Title : 
  pH - dependency of P adsorption kinetics on goethite 
 Abstract_text_1 : 
  pH - dependency of P adsorption kinetics on goethite . 
 Abstract_text_2 : 
  P adsorption was investigated on synthetic and commercially available goethite ( 99 % , Alfa Aesar ) . The composition and crystallinity was confirmed by X - ray diffraction analyses with an Empyrean powder diffractometer ( PANalytical ) with Cu Kα radiation ( λ=0.15418 nm ) . The determination of the d - values and the verification of the calculated lattice parameter were performed using the XPowder software . The specific surface area of 17.2 m2 g-1 was determined with an Autosorb-1 ( Quantachrome ) using a five - point BET measurement and N2 . The point of zero charge ( PZC ) was determined using potentiometric titration by weighing 0.5 g of goethite into 50 ml PE - tubes and adding of 10 ml of a 0.02 M , 0.2 M , and 2 M KCl solution , respectively . The KCl solutions were diluted with ultrapure water to a total volume of 20 ml , leading to final concentrations of 0.01 M , 0.1 M , and 1 M , respectively . After closing the tubes , the samples were equilibrated for 4 days at 25 ° C to reach an equilibration pH value . Following , the suspension was titrated with 0.1 M KOH , and the pH was recorded after each addition of KOH as a function of the volume . A similar procedure was used for the blank solutions . The adsorbed amounts of H+ or OH- were calculated from the amount of KOH added to the samples minus the amount of KOH used by blank titration . The titration curves of the three different concentration steps intersect at the PZC , which was 8.8 for pure goethite without adsorbed P. P adsorption on goethite was studied in a batch setup at the pH values of 4 , 6 , 8 , 10 , and 12 . Therefore , goethite sample material ( 5 g ) for the respective pH steps was adjusted to pH 4 , 6 , 8 , 10 , or 12 , respectively , in ultrapure water with 0.01 M HCl ( pH 4 ) , or 0.1 M to 5 M KOH ( pH 6 to 12 ) using a Potentiometric Compact Titrator G20S and combined DGi115 - SC pH electrode ( Mettler - Toledeo ) under continuous stirring . For P adsorption , 0.5 g of goethite and 40 ml of a 0.15 mmol l-1 KH2PO4 , solution , adjusted to pH 4 , 6 , 8 , 10 , or 12 as well , were mixed to achieve a solid - solution ratio of 1:80 . The goethite- KH2PO4 mixture was shaken horizontally at 150 motions min-1 for 24 h. The P concentration in the solution was determined after reaction times of 2 , 24 , 48 , and 168 h. The samples have been centrifuged for 15 min at 3500 rpm and the clear supernatant was filtrated by using P - poor Whatmann 512 ½ filters . The concentrations of P and Fe were determined using inductively coupled plasma atomic emission spectroscopy ( ICP - AES ) . The adsorption experiments were done in duplicate . Under real conditions in the soil , an influence of the P sorption on the pH is to be expected . Therefore , the pH change was taken into account both at the beginning and at the end of the adsorption experiments . The pH change was measured in a reference experiment that was treated exactly under the same conditions as P adsorption experiment . The amount of adsorbed P was referred to both the initial and the final pH. Considering the initial pH , P adsorption on goethite was highest for pH 8 , yielding a P surface coverage of 0.75 µmol m-2 after a reaction time of 168 h , followed by pH 6 with 0.72 µmol m-2 , and pH 4 with 0.68 µmol m-2 . The extent of adsorption decreased from pH 8 to 10 with 0.58 µmol m-2 and was lowest for pH 12 with 0.18 µmol m-2 . The solution pH decreased for the initial pH 4 to 3.9 . , and for the initial pH of 6 , the final pH was 5.0 . For initial pH values 8.0 and 10.0 , the corresponding final pH values were 6.7 and 7.6 , respectively . At initial pH 12.0 , the decrease of the solution pH weakened with a final pH of 11.6 . Research domain : Soil Sciences Research question : How does the solution - pH affect the amount of adsorbed phosphorus on the goethite surface ? 
 Keywords : 
  Soil , phosphorus , nonmetals , adsorption , Soil , Phosphorus , Adsorption , pH , Goethite , Hydroxides , Boden , Phosphor , Adsorption