Influence of the anionic and non-ionic surfactants on precipitation and transformation of the struvite to the newberyite

Struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4 ∙ 6H2O) and newberyite (magnesium hydrogen phosphate trihydrate, MgHPO4 ∙ 3H2O), considerably rare minerals, occur as constituents of renal and vesical calculi in animals and humans, mostly those related to infections of urinary tract.1, 2 Moreover, struvite is often found as scale deposit in pipes and on the surface of heat exchangers in the wastewater and sludge industry.3 It is known that struvite is formed in anaerobic digesters when the concentration of magnesium in the sludge is considerably high. In this work the influence of anionic (SDS) and nonionic (Triton X-100) surfactant on the struvite spontaneous precipitation and its transformation to newberyite was investigated. The crystals were prepared under controlled conditions by fast mixing of the cationic and anionic reactant solutions and growing them with magnetic stirring. The initial conditions were: pH = 8.5, reactant concentrations: c(NH4H2PO4) = c(MgCl2)= 4•10-2 mol dm-3 at 25 oC. The respective surfactant (cmc/2≤c(PAT)≤2cmc) was added to the anionic component prior to pH adjustment. Precipitation process was followed by pH measurements and the crystals were visualised by light microscopy. The solid phase composition was determined by thermogravimetric (TG) analysis and FT-IR spectroscopy. In all systems crystals were formed immediately after mixing the reactant solutions. Depending on added surfactant concentration newberyite starts to appear 15-90 min after commencement of precipitation. Transformation of struvite to newberyite is completed in control system after 3 hours reaction time. Addition of SDS and Tritone X-100, in all investigated concentrations, inhibits this transformation.