Investigation of the inhibitory effects of heavy metals on heterotrophic biomass activity and their mitigation through the use of natural minerals.

This study examined the inhibitory effects of lead, copper, nickel and zinc on heterotrophic biomass and their potential mitigation through the use of low-cost, natural minerals. Activated sludge was placed in batch reactors and specific heavy metal concentrations were added. Subsequently, the biomass specific oxygen uptake rate (sOUR) was determined to assess the level of biomass inhibition. Biomass inhibition by heavy metals followed the order Cu(2+)>Pb(2+)>Zn(2+)>Ni(2+), with copper being the most toxic metal, causing high inhibition of heterotrophic biomass even at relatively low concentrations (i.e. 10 mg·L(-1)). Zn had very small toxic effect at 10 mg·L(-1), while at 40 mg·L(-1) the level of biomass inhibition reached 80%. Nickel stimulated activated sludge activity at concentrations of the order of 10 mg·L(-1). The addition of 10 g·L(-1) bentonite and zeolite in activated sludge resulted in the decrease of the inhibitory effect of heavy metals on biomass respiratory activity. In some cases, mineral addition was very favorable as inhibition was reduced from 69-90% to less than 55% and even up to 12%. The beneficial action of minerals is attributed both to the adsorption of heavy metals on the mineral and on the potential aggregation between mineral and sludge particles.

Industrial wastewater pre-treatment for heavy metal reduction by employing a sorbent-assisted ultrafiltration system.

This work examined the adoption of a sorbent-assisted ultrafiltration (UF) system for the reduction of Pb(II), Cu(II), Zn(II) and Ni(II) from industrial wastewater. In such a system metals were removed via several processes which included precipitation through the formation of hydroxides, formation of precipitates/complexes among the metal ions and the wastewater compounds, adsorption of metals onto minerals (bentonite, zeolite, vermiculite) and retention of insoluble metal species by the UF membranes. At pH=6 the metal removal sequence obtained by the UF system was Pb(II)>Cu(II)>Zn(II)>Ni(II) in mg g⁻¹ with significant amount of lead and copper being removed due to chemical precipitation and formation of precipitates/complexes with wastewater compounds. At this pH, zinc and nickel adsorption onto minerals was significant, particularly when bentonite and vermiculite were employed as adsorbents. Metal adsorption onto zeolite and bentonite followed the sequence Zn(II)>Ni(II)>Cu(II)>Pb(II), while for vermiculite the sequence was Ni(II)>Zn(II)>Cu(II)>Pb(II) in mg g⁻¹. The low amount of Pb(II) and Cu(II) adsorbed by minerals was attributed to the low available lead and copper concentration. At pH=9 the adoption of UF could effectively reduce heavy metals to very low levels. The same was observed at pH=8, provided that minerals were added. The prevailing metal removal process was the formation of precipitates/complexes with wastewater compounds.

Regeneration of natural zeolite polluted by lead and zinc in wastewater treatment systems.

The aim of this work was to investigate the potential regeneration of natural zeolite which had been contaminated with lead and zinc contained in aqueous solutions, treated secondary effluent and primary treated wastewater. Several desorbing solutions were examined for the removal of Pb(II) and Zn(II) from zeolite and the highest desorption efficiency was obtained for 3M KCl and 1M KCl, respectively. The desorption process depended on the type and concentration of the desorbing solution, the metal being desorbed, the mineral selectivity towards the metal and the composition of the liquid medium where the adsorption process had taken place. Successive regeneration cycles resulted in the reduction of desorption efficiency by more than 50% after 9 and 4 cycles for lead and zinc, respectively. Kinetics examination showed that desorption was slower than adsorption, while metal ions which had been easily adsorbed were difficult to be desorbed. Adsorption was characterized by a three-stage diffusion process, while desorption followed a two-stage diffusion process.