Bài giảng soil sorbtion

Sorption   Gases or liquids being incorporated into another material of a different state and adhering to the surface of another molecule Absorption and Adsorption Importance of sorption Soil sorption Predictions of contaminant transport Sorption in filtration Filtra(on  (decontamina(on)  of  water  and  gas  in  remedia(on  technologies   What  the  difference  between  absorp2on  and   adsorp2on  and  sorp2on?     The  sorp(on  of  a  contaminant  is  one  of  the  significant  processes  that   can  hinder  the  remedia(on  of  a  ground  water  aquifer  system  Sorp(on   is  defined  in  ground  water  the  sorbing  species  ,  usually  an  organic   compound,  is  called  the  sorbate,  and  the  solid  media,  usually  soil,  to   which  the  sorbate  is  a@racted  is  known  as  the  sorbent     Sorption reactions generally occur over a short period of time, however if the adsorbed contaminant begins to be incorporated into the structure of the sorbent , a slow occurring reaction, known as absorption, begins to take place To be more precise , the difference between adsorption and absorption is that adsorption is the attraction between the outer surface of a solid particle and a contaminant, whereas absorption is the uptake of the contaminant into the physical structure of the solid     Importance of sorption   •  The  technological,  environmental  and  biological   importance  of  adsorp(on  can  never  be  in  doubt  Its   prac(cal  applica(ons  in  industry  and  environmental   protec(on  are  of  paramount  importance  The   adsorp(on  of  substrates  is  the  first  stage  in  many   cataly(c  processes   •  The  methods  for  separa(on  of  mixtures  on  a   laboratory  and  on  an  industrial  scale  are  increasingly   based  on  u(lising  the  change  in  concentra(on  of   components  at  the  interface   •  Moreover,  such  vital  problems  as  purifica(on  of  water,   sewages,  air  and  soil  are  involved  here  too     Soil sorption   •  Mechanical  sorp2on  –trapping  of  par(cles  and   colloids  in  dead-­‐end  pores  and  porenecks   •  Adsorp2on  on  interphase     •  Ion  exchange     •  Chemisorp2on  (complexa(on)   •  Biological  sorp2on  (inges(on  of  the  chem   compound  by  organisms)   Distribution of Inorganic contaminants (metals) in soil   •  Dissolved in pore water •  Adsorbed on sorption site •  Specifically adsorbed on inorganic soil constituents •  Associated with insoluble soil organic matter aqueous phase adsorbed phase •  Precipitated as solids           •  Present in the structure of minerals    solid phase Metals – „aqueous phase“   •  Free metal ions (eg Cd2+, Ni2+, Zn2+, Cr3+) or •  Complexes (eg, CdSO40, ZnCl+, CdCl3-) •  Ligands Cl-, HS-, OH-, HCO3-, SO42-, CO32- Form soluble complexes with metals Example: •  Zn2+ + Cl- = ZnCl+ ZnCl+ + Cl- = ZnCl2 These  reac2on  can  decrease  the  ionic  strength  of  a   solu2on  and  therefore  increase  solubility  of  metals  -­‐>   increase  of  contaminant  mobility   Cation exchange capacity CEC   •  •  •  •  The sum of exchangeable cations CEC = equivalent charges / mass unit Units: (me kg-1 or meq / 100 g) Sandy soils CEC < 100 meq.kg-1 Clays CEC > 100 meq.kg-1 Peat CEC up to 1500 me.kg-1 •  Adsorption of contaminants depends on soil CEC values The higher is CEC, higher the adsorption cationic contaminants to the surface, the higher the adsorption   Sorption of toxic metals   •  General order of preference or cations to adsorb Pb > Cr > Cu > Cd > Ni > Zn Higher order of pref Lower order of pref Sorption of cation is influenced by: CEC, PZC, pH, surface area, Eh, Ionic strenght   ! When  preparing  graphs  of  >sorbed=  concentra(on  versus  aqueous  concentra(on,  this   phenomenon  typically  appears  as:    a)  gradual  increase  in  sorbed  concentra(on  as  aqueous  concentra(on  increases   B)  a  lack  of  an  apparent  adsorp(on  maximum  (b)  with  a  con(nuous  increase  in  solid-­‐ phase  adsorbate  as  increasing  amounts  of  the  precipitate  form   ! Desorp2on       For  remedia(on  of  contaminated  soils,  desorp(on  is  a  more  cri(cal   phenomenon  than  adsorp(on   Major  concerns  -­‐  contamina(on  of  water  supplies,  bioavailability       Contaminants  that  are  strongly  bound  may  be  immobile  or  desorb  at  very  slow   rates       Desorp(on  is  usually  observed  to  be  slower  than  sorp(on  and  not  all  sorbed   material  may  desorb       Hysteresis  -­‐  Adsorp(on  and  desorp(on  isotherms  do  not  coincide   -­‐  Adsorp(on  is  par(ally  irreversible     Studied  most  frequently  with  sorp(on  of  organic  compounds   -­‐  Thought  to  arise  from  compounds  being  trapped  in  pores  and  diffusion-­‐ limited  or  hindered  desorp(on   -­‐  Can  also  arise  through  forma(on  of  chemical  bonds  which  limit  release  of   the  absorbate   Sorbates  sorbed  for  long  periods  usually  show  greater  hysteresis     ! [...]...  apparent  adsorp(on  maximum  (b)  with  a  con(nuous  increase  in  solid-­‐ phase  adsorbate  as  increasing  amounts  of  the  precipitate  form   ! Desorp2on       For  remedia(on  of  contaminated  soils,  desorp(on  is  a  more  cri(cal   phenomenon  than  adsorp(on   Major  concerns  -­‐  contamina(on  of  water  supplies,  bioavailability       Contaminants  that  are  strongly  bound  may  be