Starch-based hydrogel loading with carbendazim for controlled-release and water absorption

Highlights

• Starch-based hydrogel loading carbendazim was successfully synthesized.

• Largest water absorption capacity leads to the slowest release rate.

• Release mechanism can be tuned by water absorption.

• Water-holding capacity of soil is improved.

Abstract

Starch, with properties of eco-friendliness and abundance, is one of the most important natural polymers. Starch-based hydrogels were investigated as carriers of carbendazim to combine controlled-release and water absorption (WA). Three carbendazim-loaded hydrogels (CLHs) with different WA capacities were prepared by solution polymerization. The CLHs were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and liquid-chromatography mass-spectrometry (LC–MS/MS). Release kinetics of CLHs was investigated using ¹⁴C-labeling method. The diffusion parameters of CLHs were 0.47, 0.57 and 0.81 in deionized H₂O (ddH₂O). WA affected release profile significantly, the release longevity reaching 240 h when WA was 800 g/g in ddH₂O. Solution pH influenced release profiles and the lowest release rate occurred in the lowest pH. Addition of CLH (1.3 g/kg soil) markedly increased water-holding capacity (WHC) of soil by 8.2%. The study indicated that starch-based CLH was a good controlled-release agent for carbendazim and water absorbent for soil.

Introduction

Fungicides are essential to disease control and maintenance of agricultural production (Singh, Sharma, & Gupta, 2009). Carbendazim, a widely used fungicide, is used to control and prevent a wide range of fungal diseases of crops (Anastassiades and Schwack, 1998, Itak et al., 1993, Wang et al., 2009, Yu et al., 2009). When applied by conventional methods, carbendazim is invariably subject to wastages like photo-degradation, leaching, etc. (Sun et al., 2014), thus, frequent application is needed. However, excessive use of carbendazim can cause undesirable effects on humans. Its detection in tomatoes and wastewater has caused public concerns (Bollmann et al., 2014, Liu et al., 2014). Therefore, there is an urgent need to develop a new method for carbendazim application. The use of various delivery systems, including microcapsules, microspheres, and beads is effective at decreasing wastage of agrochemicals (Freiberg and Zhu, 2004, Jarosiewicz and Tomaszewska, 2003, Taki et al., 2001). The fungicides thiram and tebuconazlole have been loaded into polymers for slow-release (Asrar et al., 2004, Singh et al., 2009).

Large parts of the world, especially in China, are arid and semi-arid regions. Soil in these areas has weak WHC and does not efficiently retain rain water. Hydrogels are polymeric network structures that can absorb large amounts of water and retain water for specific periods (Liu et al., 2009, Tong and Zhang, 2005). Recently, there has been an increased interest in the use of natural polysaccharides based hydrogels in the fields of agriculture (Thakur & Thakur, 2014a) due to their enormous advantages such as economic/low cost, biodegradability, acceptable specific strength, low density, good thermal properties, recyclability, no health risk, bounty and enhanced energy (Thakur et al., 2013a, Thakur et al., 2013b; Thakur et al., 2014a, Thakur et al., 2014b, Thakur et al., 2014c). Natural polysaccharides such as starch, cellulose (Thakur et al., 2014a, Thakur et al., 2014b, Thakur et al., 2014c), chitosan (Thakur & Thakur, 2014b), lignin (Thakur and Thakur, 2015, Thakur et al., 2014a, Thakur et al., 2014b, Thakur et al., 2014c), alginate and psyllium polysaccharide (Thakur & Thakur, 2014a) are well-known examples of bio-renewable resource for environmentally friendly hydrogels. Among various types of natural polymers, starch based hydrogels have attracted great attention all over the world because of economical effectiveness, environmental friendliness and easy modification with vinyl monomers onto it. The OH on the anhydroglucose unit of starch has the potential to form complex copolymer networks, which often has the property of absorbing large amounts of water (Zhang et al., 2013).

Replacement of conventional application of agrochemicals by a combination of controlled-release and water absorption systems has received much attention. Fertilizers, pesticides cypermethrin and micronutrients copper sulfate have been successfully incorporated into hydrogels (Liang and Liu, 2006, Rudzinski et al., 2003, Yang et al., 2013). However, there are few reports on hydrogels loading carbendazim.

In this work, CLH with specific WHC and slow-release properties were prepared in two steps. The release behaviors of CLH in ddH₂O and phosphate buffer solution (PBS) were learnt. The effects of WA on release profiles and kinetics were investigated. What's more, the influences of pH and salt ion on the release rate were also determined.