Mutual Interactions of Sulfate, Oxalate, Citrate, and Phosphate on Synthetic and Natural Allophanes

Abstract

In soil environments, particularly at the soil–plant interface, organic and inorganic ligands may compete with each other for sorption sites onto soil components. The interaction between two or more ligands has a great importance for understanding the adsorption/desorption processes of a given ligand in soil. We studied the mutual interaction of sulfate (SO4), oxalate (OX), citrate (CIT), and phosphate (PO4) in binary and ternary systems on synthetic and natural allophanic samples at different pH values (4.0–7.0). The adsorption capacity of SO4, CIT, and OX (in the order listed) was much lower than that of PO4, but CIT decreased the zero point charge (ZPC) more than OX, meanwhile PO4 increased the ZPC, whereas SO4 did not change the ZPC of the sorbents. The adsorption of SO4 was strongly inhibited in the presence of equimolar amounts of OX, CIT, or PO4 even at low surface coverage of all the ligands. The lower the final pH of the binary systems, the greater the inhibition of SO4 adsorption. The capacity of CIT to prevent SO4 adsorption (ranging from 22–63%) was similar or slightly greater than that of PO4 (ranging from 25–48%) but clearly higher than OX (ranging from 20–42%). On the contrary, SO4 very poorly prevented the adsorption of the other ligands (usually lower than 10%). The effectiveness of the other ligands in retarding or inhibiting SO4 adsorption on an Andisol sample containing 40% allophane decreased with increases in the reaction time. However, even after 15 d of reaction CIT (38%) prevented SO4 adsorption more than PO4 (27%) or OX (12%) did. Our results seem to demonstrate that changes in the electric potential of the surfaces after the addition of anions, as well as competition for adsorption sites, affected the reduction in adsorption of SO4 and the other ligands. However, competition in adsorption with SO4 for common adsorption sites was greater for the organic ligands than for PO4, because the surface coverage of PO4 was particularly low with respect to that of SO4, CIT, and OX.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…Furthermore, one can see that 5-10 min is enough to achieve adsorption equilibrium under the experimental conditions used in this study, which indicates that physical adsorption rather than ion exchange contributes mainly to the removal of metal ions by natural and synthetic allophane adsorbents. This finding is in line with previous experimental results, where allophane has been successfully used in the remediation of aqueous solutions that were contaminated with, for example, heavy metals cations [15,22,33,35], metal (oxy)anions [18,23,30], heterocyclic organic components [10] and anionic surfactants and organic acids [17,23]. …”

“…Allophane "nanoballs" are characterized by a high surface area (> 300 m²/g). They are well-known to adsorb appreciable amounts of ionic or polar pollutants due to an amphoteric ligand exchange capacity [17,22,23]. To date, however, the sorption properties of allophane have not been effectively utilized in the remediation of wastewater.…”

Phosphorus accumulation and pollution potential in a hapludult fertilized with pig manure

“…The lower MPAC values in the soil subjected to the application of PM1 may be attributed to the history of P application, which favored the occupation of P adsorption sites, especially those located in inorganic reactive soil particles (Berwanger et al, 2008). However, it should be emphasized that the lowest value of MPAC found in the 20-30 cm layer in the PM1 treatment may also be related to a greater adsorption of organic ligands at the P adsorption sites derived from the decomposition of the applied organic residues (Jara et al, 2006;Ashworth & Alloway, 2008;Zhang & Zhang., 2010).…”

Competitive sorption of molybdate and phosphate in Andisols

“…In order to determine the anion nutrient availability in Andisols we studied the anion sorption , and competition between selenite and phosphate (Barrow et al, 2005), and sulphate and phosphate (Jara et al, 2006) adsorption. According previous studies (Vistoso et al, 2009;Lopez et al, 2007) The high anion retention capacity of Andisols affects the mobility and bioavailability of anions such as phosphate and molybdate, thereby resulting in the low efficiency of phosphate and molybdate fertilizers in these soils (Ribera et al, 2010).…”