash were met for reuse as fertilizer. pH below 2 in the fly ash suspension was determining the effectiveness for Cd removal. a r t i c l e i n f o a b s t r a c tDue to relatively high concentrations of Cd, biomass combustion fly ashes often fail to meet Danish legislative requirements for recycling as fertilizer. In this study, the potential of using electrodialytic remediation for removal of Cd from four different biomass combustion fly ashes was investigated with the aim of enabling reuse of the ashes. The ashes originated from combustion of straw (two ashes), wood chips, and co-firing of wood pellets and fuel oil, respectively. A series of laboratory scale electrodialytic remediation experiments were conducted with each ash. The initial Cd concentration in the ashes varied between 8.8 mg Cd/kg (co-firing ash) and 64 mg Cd/kg (pre-washed straw ash), and pH varied from 3.7 (co-firing ash) to 13.3 (wood ash). In spite of such large variations between the ashes, the electrodialytic method showed to be sufficiently robust to treat the ashes so the final Cd concentration was below 2.0 mg Cd/kg DM in at least one experiment done with each ash. This was obtained within 2 weeks of remediation and at liquid to solid (L/S) ratios of L/S 16 for the pre-washed straw ash and L/S 8 for the straw, co-firing and wood ash.

The Physicochemical Properties and Composition of Biomass Ash and Evaluating Directions of its Applications

“…Despite the general belief in the beneficial impact of biomass ash on the environment, numerous research has indicated the presence of heavy metals in biomass ash, and their content depends primarily on the type of biomass burned [14]. Concentrations of metals in wood ash varies in wide ranges: Cd 1-41 mg/kg [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], Co 3-27 mg/kg [17, 19-21, 23, 27, 29], Cr 11-9914 mg/kg [15][16][17][20][21][22][23][24][25][26][27][28][29][30][31][32], Cu 13-8793 mg/kg [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]…”

Electrodialytic remediation of fly ash from co-combustion of wood and straw

“…Previous experiments were conducted in the three/five-compartment cell on straw or wood ash separately [5,10,14,22]. Ottosen et al [10] conducted EDR experiments for 14 days at the current density of 0.8 mA cm -2 (corresponding to approximately 40 mA), and achieved removal of Cd of 75% and 45% from straw ash and wood ash, respectively.…”

“…Kirkelund et al [5] treated a straw fly ash and a pre-washed straw ash with high initial Cd concentrations of 19 and 64 mg kg -1 TS, respectively. They achieved a Cd concentration below 2 mg kg -1 TS in the raw straw fly ash after treatment at a L/S ratio of 7 and a current of 40 mA (equivalent to the current density of 0.8 mA cm -2 ) for 14 days.…”

“…The half reactions are: (1) cathode reaction 2H 2 O+2e - H 2 ↑ + 2OH -, and (2) anode reaction 2H 2 O  O 2 ↑ +4H + +4e -. Removing Cd from biomass combustion fly ash in a three-or five-compartment EDR setup was reported in [5,10,[13][14][15], and EDR turned out to be successful. Not only heavy metals but also other mobile element ions, such as K and P, are removed from ash suspensions during EDR.…”

“…This type of bio-ash usually contains plant nutrients (K and P) which have value as fertilizers [5,6]. The bottom ash can normally be spread on agricultural fields, thus using the fertilizer value directly [7].…”