Aqueous release kinetics of nitrogen and potassium from a citric acid–crosslinked lignosulfonate biopolymer matrix

Conventional nitrogen and potassium fertilisers exhibit rapid dissolution in aqueous environments, resulting in low utilisation efficiency and significant nutrient losses. In this study, a citric acid–crosslinked lignosulfonate biopolymer matrix containing nitrogen and potassium was synthesised and evaluated for its controlled nutrient-release properties in aqueous media. The biopolymer fertiliser was evaluated against conventional fertilisers (urea and muriate of potash) under varying pH (4.0, 7.0 and 9.0) and temperature (25 °C and 35 °C) and water conditions (distilled and saline at 2 dS m-1). Under neutral conditions, cumulative nutrient release reached 93.73 % for nitrogen and 83.42 % for potassium at pH 7.0, whereas release declined under acidic and alkaline pH conditions due to changes in polymer swelling and nutrient diffusion dynamics. Temperature significantly influenced nutrient release, with maximum cumulative release observed at 35 °C, reaching 97.32 % for nitrogen and 89.12 % for potassium. In contrast, saline water conditions (2 dS m-1) reduced total nutrient release to 76.31 % nitrogen and 72.53 % potassium by day 30. Conventional fertilisers such as urea and muriate of potash (MOP) released more than 99 % of their nutrients within 3 days. Kinetic modelling revealed that nutrient release from the polymer matrix followed anomalous (non-Fickian) transport and was best described by the Korsmeyer-Peppas model. Furthermore, a maize hydroponic bioassay demonstrated improved seedling performance, with 96.23 % germination, longer root and shoot lengths, higher seedling vigour index (SVI) (1673.91) and increased Soil Plant Analysis Development (SPAD) chlorophyll value (39.80). These findings demonstrate the fertiliser’s controlled-release efficiency, environmental safety and potential for precision agriculture.