ABSTRACT: This study describes a biotechnological strategy for producing and applying oxalic acid to solubilize phosphorus (P) from rock phosphate (RP). We evaluated six fungal species (Aspergillus niger FS1, Penicillium islandicum FS41, Pleurotus ostreatus PO1, Rhizoctonia solani Rhiz555, Sclerotium rolfsii Sr25, and Sclerotinia sclerotiorum Scl134) and three culture media (potato dextrose broth, Tsao and Strasser media) to maximize oxalic acid production. Among the fungal isolates tested and culture media, S. rolfsii Sr25 and Tsao medium showed efficient oxalic acid production. Tsao medium was optimized following a response surface methodology after initial screening of factors affecting RP solubilization. The optimized concentrations were 1 g L–1 NaNO3, 100 g L–1 glucose, 2 g L–1 KH2PO4, 4.5 g L–1 yeast extract, and 25 mg L–1 MgSO4·7H2O used for 20 days of incubation. Under these conditions, 71 mmol L–1 oxalic acid was obtained, representing a three-fold increase over production under non-optimized conditions (20 mmol L–1). Under optimized conditions, oxalic acid produced by S. rolfsii Sr25 reacted with low-solubility RP and solubilized 100 % of the P contained in ore. Thus, using S. rolfsii Sr25 to produce oxalic acid seems a promising biotechnological alternative for P solubilization from RP.

ABSTRACT: Although Brazil is one of the world's leading exporter of agricultural products, the country is highly dependent on the importation of potassic fertilizers. K-bearing silicate rocks are reported as potential solutions to reduce external K dependency. This work evaluated K extraction from silicate Verdete rock, a glauconite-bearing rock containing 10 % of K2O, by solubilization with organic acids. Firstly, Verdete rock was reacted during 3-120 h with solutions of citric or oxalic acid at 2 % (m/v) in Erlenmeyer flasks by shaking. Oxalic acid extracted 6.5 % of K in Verdete, while citric acid extracted 2.3 %. Another experiment was performed to evaluate the effect of various oxalic acid concentrations (2, 4, 6, 8, and 10 %) and differing reaction times (12, 24, 48, and 72 h) on K extraction from Verdete rock. Soluble K concentration nearly doubled with the increase of reaction time from 12 to 72 h, rising from 20 to 37 mg L–1. Increments in K extraction were obtained by increasing oxalic acid concentrations up to 6 % and above this concentration, no significant gain was observed. The X-ray diffraction data showed that K extraction resulted from the formation of oxalate-metal complexes with metals in Verdete rock.