Effect of biosolid-derived dissolved organic matter on orthophosphate sorption to soils depends on clay mineralogy...

Dissolved organic matter (DOM) from biosolids can alter the sorption of orthophosphate (inorganic phosphorus (IP)) to soils and, therefore, affect the bioavailability of IP. It is not clear how clay mineralogy and solution composition interfere with DOM effects on IP sorption by soils. Hence, we studied the effect of DOM on IP sorption to five semi-arid soils dominated by either illite/smectite (I/S) or kaolinite clays. IP sorption isotherms were constructed in either NaCl or CaCl₂ background solution, with and without the addition of DOM. The IP sorption capacity maxima (S_MAX, Langmuir model) of the I/S soils were 33-102% higher in the presence of CaCl₂, as compared to NaCl. Although DOM had no effect on the IP-S_MAX in the presence of CaCl₂, it increased the IP-S_MAX by 35-59% in the presence of the NaCl solution. Surprisingly, DOM sorption to the I/S soils was 30-90% greater in the presence of a Na⁺-dominated solution, as compared to a Ca²⁺-dominated solution. In contrast to the I/S soils, the S_MAX of the kaolinitic soil was not affected by the background electrolyte (Na⁺, Ca²⁺) or the addition of DOM. Furthermore, the addition of IP reduced the sorption of DOM to the kaolinitic soil (by up to 50%) in both background electrolyte solutions. These results highlight the contrasting roles of divalent and monovalent cations in conjunction with DOM in IP sorption to semi-arid I/S soils. We propose a new approach based on two conceptual mechanisms to explain the DOM's enhancement of IP sorption to I/S soils. (1) Under dispersion conditions in the Na⁺-dominated solutions, Ca²⁺-mediated DOM-IP complexes bind to the clay's negative planar surfaces. (2) Under flocculation conditions in the Ca⁺-dominated solutions, the distance between adjacent platelets decreases, reducing both the electronegative charge spillover and Ca²⁺ bridge-mediated DOM sorption. In contrast, the addition of DOM to kaolinite, a multi-platelet clay with a low isomorphic negative charge, reduces IP sorption due to competitive sorption on the clay's broken edges.